ORCID Profile
0000-0002-5295-635X
Current Organisation
University of Western Australia
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Publisher: Springer Science and Business Media LLC
Date: 13-10-2007
Publisher: Elsevier BV
Date: 10-2003
Publisher: IOP Publishing
Date: 19-10-2018
Publisher: MDPI AG
Date: 28-02-2023
Abstract: To evaluate novel turbine designs, the wind energy sector extensively depends on computational fluid dynamics (CFD). To use CFD in the design optimization process, where lower-fidelity approaches such as blade element momentum (BEM) are more popular, new tools to increase the accuracy must be developed as the latest wind turbines are larger and the aerodynamics and structural dynamics become more complex. In the present study, a new concurrent aerodynamic shape optimization approach towards multidisciplinary design optimization (MDO) that uses a Reynolds-averaged Navier–Stokes solver in conjunction with a numerical optimization methodology is introduced. A multidisciplinary design optimization tool called DAFoam is used for the NREL phase VI turbine as a baseline geometry. Aerodynamic design optimizations in terms of five different schemes, namely, cross-sectional shape, pitch angle, twist, chord length, and dihedral optimization are conducted. Pointwise, a commercial mesh generator is used to create the numerical meshes. As the adjoint approach is strongly reliant on the mesh quality, up to 17.8 million mesh cells were employed during the mesh convergence and result validation processes, whereas 2.65 million mesh cells were used throughout the design optimization due to the computational cost. The Sparse Nonlinear OPTimizer (SNOPT) is used for the optimization process in the adjoint solver. The torque in the tangential direction is the optimization’s merit function and excellent results are achieved, which shows the promising prospect of applying this approach for transient MDO. This work represents the first attempt to implement DAFoam for wind turbine aerodynamic design optimization.
Publisher: American Physical Society (APS)
Date: 26-01-2004
Publisher: Ovid Technologies (Wolters Kluwer Health)
Date: 11-2004
DOI: 10.1097/01.MAT.0000144363.92963.CB
Abstract: In previous studies, the radial and tangential leakage flow velocities in the gap between the rotating impeller and the pump casing of a biocentrifugal ventricular assist device (VAD) model were obtained by hot wire. Based on the velocities obtained, the leakage flow rate through the clearance gap between the impeller and the stationary casing, as well as the wall shear stress distributions on the inner surface of the stationary casing, can be determined. By integrating the radial velocities numerically, the leakage flow rate through the clearance gap was found to be 6.43 x 10(-4) m3/s under operating conditions, which is 25.7% of the inlet flow. This is equivalent to approximately 1.7 L/min in the prototype. The leakage flow rate was found to be 5.22 x 10(-4) m3/s and 3.66 x 10(-4) m3/s under fully opened and fully closed conditions, respectively. The double volute design significantly affected wall shear stress distributions, with the high wall shear stress region concentrated at the beginning of the splitter plate under all three flow conditions. In contrast, the high wall shear stress region could only be observed at the cutwater in the fully closed condition. The highest wall shear stress was found to be 44.1 Pa, which is much lower than the threshold values that cause hemolysis. On the other hand, the lowest wall shear stress was found to be 21.31 Pa. This wall shear stress level is much higher than the shear stress required to disrupt aggregates of blood cells and platelets. These findings explain why hemolysis and thrombosis are at the low level in the clearance gap of this VAD.
Publisher: Elsevier BV
Date: 05-2021
Publisher: Elsevier BV
Date: 07-2009
Publisher: American Physical Society (APS)
Date: 02-10-2007
Publisher: AIP Publishing
Date: 10-2022
DOI: 10.1063/5.0111570
Abstract: Nonlinear dynamic response of catenary flexible risers excited by ocean currents is the main reason for fatigue failures. In this work, a series of experiments are conducted to improve the understanding of the intrinsic relationship between the in-plane and out-of-plane responses of a catenary flexible riser subjected to a sheared current in the depth-averaged Reynold number range of 150–1880. It is found that the interaction between the in-plane and out-of-plane responses is sensitive to the reduced velocity. Four patterns are identified, including the weak coupling, the partial coupling, the strong coupling, and the strong coupling+. The interaction is negligible at low reduced velocities. In contrast, when the reduced velocity is sufficiently large, the in-plane dominant frequency coincides with the out-of-plane one, signifying a strong coupling. As the reduced velocity increases, only a part of the riser possesses the same dominant frequency in the in-plane and out-of-plane directions, termed partial coupling. At higher reduced velocities, the coupled pipe section is shortened and the interaction is finally attenuated.
Publisher: Springer Berlin Heidelberg
Date: 18-12-2015
Publisher: Elsevier BV
Date: 04-2022
Publisher: Elsevier BV
Date: 07-2022
Publisher: American Institute of Aeronautics and Astronautics (AIAA)
Date: 03-2004
DOI: 10.2514/1.9111
Publisher: Cambridge University Press (CUP)
Date: 09-11-2016
DOI: 10.1017/JFM.2016.664
Abstract: The transport of momentum and heat in the turbulent intermediate wake of a circular cylinder is inherently three-dimensional (3-D). This work aims to gain new insight into the 3-D vorticity structure, momentum and heat transport in this flow. All three components of the velocity and vorticity vectors, along with the fluctuating temperature, are measured simultaneously, at nominally the same point in the flow, with a probe consisting of four X-wires and four cold wires. Measurements are made in the ( $x$ , $y$ ) or mean shear plane at $x/d=10$ , 20 and 40 at a Reynolds number of $2.5\\times 10^{3}$ based on the cylinder diameter $d$ and the free-stream velocity. A phase-averaging technique is developed to separate the large-scale coherent structures from the remainder of the flow. It is found that the effects of vorticity on heat transport at $x/d=10$ and $x/d=20{-}40$ are distinctly different. At $x/d=10$ , both spanwise and streamwise vorticity components account significantly for the heat flux. At $x/d=20$ and 40, the spanwise vortex rollers play a major role in inducing the coherent components of the heat flux vector, while the ribs are responsible for the small-scale heat diffusion out of the spanwise vortex rollers. The present data indicate that, if the spanwise-velocity-related terms are ignored, the estimated values of the production can have errors of approximately 22 % and 13 % respectively for the turbulent energy and temperature variance at $x/d=40$ , and the errors are expected to further increase downstream. A conceptual model summarizing the 3-D features of the heat and momentum transports at $x/d=10$ is proposed. Compared with the previous two-dimensional model of Matsumura & Antonia ( J. Fluid Mech. , vol. 250, 1993, pp. 651–668) or MA, the new model provides a more detailed description of the role the rib-like structures undertake in transporting heat and momentum, and also underlines the importance of the upstream half of the spanwise vortex rollers, instead of only one quadrant of these rollers, as in the MA model, in diffusing heat out of the vortex.
Publisher: Elsevier BV
Date: 10-2011
Publisher: AIP Publishing
Date: 03-2023
DOI: 10.1063/5.0138364
Abstract: This paper reports the experimental results of the vortex-induced vibration (VIV) response of an inclined flexible pipe with various oblique angles (θ) ranging from 0° to 60°. The flexible pipe with an aspect ratio of 75 was fixed at both ends. The VIV response was examined in the reduced velocity range of 4.02–17.55 to evaluate the IP (independence principle) validity. The experimental results illustrate that the spatial distribution of response litudes and associated modal weights varies with θ, and the differences are enlarged in the mode transition cases. With increasing the θ, the onset normal reduced velocity (Urn) of mode transition shifts to a lower value and the Urn range of mode transition is narrowed gradually. Additionally, the mode competition is enhanced, resulting in pronounced traveling waves with accelerated propagation speed. The vortex shedding pattern varies along the span, presenting the 2S (two vortices are shed per cycle) and P + S (a pair of vortices and a single vortex are shed from two sides of the cylinder in one cycle) patterns at the upstream and downstream pipe segments, respectively. The streamwise distance between two adjacent vortices is enlarged with the increase in θ. The pipe placed with different inclined angles presents different partitions of coupling pattern, and the Urn range of each sub-region is different. By comprehensive consideration of the spatial–temporal evolution of VIV response, the mode transition features, the coupling pattern, and vortex shedding characteristics, the IP is invalid in predicting the VIV of flexible pipe.
Publisher: Elsevier BV
Date: 09-2014
Publisher: Elsevier BV
Date: 06-2021
Publisher: Wiley
Date: 07-2007
DOI: 10.1111/J.1525-1594.2007.00422.X
Abstract: The axial blood pump with a magnetically suspended impeller is superior to other artificial blood pumps because of its small size. In this article, the distributions of velocity, path line, pressure, and shear stress in the straightener, the rotor, and the diffuser of the axial blood pump, as well as the gap zone were obtained using the commercial software, Fluent (version 6.2). The main focus was on the flow field of the blood pump. The numerical results showed that the axial blood pump could produce 5.14 L/min of blood at 100 mm Hg through the outlet when rotating at 11,000 rpm. However, there was a leakage flow of 1.06 L/min in the gap between the rotor cylinder and the pump housing, and thus the overall flow rate the impeller could generate was 6.2 L/min. The numerical results showed that 75% of the scalar shear stresses (SSs) were less than 250 Pa, and 10% were higher than 500 Pa within the whole pump. The high SS region appeared around the blade tip where a large variation of velocity direction and magnitude was found, which might be due to the steep angle variation at the blade tip. Because the exposure time of the blood cell at the high SS region within the pump was relatively short, it might not cause serious damage to the blood cells, but the improvement of blade profile should be considered in the future design of the axial pump.
