ORCID Profile
0000-0002-8452-5773
Current Organisations
Beijing Institute of Technology
,
University of Nottingham
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Publisher: Springer International Publishing
Date: 2019
Publisher: American Physical Society (APS)
Date: 21-02-2017
Publisher: Cambridge University Press (CUP)
Date: 06-03-2019
DOI: 10.1017/JFM.2019.91
Abstract: Linear perturbation analyses of zero-pressure-gradient boundary layers at subcritical Reynolds numbers predict that transient disturbance lification can take place due to the lift-up mechanism. Upstream, streamwise-elongated vortices yield the largest response per unit of inflow disturbance energy, which takes the form of streamwise-elongated streaks. In this work, we compute the linear and also nonlinear inflow disturbances that generate the largest response inside the boundary layer, for flow over a thin flat plate with a slender leading edge. In order to compare our results with earlier linear analyses, we constrain the inlet disturbance to be monochromatic in time, or a single frequency. The boundary layer effectively filters high frequencies, and only low-frequency perturbations induce a strong response downstream. The low-frequency optimal inflow disturbance has a spanwise wavenumber that scales with $\\sqrt{Re}$ , and it consists of streamwise and normal vorticity components: the latter is tilted around the leading edge into the streamwise direction and, further downstream, generates streaks. While none of the computed monochromatic disturbances alone can lead to breakdown to turbulence, secondary instability analyses demonstrate that the streaky base state is unstable. Nonlinear simulations where the inflow disturbance is supplemented with additional white noise undergo secondary instability and breakdown to turbulence.
Publisher: American Physical Society (APS)
Date: 31-10-2023
Publisher: Cambridge University Press (