Publisher: Elsevier BV
Date: 11-2020
Publisher: AIP Publishing
Date: 04-2021
DOI: 10.1063/5.0050385
Abstract: This paper reports the results of a numerical investigation into the flow around three isodiametric circular cylinders placed in an in-line configuration with identical spacing and the associated wake structures. The effect of spacing ratio (L/D, where L is the center-to-center spacing between two adjacent cylinders and D is the cylinder diameter) ranging from 1.5D to 10D with increment of 0.5D is examined at a low Reynolds number of 160. The results indicate that the wake structure experiences three evolutions as L/D increases, exhibiting four combined flow patterns: the overshoot, continuous reattachment-alternate reattachment (CR-AR), quasi-co-shedding (QCS), and co-shedding-co-shedding regimes. More than one frequency participates in the lift fluctuation for all three cylinders in the latter three patterns, signifying the wake interference. The evolution of the flow regime results in the variations of drag and lift coefficients, the alteration of pressure distribution around the cylinders' surface, and the switching of the phase lags of fluctuating lifts as well as the modification of vortex shedding characteristics, including the vortex formation location, wake width, and shedding frequencies. Particularly, the fluid forces and Strouhal number for the upstream and middle cylinders present a jump in the transition from the CR-AR to QCS regimes, while the fluid forces are reduced for the downstream cylinder. Furthermore, the transition from two layered vortices to the secondary vortex street is observed at 3.5 & L/D ≤ 6.5, where the downstream cylinder is sandwiched between the two shear layers detached from the middle cylinder, and two same-sign vortices in the same layer merge into a new vortex in the far wake.
Publisher: Springer Science and Business Media LLC
Date: 04-05-2006
Publisher: Wiley
Date: 06-2006
DOI: 10.1111/J.1525-1594.2006.00243.X
Abstract: Implantable ventricular assist devices have been regarded as a promising instrument in the clinical treatment of patients with severe heart failures. In this article, a three-dimensional model of the Kyoto-NTN magnetically suspended centrifugal blood pump was generated and a computational fluid dynamics solution of the inner flow field of the pump including the static pressure distributions, velocity profiles, and the shear stress distributions of the blood was presented. The results revealed that reverse flow generally occurred in the impeller blade channels during the operation of the pump, due to the imbalance of the flow and the pressure gradient generated in the blade channels. The flow pattern at the exit of the blade channels was varying with its angular positions in the pump. The reverse flow at the exit of the impeller blade channels was found to be closely related with the static pressure distribution in the volute passage. Higher pressure in the volute caused severe backflow from the volute into the blade channels. To clarify the effects of a moving impeller on the blood, shear stresses of the blood in the pump were investigated according to the simulation results. The studies indicated that at the beginning of the splitter plate and the cutwater, the highest shear stress exceeded 700 Pa. At other regions such as the inlet and outlet of the impeller blade channels and some regions in the volute passage, shear stresses were found to be about 200 Pa. These areas are believed to have a high possibility of rendering blood trauma.
Publisher: Springer Science and Business Media LLC
Date: 31-08-2020
DOI: 10.1186/S13256-020-02451-4
Abstract: Anaplastic astrocytoma is a rare disorder in children from 10 to 14 years of age, with an estimated 0.38 new cases per 100,000 people per year worldwide. Panel-based next-generation sequencing opens new possibilities for diagnosis and therapy of rare diseases such as this one. Because it has never been genetically studied in the Ecuadorian population, we chose to genetically characterize an Ecuadorian pediatric patient with anaplastic astrocytoma for the first time. Doing so allows us to provide new insights into anaplastic astrocytoma diagnosis and treatment. Our patient was a 13-year-old Mestizo girl with an extensive family history of cancer who was diagnosed with anaplastic astrocytoma. According to ClinVar, SIFT, and PolyPhen, the patient harbored 354 genomic alterations in 100 genes. These variants were mostly implicated in deoxyribonucleic acid (DNA) repair. The top five most altered genes were FANCD2 , NF1 , FANCA , FANCI , and WRN. Even though TP53 presented only five mutations, the rs11540652 single-nucleotide polymorphism classified as pathogenic was found in the patient and her relatives interestingly, several reports have related it to Li-Fraumeni syndrome. Furthermore, in silico analysis using the Open Targets Platform revealed two clinical trials for pediatric anaplastic astrocytoma (studying cabozantinib, ribociclib, and everolimus) and 118 drugs that target the patient’s variants, but the studies were not designed specifically to treat pediatric anaplastic astrocytoma. Next-generation sequencing allows genomic characterization of rare diseases for instance, this study unraveled a pathogenic single-nucleotide polymorphism related to Li-Fraumeni syndrome and identified possible new drugs that specifically target the patient’s variants. Molecular tools should be implemented in routine clinical practice for early detection and effective preemptive intervention delivery and treatment.
Publisher: Elsevier BV
Date: 12-2021
Publisher: Springer Science and Business Media LLC
Date: 09-2002
Publisher: Elsevier BV
Date: 05-2018
Publisher: Springer Science and Business Media LLC
Date: 23-09-1999
Publisher: Informa UK Limited
Date: 2007
Publisher: American Society of Mechanical Engineers
Date: 03-08-2020
Abstract: Water Intake Riser (WIR), conveying cooling water from the sea, is key to liquefaction of natural gas in the Floating Liquefied Natural Gas (FLNG) facility. Due to the wave-induced vessel motion, WIRs may experience resonant vibrations, which influence its fatigue life. In such situations, the estimate of hydrodynamic d ing is critical to the prediction of fatigue life. Due to its small motion litudes compared to the diameter of WIR, the Keulegan–Carpenter (KC) for motion-induced flow around WIR is normally small (e.g. KC & 5). For small KC values, the effect of steady current on the hydrodynamic d ing is not well understood and the current practice of using the relative velocity Morison model for predicting the hydrodynamic d ing with in-line steady current is challenged by guidelines such as DNVGL-RP-C205 and ISO-19902. In this study, the hydrodynamic d ing of a smooth WIR oscillating in still water or in steady currents is measured with a series of experiments at KC & 5 and the Reynolds number (Re) in the range of 103 ∼ 105. The effect of in-line or cross steady currents on the in-line hydrodynamic d ing is investigated and the performance of the relative velocity Morison model for predicting the hydrodynamic d ing at low KC is examined. Experiments are also conducted for a WIR with helical strakes in in-line or cross currents. Based on these experimental results, recommendations are made for predicting hydrodynamic d ing in the WIR design.
Publisher: American Society of Mechanical Engineers
Date: 07-2012
Abstract: Vortex-induced vibrations of two side-by-side cylinders of different diameters in steady incompressible flow are studied. The diameter ratio of cylinders is fixed at 0.1. The Reynolds number is fixed at 5000 based on the large cylinder diameter and free stream velocity. A Petrov-Galerkin finite element method is used to solve the two dimensional Reynolds-averaged Navier Stokes equations using the Arbitrary Lagrangian Eulerian scheme with a SST k-ω turbulence model closure. The numerical method has been validated against available experimental results. Then, the effects of natural frequencies of the cylinders on the vibration litude and vortex shedding regimes are investigated. It is found that for the range of considered parameters, collision of the cylinders is dependent on the difference of the natural frequencies of the cylinders.
Publisher: The University of Queensland
Date: 11-12-2020
DOI: 10.14264/23C4C62
Publisher: Informa UK Limited
Date: 2010
Publisher: Wiley
Date: 26-09-2010
DOI: 10.1111/J.1525-1594.2010.01099.X
Abstract: Generally, there are two types of impeller design used in the axial flow blood pumps. For the first type, which can be found in most of the axial flow blood pumps, the magnet is embedded inside the impeller hub or blades. For the second type, the magnet is embedded inside the cylindrical impeller shroud, and this design has not only increased the rotating stability of the impeller but has also avoided the flow interaction between the impeller blade tip and the pump casing. Although the axial flow blood pumps with either impeller design have been studied in idually, the comparisons between these two designs have not been conducted in the literature. Therefore, in this study, two axial flow blood pumps with and without impeller shrouds were numerically simulated with computational fluid dynamics and compared with each other in terms of hydraulic and hematologic performances. For the ease of comparison, these two models have the same inner components, which include a three-blade straightener, a two-blade impeller, and a three-blade diffuser. The simulation results showed that the model with impeller shroud had a lower static pressure head with a lower hydraulic efficiency than its counterpart. It was also found that the blood had a high possibility to deposit on the impeller shroud inner surface, which greatly enhanced the possibility of thrombus formation. The blood damage indices in both models were around 1%, which was much lower than the 13.1% of the axial flow blood pump of Yano et al. with the corresponding experimental hemolysis of 0.033 g/100 L.
Publisher: Wiley
Date: 31-03-2021
Publisher: AIP Publishing
Date: 2013
DOI: 10.1063/1.4788934
Abstract: Vortex shedding from an inclined circular cylinder at low values of Reynolds number (Re) is investigated numerically. The aim of the study is to investigate the effect of cylinder oblique angle on the transition from two-dimensionality (2D) to three-dimensionality (3D) of the wake flow. The Navier-Stokes equations are solved by the Petrov-Galerkin finite element method for Reynolds numbers ranging from 100 to 1000 and the flow attack angles of α = 0° and 45°. For the right attack angle case (α = 0°), the predicted wavy spanwise vortices in the early stage of the transition mode A, the vortex dislocation in the late stage of the transition mode A and the streamwise-vortex dominant wake flow structure in the transition mode B are found to agree well with independent experimental observations and measurements. The transition from 2D to 3D at α = 45° was found distinctively different from that at α = 0°. For α = 45°, no clear-cut transition modes are observed. The wake is characterized by wavy spanwise vortices close to the lower boundary of the transition Reynolds number regime, which are similar to those in the early stage of the transition mode A at α = 0°. The vortex-dislocation in the transition mode A was not observed at α = 45°. It appears that the fluid flow in the spanwise direction in the primary vortices at α = 45° does not allow the instability to sustain at a specific spanwise location and trigger the vortex dislocation. Although the wake flow structure is different, the variation of the normal Strouhal number with the normal Reynolds number (both based on the velocity component perpendicular to the cylinder span) at α = 45° is close to that at α = 0° in the transitional Reynolds number regime. The root mean square of the lift coefficient normalized by the velocity component perpendicular to the axial direction of the cylinder at α = 45° is about 20% to 25% larger than that at α = 0° in the Reynolds number regime between 250 and 500.
Publisher: Elsevier BV
Date: 05-2022
Publisher: The University of Queensland
Date: 11-12-2020
DOI: 10.14264/3EB5472
Publisher: AIP Publishing
Date: 02-2013
DOI: 10.1063/1.4792351
Abstract: Vortex-induced vibrations (VIV) of a square cylinder at a Reynolds number of 100 and a low mass ratio of 3 are studied numerically by solving the Navier-Stokes equations using the finite element method. The equation of motion of the square cylinder is solved to simulate the vibration and the Arbitrary Lagrangian Eulerian scheme is employed to model the interaction between the vibrating cylinder and the fluid flow. The numerical model is validated against the published results of flow past a stationary square cylinder and the results of VIV of a circular cylinder at low Reynolds numbers. The effect of flow approaching angle (α) on the response of the square cylinder is investigated. It is found that α affects not only the vibration litude but also the lock-in regime. Among the three values of α (α = 0°, 45°, and 22.5°) that are studied, the smallest vibration litude and the narrowest lock-in regime occur at α = 0°. It is discovered that the vibration locks in with the natural frequency in two regimes of reduced velocity for α = 22.5°. Single loop vibration trajectories are observed in the lock-in regime at α = 22.5° and 45°, which is distinctively different from VIV of a circular cylinder. As a result, the vibration frequency in the in-line direction is the same as that in the cross-flow direction.
Publisher: Cambridge University Press (CUP)
Date: 17-05-2019
DOI: 10.1017/JFM.2019.296
Abstract: This work focuses on the temperature (passive scalar) and velocity characteristics within a turbulent Kármán vortex using a phase-averaging technique. The vortices are generated by a circular cylinder, and the three components of the fluctuating velocity and vorticity vectors, $u_{i}$ and $\\unicode[STIX]{x1D714}_{i}$ ( $i=1,2,3$ ), are simultaneously measured, along with the fluctuating temperature $\\unicode[STIX]{x1D703}$ and the temperature gradient vector, at nominally the same spatial point in the plane of mean shear at $x/d=10$ , where $x$ is the streamwise distance from the cylinder axis and $d$ is the cylinder diameter. We believe this is the first time the properties of fluctuating velocity, temperature, vorticity and temperature gradient vectors have been explored simultaneously within the Kármán vortex in detail. The Reynolds number based on $d$ and the free-stream velocity is $2.5\\times 10^{3}$ . The phase-averaged distributions of $\\unicode[STIX]{x1D703}$ and $u_{i}$ follow closely the Gaussian distribution for $r/d\\leqslant 0.2$ ( $r$ is the distance from the vortex centre), but not for $r/d .2$ . The collapse of the distributions of the mean-square streamwise derivative of the velocity fluctuations within the Kármán vortex implies that the velocity field within the vortex tends to be more locally isotropic than the flow field outside the vortex. A possible physical explanation is that the large and small scales of velocity and temperature fields are statistically independent of each other near the Kármán vortex centre, but interact vigorously outside the vortex, especially in the saddle region, due to the action of coherent strain rate.
Publisher: Elsevier BV
Date: 07-2021
Publisher: American Society of Mechanical Engineers
Date: 08-06-2014
Abstract: The resonant behavior of the fluid trapped in the narrow gap between a floating LNG and an LNG carrier in a side-by-side offloading operation is investigated in this study employing a numerical wave flume. The wave flume is based on the finite volume solution of the Navier-Stokes equations to account for the viscous dissipation. The waveFoam toolbox, a modified version of the standard OpenFOAM multiphase flow solver interFoam developed by Jacobson et al (2011) has been used for the purpose of wave generation and relaxation inside the computational domain. This method has a quite high efficiency as it takes advantage of the potential flow theory for wave generation purpose and the viscous flow theory for inside the wave tank, respectively. The volume of fluid (VOF) method first introduced by Hirt and Nichols (1981) is used to capture the free surface oscillations at the air and water interface. Water waves are generated at a reasonable distance from the inlet boundary and two rectangular relaxation zones at the inlet and outlet boundaries of the domain have been implemented to suppress wave reflection at the outer boundaries as well as waves reflected internally in the computational domain. The influence of incident wave frequency on resonance wave height and frequency is examined. Numerical results of free surface evolution at different incident wave frequency seem to agree well with the experimental results of Saitoh et al (2006) and numerical results of Lu et al (2008). In order to justify the effect of bilge keels on flow separation at the bottom corner of the ship, four different corner configurations have been investigated and compared to the base sharp edged case. It is observed that the magnitude of the free surface elevation at the resonance frequency increases significantly by about 10 times the incoming wave height while the peak of resonant frequency curves shifts to higher frequencies in the higher curvature modes compared with the base case.
Publisher: IOP Publishing
Date: 12-2017
Publisher: Elsevier BV
Date: 08-2014
Publisher: Cambridge University Press (CUP)
Date: 24-11-2022
DOI: 10.1017/JFM.2022.872
Abstract: Understanding the fluid dynamics associated with a circular cylinder oscillating normal to a plane wall is important for safe design of offshore infrastructure, such as power cables and pipeline risers. This paper investigates the fluid dynamics of an oscillating cylinder with no imposed incident current experimentally using flow visualisation and force measurements where the ratio of the cylinder Reynolds number ( $Re$ ) to Keulegan–Carpenter number ( $KC$ ) is $\\beta =500$ and $KC$ varies between 2 and 12. The minimum distance between the cylinder and wall was between 12.5 % and $50\\,\\%$ of the diameter. Across this parameter space three primary vortex flow regimes were observed: (i) for $KC\\leq 5$ , the flow field is approximately symmetric about the cylinder centreline and the velocity field between the cylinder and the wall resembled a pumping flow in phase with cylinder motion, which is well predicted by potential theory for most of the cycle (ii) for $5 KC $ , the flow field is increasingly asymmetric but with frequent switching of the side associated with vortex shedding and (iii) for $KC\\geq 8$ , the flow field is consistently asymmetric due to vortex shedding. The in-line force increases when the cylinder is near the wall due to dynamic pressures associated with pumping. This increase can be estimated using potential theory superimposed onto the force time history for an isolated cylinder at the same $KC$ and $Re$ . This study complements recent numerical modelling focused on low Reynolds number conditions and provides important insights into the fluid mechanics associated with trenching beneath cable and pipeline risers.
Publisher: American Society of Mechanical Engineers
Date: 07-2012
Abstract: Vortex induced vibration (VIV) due to steady current flow can be a significant driver in the design of offshore riser systems, affecting riser global configuration, component details and overall subsea architecture. Helical strakes are known to reduce VIV but the degree of effectiveness can vary considerably depending on strake pitch, fin height and more importantly, current flow regime. In addition, the litude of VIV and the effectiveness of VIV suppression strakes depends on the inclination of flow to the riser (incidence angle) and presence of wake effects from adjacent risers. Test and field data regarding suppression of riser VIV by strakes is not extensively available in the public domain. This is primarily due to the proprietary nature of the tests conducted in industry. In this paper, a program of testing is devised to better understand strake effectiveness as a function of current incidence angle and the presence of adjacent risers. Experiments have been conducted on single and tandem pipe arrangements in air in order to evaluate strake suppression efficiency. Aluminium cylinders are tested in a wind tunnel in the structures laboratory of The University of Western Australia (UWA). Two sets of experiments are conducted: the first to evaluate cylinder VIV response at angles of incidence ranging from 30 to 90 degrees and the second to evaluate VIV response of the downstream pipe in a dual pipe arrangement with varying spacing between the pipes. In both cases the bare cylinders are first tested at varying flow speeds. Helical strakes are then added to the single cylinder, and downstream cylinder in the tandem pipe test, and the vibration response is recorded at varying flow speeds. From the experimentation, it can be seen that downstream cylinder motions are lified by wake induced instability. This phenomenon is of particular concern for tightly spaced top-tensioned risers (TTR) in wellbays of tension leg platforms (TLP) and deep draft floaters. The VIV motion of the downstream, bare, wake-affected pipe, is magnified to approximately 1.3–2 times the motion of a single bare pipe. When strakes are added to the downstream cylinder, the magnification factor of the downstream cylinder response is largely increased due to the wake of the upstream bare cylinder. However, the actual VIV motions of the downstream cylinder are largely reduced when strakes are incorporated. The present work demonstrates that helical strakes provide an effective means of suppressing vortex induced vibrations of risers in riser arrays, though the degree of effectiveness is reduced in a downstream tubular compared to suppression levels for single pipes.
Publisher: Elsevier BV
Date: 08-2014
Publisher: AIP Publishing
Date: 07-2023
DOI: 10.1063/5.0156623
Abstract: This paper reports the numerical results of flow past a round-ended cylinder with various incidence angles in a low Reynolds number range of Re = 60–160. Mapping of the flow structure and hydrodynamic properties is examined in the incidence angle range of α = 0°–90° with increment of 15°. Three wake patterns are identified, including the steady and symmetric wake without vortex shedding (Pattern I), the Karman vortex street (Pattern II), and the Karman vortex street with the occurrence of subordinate vortex (Pattern III). The reattachment of boundary layers results in the occurrence of a subordinate vortex and hence the non-single-frequency fluctuation of hydrodynamic coefficients (CL and CD, which are lift and drag coefficients, respectively). Elliptical and figure-eight CL–CD curves are observed, depending on the frequency ratios of the two coefficients and their weights. Non-zero time-averaged CL occurs when 0° & α & 90°, due to the asymmetric boundary layer separation. The backward migration of boundary layer separation point contributes to the reduction of frictional drag and the vortex formation length. The shortening of the vortex formation length results in the enhanced fluctuations of CL and CD.
Publisher: IOP Publishing
Date: 22-11-2017
Publisher: Springer Science and Business Media LLC
Date: 08-2002
Publisher: Informa UK Limited
Date: 19-03-2020
Publisher: AIP Publishing
Date: 08-2021
DOI: 10.1063/5.0059089
Abstract: This work examined the development of the large-scale vortex structures in the wake of two screen cylinders with porosities β = 48% and 61% in the intermediate region at a Reynolds number of 7000 using two X-type hot-wire probes. The screen cylinders were made of stainless steel screen meshes rolled into a cylindrical shape. The results were compared with those of a solid cylinder (β = 0%) under the same flow conditions. It was shown that the formation of the large-scale vortices in the screen cylinder wakes involves different mechanisms. For the screen cylinder with β = 48%, they were generated due to wake instability (Kármán vortex), which closely resembled that of a solid cylinder. In contrast, for the screen cylinder with β = 61%, the vortices were formed owing to the shear-layer (Kelvin–Helmholtz) convective instability. These findings were justified by the streamwise evolution of the vortex shedding frequency, a stability analysis in the near wake using the mean streamwise velocity profiles, and the coherent and incoherent wake structures obtained by the phase-averaged analysis at different downstream locations.
Publisher: Cambridge University Press (CUP)
Date: 15-07-2015
DOI: 10.1017/JFM.2015.342
Abstract: The transport equation for the mean turbulent energy dissipation rate $\\overline{{\\it\\epsilon}}$ along the centreline of a fully developed channel flow is derived by applying the limit at small separations to the two-point budget equation. Since the ratio of the isotropic energy dissipation rate to the mean turbulent energy dissipation rate $\\overline{{\\it\\epsilon}}_{iso}/\\overline{{\\it\\epsilon}}$ is sufficiently close to 1 on the centreline, our main focus is on the isotropic form of the transport equation. It is found that the imbalance between the production of $\\overline{{\\it\\epsilon}}$ due to vortex stretching and the destruction of $\\overline{{\\it\\epsilon}}$ caused by the action of viscosity is governed by the diffusion of $\\overline{{\\it\\epsilon}}$ by the wall-normal velocity fluctuation. This imbalance is intrinsically different from the advection-driven imbalance in decaying-type flows, such as grid turbulence, jets and wakes. In effect, the different types of imbalance represent different constraints on the relation between the skewness of the longitudinal velocity derivative $S_{1,1}$ and the destruction coefficient $G$ of enstrophy in different flows, thus resulting in non-universal approaches of $S_{1,1}$ towards a constant value as the Taylor microscale Reynolds number, $R_{{\\it\\lambda}}$ , increases. For ex le, the approach is slower for the measured values of $S_{1,1}$ along either the channel or pipe centreline than along the axis in the self-preserving region of a round jet. The data for $S_{1,1}$ collected in different flows strongly suggest that, in each flow, the magnitude of $S_{1,1}$ is bounded, the value being slightly larger than 0.5.
Publisher: Springer Science and Business Media LLC
Date: 27-12-2021
Publisher: Elsevier BV
Date: 10-2020
Publisher: Springer Science and Business Media LLC
Date: 13-02-2009
Publisher: Springer International Publishing
Date: 2016
Publisher: Elsevier BV
Date: 05-2015
Publisher: Elsevier BV
Date: 02-2005
Publisher: Elsevier BV
Date: 04-2005
Publisher: AIP Publishing
Date: 1979
DOI: 10.1063/1.862446
Abstract: For a one-dimensional flow situation involving a source, a simple closed form solution for the mass density is obtained for the mass continuity equation by introducing a change in variable. Instead of functionalizing the flow in terms of the velocity, the starting time or transit time is used. This type of functionalization allows the in idual trajectories to be obtained by algebraic or geometric means, avoiding the nonclosed form solution associated with integrating the velocity. For a time varying system, the integrals of mass, momentum, and energy all involve the mass density expression. By eliminating the mass density expression from these integrals and expressing the velocity in terms of one of the new time parameters, a set of equations is obtained which may be easier to solve than the original set of equations.
Publisher: IOP Publishing
Date: 02-2001
Publisher: Elsevier BV
Date: 05-2022
Publisher: Springer Science and Business Media LLC
Date: 24-03-2011
Publisher: Elsevier BV
Date: 03-2008
DOI: 10.1016/J.MEDENGPHY.2007.02.004
Abstract: Hemodynamics is widely believed to influence coronary artery bypass graft (CABG) stenosis. Although distal anastomosis has been extensively investigated, further studies on proximal anastomosis are still necessary, as the extent and initiation of the stenosis process may be influenced by the flow of the proximal anastomosis per se. Therefore, in this study, two models (i.e. 90 degrees and 135 degrees anastomotic models) were designed and constructed to simulate a proximal anastomosis of CABG for the left and right coronary arteries, respectively. Flow characteristics for these models were studied experimentally in order to validate the simulation results found earlier. PIV measurements were carried out on two Pyrex glass models, so that the disturbed flow (stagnation point, flow separation and vortex) found in both proximal anastomosis models using numerical simulation, could be verified. Consequently, a fair agreement between numerical and experimental data was observed in terms of flow characteristics, velocity profiles and wall shear stress (WSS) distributions under both steady and pulsatile flow conditions. The discrepancy was postulated to be due to the difference in detailed geometry of the physical and computational models, due to manufacturing limitations. It was not possible to reproduce the exact shape of the computational model when making the Pyrex glass model. The analysis of the hemodynamic parameters based on the numerical simulation study also suggested that the 135 degrees proximal anastomosis model would alleviate the potential of intimal thickening and/or atherosclerosis, more than that of a 90 degrees proximal anastomosis model, as it had a lower variation range of time-averaged WSS and the lower segmental average of WSSG.
Publisher: Springer International Publishing
Date: 2019
Publisher: Elsevier BV
Date: 04-2006
Publisher: IOP Publishing
Date: 10-1999
Publisher: Elsevier BV
Date: 06-2016
Publisher: AIP Publishing
Date: 11-2021
DOI: 10.1063/5.0073783
Abstract: This paper reports the results of a numerical investigation into the flow over two tandem semi-circular cylinders with flat surfaces facing each other and the associated wake flow characteristics. The effect of spacing ratio (L/D, L is the in-line spacing between two flat surfaces and D is the diameter of cylinders) ranging from 1 to 16 is examined in a low Reynolds number range of Re = 60–180. Five flow patterns are identified including the over-shoot, continuous reattachment, alternate reattachment, quasi-co-shedding, and co-shedding regimes. The difference between quasi-co-shedding and co-shedding regimes depends on the origin of the vortices shed from the downstream cylinder. Unlike the vortices generated due to the roll-up of shear layers detached from both upstream and downstream cylinders in the co-shedding regime, the vortices shed from the downstream cylinder come from upstream instead of the roll-up of shear layers for the quasi-co-shedding regime. The wake flow characteristics are discussed in terms of the vortex formation and shedding, wake structure and evolution, boundary layer separation and shear layer development, the occurrence of secondary vortex street, and the associated migrating vortex modes. The variations of hydrodynamic coefficients and Strouhal number are associated with the flow regime transition. When the alternate reattachment and quasi-co-shedding regimes emerge, a small pair of peaks of the root-mean-squared streamwise flow velocity (u*rms) fixedly occurs at the two corners of the downstream cylinder, illustrating the strong velocity fluctuation caused by vortices impingement and the secondary separation. Three types of vortex street are observed in the wake, where the secondary vortex street region is further ided into three parts in terms of the vortex structure, including S + P (a single vortex and a pair of vortices are formed per cycle in the secondary vortex street), P (a pair of vortices is formed per cycle in the secondary vortex street), and 2S (two vortices in opposite directions are formed per cycle in the secondary vortex street) modes. Two partitions in Re–L/D map (Re and L/D as the vertical and horizontal ordinates, respectively) are proposed in this work for the flow regime as well as the vortex street.
Publisher: Cambridge University Press (CUP)
Date: 25-06-2003
Publisher: WIT Press
Date: 10-07-2018
DOI: 10.2495/AFM180061
Publisher: Elsevier BV
Date: 2008
Publisher: Elsevier BV
Date: 06-2022
Publisher: AIP Publishing
Date: 09-2023
DOI: 10.1063/5.0168025
Publisher: American Physical Society (APS)
Date: 13-07-2018
Publisher: AIP Publishing
Date: 08-2015
DOI: 10.1063/1.4928306
Abstract: Flow past two cylinders of different diameters in close proximity is simulated numerically for a constant diameter ratio of 0.45, a gap ratio of 0.0625, and a Reynolds number of 1000 (defined using the diameter of the main cylinder). The effect of the position angle α of the small cylinder relative to the large one on force coefficients and wake flow patterns are studied. Depending on the position angle α of the small cylinder, four wake flow modes are identified: the upstream interference mode for α = 0°, 22.5°, and 45°, the intermittent attached gap flow mode for α = 67.5° and 90°, the attached gap flow mode for α = 112.5° and 135°, and the wake interference mode for α = 157.5° and 180°. The RMS lift coefficients of both cylinders are reduced significantly compared with that of a single cylinder, regardless of the position angle of the small cylinder. Although the variation trends of the mean drag and lift coefficients with the position angle of the small cylinder obtained from the two-dimensional (2D) and three-dimensional (3D) simulations are similar, the 2D simulations overestimate the mean drag coefficient, the RMS drag and lift coefficients compared with those obtained from the 3D simulations.
Publisher: Informa UK Limited
Date: 19-07-2018
Publisher: Elsevier BV
Date: 10-2023
Publisher: AIP Publishing
Date: 10-2020
DOI: 10.1063/5.0021043
Abstract: This paper reports the results of a numerical investigation into the flow around trapezoidal cylinders with various base length ratios and the associated wake structures. Seven base length ratios (d/D, where d and D represent the shorter and longer bases, respectively) ranging from 0 to 1 and three typical attack angles (θ) of 0° (shorter base facing flow), 90°, and 180° are examined at a low Reynolds number of 150. The results indicate that the hydrodynamic forces increase significantly as θ increases from 0° to 180°. Compared with a square cylinder, trapezoidal cylinders with d/D = 0.1 and d/D = 0.3 produce the maximum growth in the time-averaged drag coefficient (up to 54.5%) and the root-mean-square lift coefficient (up to 451.3%), respectively. As d/D increases, the roll-up of the shear layers occurs further downstream, resulting in an increased vortex formation length and subsequent deceleration of vortex shedding. Besides the trailing-edge separation observed in all cylinders at θ = 0°, leading-edge separation in the square cylinder leads to a pair of trapped vortices symmetrically distributed on its lateral sides. At θ = 90° and θ = 180°, the occurrence of trapped and secondary vortices may contribute to the appearance of second and third harmonic frequencies, although the location at which these vortices are formed varies with the trailing edge and attack angle.
Publisher: AIP Publishing
Date: 02-2022
DOI: 10.1063/5.0080268
Abstract: Direct numerical simulations for flow over six tandem circular cylinders at Reynolds numbers (Re) from 40 to 180 and gap ratios (G/D) from 0.5 to 18 were conducted. The main aims are to identify vortex transitions from primary to secondary vortices and from secondary to tertiary vortices and to explore the mechanisms for these transitions. In addition, the variations of the force coefficients and Strouhal numbers with the transitions are also examined. Based on the characteristics of vortex shedding frequency, flow patterns, and force coefficients on the cylinders, four flow regimes, namely, no-shedding, primary shedding, secondary shedding, and tertiary shedding regimes, were mapped out in the Re–G/D domain. The three shedding regimes always initiate from the last cylinder. With the increase of Re and/or G/D, the onset locations of the regimes shift upstream. The secondary and tertiary vortices are formed after a region of convective instability characterized by two-row parallel vortex streets or shear layers. Two physical mechanisms for the transition from primary to secondary vortices are identified. The first one is associated with shear layer instabilities developed after the decay of the primary vortices, and the second one is the merging of the primary vortices. The transition from secondary to tertiary vortices involves vortex merging only. Whereas no-shedding or primary shedding regimes occur at smaller gap ratios or lower Reynolds numbers, the three vortex shedding patterns (primary, secondary, and tertiary) can co-exist for larger gap ratios or higher Reynolds numbers, and each pattern dominates a region around the group of cylinders. The variations of the flow patterns, vortex shedding frequency, and force coefficients on the cylinders in each of the flow regimes are quantified and interpreted. When the number of cylinders is increased to more than 6 at a gap ratio of 15, no further transition is observed after the transition to tertiary wake, and the vortex shedding processes on the downstream cylinders become chaotic without a dominant frequency, suggesting that for multiple tandem cylinders, only two transitions, corresponding to the generations of the secondary and tertiary vortices, can be identified. The change in Reynolds number or gap ratio influences the onset locations of the transitions only.
Publisher: American Society of Mechanical Engineers
Date: 03-08-2014
Abstract: The wake of a circular cylinder enclosed in various shrouds is experimentally investigated in a wind tunnel at Reynolds number of 7000. The aim of the present work is to understand the effect of different shroud types on the vortex shedding frequency and vortex structures from the shrouded cylinders. The tested shrouds are porous screen cylinders and a circular-holed shroud at various porosities of 37%, 48% and 40%, respectively, with the diameter ratio between the shroud and the inner cylinder of 1.3. Phase-averaged analysis is used to examined the large-scale coherent structures with one hot-wire probe moving across the wake in the y-direction to measure the velocity components and another fixed at y/d=1 to 2 from the wake centerline to provide a phase reference to the measured velocity signals. It was found that the vortex shedding persists to some great distance downstream in the wake of the tested shrouds. While the strength of the coherent structures in the wakes of the bare cylinder and tested shrouds are comparable, those in the circular-holed shroud and screen shroud of 48% porosity are 40% higher than the former two at x/d=10.
Publisher: Springer Science and Business Media LLC
Date: 2004
Publisher: Elsevier BV
Date: 2017
Publisher: American Physical Society (APS)
Date: 29-11-2006
Publisher: Elsevier BV
Date: 02-2016
Publisher: Elsevier BV
Date: 06-2019
Publisher: Wiley
Date: 10-2003
DOI: 10.1046/J.1525-1594.2003.00033.X
Abstract: The magnetically suspended impeller centrifugal blood pump (MSCBP) has shown its superiority as compared to other artificial hearts. However, it is still plagued by hemolysis and thrombus formation in the clearance gap between the impeller shroud surface and the pump casing. In an early study, a 5 : 1 scaled up model was built in order to measure the radial and tangential velocities at the gap under the operating condition of a flow coefficient (Phi) of 0.04 using a hot wire anemometer. In this study, velocity profiles were obtained at two critical flow coefficients, Phi = 0.078 (fully opened condition) and 0 (fully closed condition), to further the understanding of the flow field in the pump. Similar to the operating condition, the vector plot of the resultant velocity under these two flow coefficients showed that there were no vortices in the gap and a washout mechanism presented at the region around the starting of the splitter plate of the double volute. It was found that the washout mechanism was more obvious in the fully closed condition than in the fully opened condition. This has suggested that with real life operation, the pump could occasionally be operated at a relatively lower flow rate (e.g., under the resting condition of the patient) to obtain a good washout in the gap and thus reduce thrombus formation at the pump casing.
Publisher: Elsevier BV
Date: 08-2022
Publisher: Elsevier BV
Date: 11-2018
Publisher: AIP Publishing
Date: 06-2022
DOI: 10.1063/5.0092018
Abstract: This work studied the very-large-scale motions (VLSMs), which were first observed in wall-bounded flows, in a turbulent cylinder near wake using two experimental datasets from Chen et al. [“Three-dimensional vorticity, momentum and heat transport in a turbulent cylinder wake,” J. Fluid Mech. 809, 135–167 (2016)] and Tang et al. [“Secondary vortex street in the intermediate wake of a circular cylinder,” Exp. Fluids 59, 119 (2018)]. It is found that the premultiplied spectra of both the streamwise velocity and temperature fluctuations measured with hot and cold wires by Chen et al. [“Three-dimensional vorticity, momentum and heat transport in a turbulent cylinder wake,” J. Fluid Mech. 809, 135–167 (2016)] display a peak at a low wavenumber corresponding to a frequency of about 1/4 of that of the Kármán vortices in the flow. A proper orthogonal decomposition method was utilized to decompose the velocity field measured with particle image velocimetry by Tang et al. [“Secondary vortex street in the intermediate wake of a circular cylinder,” Exp. Fluids 59, 119 (2018)] into modes with different frequencies. A prolonged coherent structure with a wavelength of about four times the Kármán vortices is observed in the visualization of the energetic velocity modes with the same frequency of the spectral peak, which is consistent with the spectral data and confirms the existence of VLSMs in the wake.
Publisher: Cambridge University Press (CUP)
Date: 25-05-2002
DOI: 10.1017/S0022112002007942
Abstract: Measurements have been made in nominally two-dimensional turbulent wakes generated by five different bluff bodies. Each wake has a different level of large-scale organization which is reflected in different amounts of large-scale anisotropy. Structure functions of streamwise ( u ) and lateral ( v ) velocity fluctuations at approximately the same value of R λ , the Taylor microscale Reynolds number, indicate that inertial-range scales are significantly affected by the large-scale anisotropy. The effect is greater on v than u and more pronounced for the porous-body wakes than the solid-body wakes. In particular, ‘relative’ values of the scaling (or power-law) exponents indicate that the magnitude of the transverse exponents can exceed that of the longitudinal ones in the porous-body wakes. This is supported by the inertial-range behaviour of the spectra of u and v . The difference between the transverse and longitudinal exponents appears to depend on the large-scale anisotropy of the flow, as measured by the ratio of the variances of v and u and ratio of the integral length scales of v and u . The spanwise vorticity spectra are much less affected by the anisotropy than the spectra of u and v .
Publisher: AIP Publishing
Date: 05-2023
DOI: 10.1063/5.0151654
Abstract: This work presents numerical investigations of flow-induced rotation of three different polygonal prisms, including triangular, square, and pentagonal ones. Taking into account the torsional stiffness and d ing, the rotation response is examined in a reduced velocity range of Ur = 3–15 at Re = 100. Four distinct oscillating regimes and associated vortex shedding modes are identified: 2S mode in the bifurcated-steady oscillating regime (3 ≤ Ur & 5 and 10 & Ur ≤ 15 for the triangular prism, 7 & Ur ≤ 11 for the square prism), 4S mode in the bifurcated-unsteady oscillating regime (12 ≤ Ur ≤ 15 for the square prism), 6S mode in the non-bifurcated oscillating regime (5 ≤ Ur ≤ 10 for the triangular prism), and 2S mode in the quasi-static regime (3 ≤ Ur & 8 for the square prism, the whole Ur range for the pentagonal prism). Moreover, the competitions between the pressure-induced torque and shear-induced torque vary in these four oscillating regimes. Specifically, the dominant component of the flow-induced torque is the pressure-induced torque in bifurcated-steady oscillating and bifurcated-unsteady oscillating regimes. By contrast, these two kinds of torques owning the nearly equal proportions possess the same and opposite sign in non-bifurcated oscillating and quasi-static regimes, respectively. The root-mean-squared rotary angle (θrms) depends on the cross-sectional shape and reduced velocity. The θrms of the triangular prism suddenly drops to the level of freely rotating case before two rising stages. The square prism starts to rotate violently when it enters the bifurcated-unsteady oscillating regime. For the pentagonal prism, a vortex-induced vibration-like rotation response is observed. Due to the different oscillating regimes, the general θrms of the triangular prism is the largest group, followed by the square prism and pentagonal prism in sequence.
Publisher: Elsevier BV
Date: 2021
Publisher: Springer Science and Business Media LLC
Date: 25-01-2009
Publisher: Elsevier BV
Date: 12-2022
Publisher: AIP Publishing
Date: 08-2022
DOI: 10.1063/5.0103331
Abstract: Hydrodynamic characteristics and wake structures of a submarine gas transmission pipe with the presence of gas leakage are of significance from a scientific and practical viewpoint. In this paper, we present a numerical investigation of flow past a leaking pipe at a low Reynolds number of 160 using the Eulerian–Eulerian multi-fluid volume of fluid model. The focus is on the effects of gas buoyancy and the location of the leak hole on the wake flow structures, bubble–vortex interference, and hydrodynamic forces. The variation of drag and lift coefficients is highly associated with the evolution of gas bubbles and the interaction between the bubble-induced vortices and the shear layers. When the gas buoyancy is ignored, the alterations of the main vortex structure and hydrodynamic forces are not sensitive to the location of the leak hole. The bubble-induced vortices are encompassed by the two shear layers and quickly dissolved in the main vortices. Finally, the released gas bubbles are locked in the center of main vortices and convected downstream with them. In contrast, when the buoyancy is considered, the gas bubbles line up in the upper shear layer, strongly interfering with the formation of the upper main vortex. Each gas bubble introduces a pair of small vortices that experience complicated merging or splitting during the migration. Consequently, the upper main vortex is suppressed at θ = 90° and vanished at θ = 180° and θ = 270° (θ is measured clockwise from the forward stagnation point), leading to the negative time-averaged lift force and the same-frequency oscillation of drag and lift coefficients. Due to the upward migration of gas bubbles from both front and rear surfaces at θ = 270°, the evolution of bubble-induced vortices is more complicated and the oscillation of hydrodynamic forces is significantly enhanced in comparison with other cases.
Publisher: Elsevier BV
Date: 10-2019
Publisher: Cambridge University Press (CUP)
Date: 26-01-2017
DOI: 10.1017/JFM.2016.868
Abstract: This work compares the enstrophy with the scalar dissipation rate, as well as the passive scalar variance with the turbulent kinetic energy, in the presence of coherent Kármán vortices in the intermediate wake of a circular cylinder. Measurements are made at $x/d=10$ , 20 and 40, where $x$ is the streamwise distance from the cylinder axis and $d$ is the cylinder diameter, with a Reynolds number of $2.5\\times 10^{3}$ based on the cylinder diameter and the free-stream velocity. A probe consisting of eight hot wires (four X-wires) and four cold wires is used to measure simultaneously the three components of the fluctuating velocity and vorticity vectors, as well as the fluctuating temperature gradient vector at nominally the same point in the plane of the mean shear. It is found that the enstrophy and scalar dissipation spectra collapse approximately at all wavenumbers except around the Kármán vortex street wavenumber for $x/d\\geqslant 20$ . The spectral similarity between the streamwise velocity fluctuation $u$ and the passive scalar $\\unicode[STIX]{x1D703}$ is better than that between the velocity fluctuation vector $\\boldsymbol{q}$ and $\\unicode[STIX]{x1D703}$ . This is closely related to the highly organized lateral velocity fluctuation $v$ in this flow. The present observations are fully consistent with the expectation that small scales of the velocity and temperature fields are more likely to exhibit a close relationship than scales associated with the bulk of the turbulent energy or scalar variance. The variation across the wake of the time scale ratio between scalar and velocity fields is significantly smaller than that of the turbulent Prandtl number.
Publisher: AIP Publishing
Date: 07-2013
DOI: 10.1063/1.4811403
Abstract: The present paper examines the effect of Reynolds number on turbulence properties in the transition region of a circular jet issuing from a smoothly contracting nozzle. Hot-wire measurements were performed for this investigation through varying the jet-exit Reynolds number Red (≡ Ujd/ν, where Uj, d, and ν are the jet-exit mean velocity, nozzle diameter, and kinematic viscosity) approximately from Red ≈ 4 × 103 to Red ≈ 2 × 104. Results reveal that the rates of the mean flow decay and spread vary with Reynolds number for Red & 104 and tend to become Reynolds-number independent at Red ≥ 104. Even more importantly, the small-scale turbulence properties, e.g., the mean rate of dissipation of kinetic energy (ɛ), the Kolmogorov and Taylor microscales, are found to vary in different forms over the Red ranges of Red & 104 and Red & 104. Namely, the critical Reynolds number appears to occur at Red,cr ≈ 104 across which the jet turbulence behaves distinctly. Two turbulence regimes are therefore identified: (i) developing or partially developed turbulence at Red & Red,cr and (ii) fully developed turbulence at Red ≥ Red,cr. It is suggested that the energy dissipation rate (DR) can be expressed as $\\varepsilon \\sim \\nu U_c^2 /R^2$ɛ∼νUc2/R2 in regime (i) and $\\varepsilon \\sim U_c^3 /R$ɛ∼Uc3/R in regime (ii), where Uc and R are the centerline (or maximum) mean velocity and half-radius at which the mean velocity is 0.5Uc. In addition, the critical Reynolds number appears to vary from flow to flow.
Publisher: American Physical Society (APS)
Date: 09-03-2006
Publisher: Akademia Baru Publishing
Date: 17-04-2021
Abstract: Flow over bluff bodies produces vortex shedding in their wake regions, leading to structural failure from the flow-induced forces. In this study, a passive flow control method was explored to suppress the vortex shedding from a circular cylinder that causes many problems in engineering applications. Perforated shrouds were used to control the vortex shedding of a circular cylinder at Reynolds number, Re = 200. The shrouds were of non-uniform and uniform holes with 67% porosity. The spacing gap ratio between the shroud and the cylinder was set at 1.2, 1.5, 2, and 2.2. The analysis was conducted using ANSYS Fluent using a viscous laminar model. The outcomes of the simulation of the base case were validated with existing studies. The drag coefficient, Cd, lift coefficient, Cl and the Strouhal number, St, as well as vorticity contours, velocity contours, and pressure contours were examined. Vortex shedding behind the shrouded cylinders was observed to be suppressed and delayed farther downstream with increasing gap ratio. The effect was significant for spacing ratio greater than 2.0. The effect of hole types: uniform and non-uniform holes, was also effective at these spacing ratios for the chosen Reynolds number of 200. Specifically, a spacing ratio of 1.2 enhanced further the vortex intensity and should be avoided.
Publisher: Cambridge University Press (CUP)
Date: 02-08-2022
DOI: 10.1017/JFM.2022.587
Abstract: The turbulent kinetic energy and energy dissipation rate in the wake of a circular cylinder are examined at a Reynolds number of 1000. The turbulence characteristics are quantified using direct numerical simulation, which provides a comprehensive dataset that is almost impossible to acquire from physical experiments. The energy dissipation rate is decomposed into the components due to the mean flow, the coherent primary vortices and the remainder. It is found that the remainder component, which develops only in a three-dimensional turbulent wake and resides mainly in the regions of vortices, accounts for 95 % and 97 % of the total dissipation rate for 10 and 20 cylinder diameters downstream of the cylinder, respectively (while the remainder accounts for 62 % and 83 % of the total turbulent kinetic energy). Based on the remainder component, the validity of local isotropy, local axisymmetry, local homogeneity and homogeneity in the y – z plane for the turbulent dissipation in the wake is examined. The analysis reveals that the turbulent dissipation is largely locally homogeneous, but not locally isotropic or axisymmetric, even after the annihilation of the primary vortex street. In addition, the performances of the four corresponding surrogates to the true dissipation rate are evaluated. Owing to the general validity of local homogeneity, the surrogates of local homogeneity and homogeneity in the y – z plane perform well. Although local axisymmetry does not hold, the corresponding surrogate performs well, because errors from different terms largely cancel out. However, the surrogate of local isotropy generally underestimates the true dissipation rate.
Publisher: Elsevier BV
Date: 03-2018
Publisher: Springer Science and Business Media LLC
Date: 14-06-2016
Publisher: AIP Publishing
Date: 08-2022
DOI: 10.1063/5.0107651
Abstract: Hydrodynamic characteristics and wake structures of the flow over a round-ended cylinder are invaluable for the design of associated cylindrical structures such as bridge piers and submerged floating tunnels. This paper reports the results of a numerical investigation into the flow past a round-ended cylinder and the associated hydrodynamic forces as well as the evolution of vortex structure and boundary layer separation. The effect of incidence angle α, ranging from 0° to 90°, is examined at a low Reynolds number of 100 based on the projected length. Both the drag and lift forces are sensitive to the incidence angle. When 0° & α & 90°, boundary layers asymmetrically separate from two sides of the cylinder, giving rise to a time-mean pressure difference that is not perpendicular to the incoming flow direction. There are two directly related results, one is the non-zero time-mean lift coefficient, and the other is the occurrence of a secondary frequency of drag coefficient, the same as that of lift coefficient. The most forward separation point of the upper boundary layer and the maximum size difference of vortices generated from two sides contribute to the maximum time-mean lift coefficient occurring at α = 45°. The vortex formation length is shortened with increasing α, resulting in the augment of the fluctuation litudes of fluid forces. The enlargement of drag force is mainly attributed to the broadened wake width with the increase in α.
Publisher: AIP Publishing
Date: 2023
DOI: 10.1063/5.0131257
Abstract: This paper reports the experimental results of the vortex-induced vibration of a catenary flexible riser placed in the concave orientation. The flexible riser with an aspect ratio of 125 was hung by its own weight with its top end hinged and bottom end fixed. The non-intrusive optical measurement with high-speed cameras was employed to capture the in- and out-of-plane responses simultaneously. Compared to the fixed–fixed case, the change of top boundary leads to the alterations of response litudes and excited mode ranges, and the influence on the in-plane response is greater than that on the out-of-plane one, presenting the pronounced traveling wave components and more frequencies during the mode transition. The coupling response between the top joint and flexible riser is examined in terms of the coincidence of the dominant vibration frequency, which is closely related to the spatial mode competition, presenting the time-varying response profile and vibration frequency as well as the spatial evolution of dominant frequency and modal weight. With increasing the depth-averaged reduced velocity, the out-of-plane mode transition is not synchronized with that of in-plane in spite of the coincidence of vibration frequency. To illustrate the spatial evolution of coupling patterns, four coupling patterns are proposed: strong coupling+, strong coupling, weak coupling, and weak coupling−. More violent mode competition contributes to more spatial partitions of coupling pattern.
Publisher: SAGE Publications
Date: 03-2008
Abstract: This study was designed to examine the effects of the anastomotic angle on the flow and haemodynamic parameter distribution patterns of the proximal anastomoses, with emphasis on identifying site-specific haemodynamic features that could reasonably be expected to trigger the initiation and further development of anastomotic intimal hyperplasia. Particle image velocimetry measurements were carried out with three simplified glass proximal models under a physiological flow condition. The results revealed that the disturbed flow and the induced shear stress patterns including low recirculation flow, stagnation point, high wall shear stress, high temporal wall shear stress gradient, low time-averaged wall shear stress (TAWSS), and high oscillating shear index (OSI) occurred around the anastomotic joints and the flow field at proximal anastomosis was strongly affected by the anastomotic angle. Among the three models investigated, the 45° backward anastomosis is found to have a smaller low-recirculation-flow region along the graft inner wall, non-stationary stagnation, and separation points, a higher TAWSS and smaller high-OSI low-TAWSS and low-OSI high-TAWSS regions.
Publisher: Elsevier BV
Date: 07-2014
Publisher: Elsevier BV
Date: 09-2023
Publisher: EDP Sciences
Date: 2020
DOI: 10.1051/E3SCONF/202016001005
Abstract: In this paper, flow-induced vibrations of bluff bodies with four different cross-sectional geometries (circle, square, triangle and semi-circle) arranged both in single and tandem (gap ratio equals to 3 and 5) configurations are investigated in a wind tunnel. It is found that triangular and square cylinders have the higher litude than that of the semi-circular and the circular cylinders in the single configuration. When two cylinders are arranged in tandem, the circular cylinders have the highest litude among all tested cylinders. Furthermore, the semi-circular cylinder shows that its vibrating litude increases with the reduced velocity in the tandem system due to the galloping effect.
Publisher: Elsevier BV
Date: 2020
Publisher: Elsevier BV
Date: 07-2020
Publisher: AIP Publishing
Date: 02-2014
DOI: 10.1063/1.4866593
Abstract: This paper presents a numerical study of three-dimensional (3D) vortex shedding flow in the wake of four circular cylinders in a square configuration with a constant space-to-diameter ratio of 2. Numerical tests are carried out for Reynolds number (Re) in the range from 100 to 500. Four wake flow regimes are identified at this spacing ratio. Regime 1 ($100 \\le {\\mathop{\\rm Re}\\nolimits} \\le 220$100≤ Re ≤220) is characterized by the inclination of weak spanwise vortices and weak streamwise vortices, where the wake behind four cylinder array shares similar features to that behind a single cylinder with a large equivalent diameter. It is observed that the onset of three-dimensionality in the wake behind four cylinder array occurs at lower Re than that behind a single cylinder. Regime 2 ($240 \\le {\\mathop{\\rm Re}\\nolimits} \\le 300$240≤ Re ≤300) is characterized by the appearance of the regular wavy spanwise vortices and rib-shaped streamwise vortices. The wavelength of the spanwise vortices is about 1.2 and the wake flow is similar to the transition mode B of a single cylinder. Regime 3 ($320 \\le {\\mathop{\\rm Re}\\nolimits} \\le 380$320≤ Re ≤380) is characterized by severe vortex dislocations in the wake of the cylinders and regime 4 ($400 \\le {\\mathop{\\rm Re}\\nolimits} \\le 500$400≤ Re ≤500) is characterized by the absence of vortex dislocations and the strong streamwise vortices. The flow between the upstream and the downstream cylinders is predominantly two-dimensional in regimes 1, 2, and 3 and becomes 3D in regime 4. Physical mechanisms responsible for different flow regimes are proposed and discussed in details. Significant changes in the root-mean-square force coefficients, wake formation length, and phase angle of the lift coefficients on the downstream cylinders are observed when the flow transits from one regime to another.
Publisher: Springer Science and Business Media LLC
Date: 17-02-2009
Publisher: AIP Publishing
Date: 07-2022
DOI: 10.1063/5.0096549
Abstract: SARS-CoV-2 can be transmitted through contact with fomite, respiratory droplets, and aerosolized viruses. Recent evidence suggests that aerosol transmission represents a significant route of infection. In relation to healthcare workers (HCWs), much attention has been focused on personal protective equipment, yet this is the lowest level of the Centers for Disease Control and Prevention hierarchy of controls. Although engineering controls are prominent in the hierarchy, little attention has been given to developing effective interventions. This study aims to evaluate the performance of a simple extraction device in a clinical setting. This was accomplished by using a high flow local extraction (HFLE) that was connected to the existing ventilation system of the hospital on one end and to an intake nozzle near the patient's airway on the other end. Propylene glycol was aerosolized through a physiological test apparatus to simulate the breath of a patient. The field of interest was illuminated using a laser sheet in two planes from the model, namely, the sagittal plane and the transverse plane, and the movement of the simulated aerosol was recorded using a video camera to assess the dispersion of the aerosol qualitatively. In the meantime, the concentration of the aerosol particles was measured using a particle meter to evaluate the effectiveness of the extraction quantitatively. It was found that the HFLE device could effectively reduce the dispersion of the exhaled aerosols to undetectable levels when it was positioned within 250 mm from the mouth. This result has significance in the safety of HCWs involved in the management of patients with infectious diseases and may also have potential applications in other clinical areas with high airflow in the ventilation systems.
Publisher: Elsevier BV
Date: 02-2023
Publisher: Elsevier BV
Date: 2017
Publisher: Springer Science and Business Media LLC
Date: 07-02-2003
Publisher: IOP Publishing
Date: 22-12-2011
Publisher: Informa UK Limited
Date: 2005
Publisher: Cambridge University Press (CUP)
Date: 10-03-2001
Publisher: Japan Society of Mechanical Engineers
Date: 2006
Publisher: Cambridge University Press (CUP)
Date: 10-11-1998
Publisher: Springer Science and Business Media LLC
Date: 11-2003
Publisher: Elsevier BV
Date: 12-2011
Publisher: IOP Publishing
Date: 09-2017
Publisher: Elsevier BV
Date: 11-2012
Publisher: Elsevier BV
Date: 08-2020
Publisher: MDPI AG
Date: 17-08-2022
DOI: 10.3390/JMSE10081128
Abstract: The effects of yaw angle on wake characteristics of a stationary square cylinder were investigated in terms of the hydrodynamic forces, the vortex shedding frequency, and the vortical structures using direct numerical simulations (DNS) at a Reynolds number of 1000. In total, four yaw angles, namely, α = 0°, 15°, 30°, and 45°, were considered. The three-dimensional (3D) Navier–Stokes equations were solved directly using the finite volume method in OpenFOAM. It was found that the first-order statistics of the drag coefficient and the Strouhal number satisfied the independence principle (IP) closely. However, the second-order statistics of the drag and lift coefficients deviated apparently from the IP for α ≥ 25°. The iso-surfaces of the spanwise vorticity gradually disorganized and the magnitudes of the spanwise vorticity contour decreased as the yaw angle α was increased from 0° to 45°. By contrast, the streamwise vorticity iso-surfaces were found to become more organized and the magnitudes of the spanwise velocity contour became larger as a result of the increase in yaw angle, indicating the impairment of the quasi-two-dimensionality and the enhancement of the three-dimensionality of the wake flow. Extensive comparisons were also made with previous DNS results for a yawed circular cylinder, and both similarities and differences between these two kinds of cylinder wakes are discussed.
Publisher: MDPI AG
Date: 03-06-2022
Abstract: The effect of triple helical grooves on the suppression of vortex-induced vibration (VIV) of a circular cylinder was investigated experimentally in a wind tunnel over Reynolds number in the range of 1 × 104 Re 4 × 104. It was found that the helical grooves were effective in suppressing VIV with the peak litude reduction of approximately 36%. In addition, the lock-on region was also reduced. To explore the mechanism for the suppression of VIV, experiments on flow structures for a stationary grooved cylinder were also conducted in a wind tunnel at a free stream velocity U∞ of 4.37 m/s, corresponding to a Reynolds number based on the bare cylinder diameter of about 3500. The data were then analyzed using the phase-averaged method to evaluate the coherent vortex structures in the wakes. The results for the stationary grooved cylinder showed that the grooves weakened vortex shedding in the near wake. In addition, the grooves also reduced the drag coefficient by 6.6%. These results help explain the reduction of VIV using helical grooves.
Publisher: Elsevier BV
Date: 07-2015
Publisher: AIP Publishing
Date: 11-2000
DOI: 10.1063/1.1314336
Abstract: Accounting for the streamwise inhomogeneity of turbulence in Kolmogorov’s equation for the third-order moment of the velocity increment between two points allows compliance with two important classical results. These are retrieved when the separation between the points either exceeds the integral length scale or becomes comparable to the Kolmogorov length scale. In the context of decaying grid turbulence, the results correspond to the mean turbulent energy equation and the Batchelor–Townsend equation for the decay of the mean-squared vorticity. Analogous results are obtained when the streamwise inhomogeneity is included in Yaglom’s equation. The form of the inhomogeneous term is illustrated and discussed in the context of measurements in a turbulent grid flow.
Publisher: Elsevier BV
Date: 06-2020
Publisher: AIP Publishing
Date: 11-2000
DOI: 10.1063/1.1314339
Abstract: The Reynolds number dependence of measured (and corrected) second-order longitudinal and transverse velocity structure functions is examined by fitting an expression that extends from the smallest dissipative scales to inertial range scales. Results obtained from fitting to data for decaying grid turbulence and on the centerline of turbulent jets indicate an increase, with respect to the Reynolds number, of both longitudinal and transverse scaling exponents. The results should provide a relatively reliable description for the Reynolds number evolution of Kolmogorov-normalized second-order structure functions from viscous to inertial range scales.
Publisher: American Physical Society (APS)
Date: 21-06-2005
Publisher: Springer Berlin Heidelberg
Date: 18-12-2015
Publisher: American Institute of Aeronautics and Astronautics (AIAA)
Date: 05-2004
DOI: 10.2514/1.975
Publisher: Cambridge University Press (CUP)
Date: 27-11-2017
DOI: 10.1017/JFM.2017.765
Abstract: This work aims to improve our understanding of the turbulent energy dissipation rate in the wake of a circular cylinder. Ten of the twelve velocity derivative terms which make up the energy dissipation rate are simultaneously obtained with a probe composed of four X-wires. Measurements are made in the plane of mean shear at $x/d=10$ , 20 and 40, where $x$ is the streamwise distance from the cylinder axis and $d$ is the cylinder diameter, at a Reynolds number of $2.5\\times 10^{3}$ based on $d$ and free-stream velocity. Both statistical and topological features of the velocity derivatives as well as the energy dissipation rate, approximated by a surrogate based on the assumption of homogeneity in the transverse plane, are examined. The spectra of the velocity derivatives indicate that local axisymmetry is first satisfied at higher wavenumbers while the departure at lower wavenumbers is caused by the Kármán vortex street. The spectral method proposed by Djenidi & Antonia ( Exp. Fluids , vol. 53, 2012, pp. 1005–1013) based on the universality of the dissipation range of the longitudinal velocity spectrum normalized by the Kolmogorov scales also applies in the present flow despite the strong perturbation from the Kármán vortex street and violation of local isotropy at small $x/d$ . The appropriateness of the spectral chart method is consistent with Antonia et al. ’s ( Phys. Fluids , vol. 26, 2014, 45105) observation that the two major assumptions in Kolmogorov’s first similarity hypothesis, i.e. very large Taylor microscale Reynolds number and local isotropy, can be significantly relaxed. The data also indicate that vorticity spectra are more sensitive, when testing the first similarity hypothesis, than velocity spectra. They also reveal that the velocity derivatives $\\unicode[STIX]{x2202}u/\\unicode[STIX]{x2202}y$ and $\\unicode[STIX]{x2202}v/\\unicode[STIX]{x2202}x$ play an important role in the interaction between large and small scales in the present flow. The phase-averaged data indicate that the energy dissipation is concentrated mostly within the coherent spanwise vortex rollers, in contrast with the model of Hussain ( J. Fluid Mech. , vol. 173, 1986, pp. 303–356) and Hussain & Hayakawa ( J. Fluid Mech. , vol. 180, 1987, p. 193), who conjectured that it resides mainly in regions of strong turbulent mixing.
Publisher: Elsevier BV
Date: 02-2018
Publisher: Informa UK Limited
Date: 2002
Publisher: MDPI AG
Date: 20-10-2022
DOI: 10.3390/PR10102133
Abstract: The vibration excited by gas-liquid multiphase flow endangers the structural instability and fatigue life of subsea jumpers due to the cyclic behavior. In this paper, the multiphase flow-induced vibration (MFIV) of an M-shaped jumper is numerically investigated using a two-way fluid-structure interaction (FSI) approach. The effect of gas-liquid ratios (β) ranging from 1:1 to 1:5 is examined with a fixed flow velocity of 3 m/s, and the influence of mixture velocity (vm) in the range 2–6 m/s is evaluated with a gas-liquid ratio of 1:1. The numerical results reveal the detailed flow evolution of the gas-liquid mixture along the jumper. With inflow of slugs, the pattern successively experiences the slug flow, wavy flow, imperfect annular flow, stratified flow, churn flow, wavy flow and imperfect annular flow in the pipe segments when β = 1:1 and vm = 3 m/s. This development of mixture flow is significantly altered by changing either the gas-liquid ratio or the mixture velocity. In comparison with the flow evolution in a stationary jumper, the pattern in each pipe segment is not been substantially changed due to the limited response litude of order of 10−3D (D is the outer diameter of the jumper). Due to the complex flow evolution, the pressure acting on the six bends of the jumper fluctuate in multiple frequencies. Nevertheless, the dominant fluctuation frequency is approximately equal to the inflow slug frequency. Moreover, the inflow slug frequency also dominates the in-plane response of the jumper. Both the in-plane and out-of-plane responses of the jumper exhibit spatial-temporal variation characteristics. The most vigorous oscillation occurs at the midspan of the jumper. As β is reduced, the out-of-plane response of the jumper midspan is suppressed while the in-plane response is enhanced. In contrast, both the in-plane and out-of-plane oscillations of the jumper midspan are lified with the increase of vm.
Publisher: Elsevier BV
Date: 02-2005
No related grants have been discovered for Tongming Zhou.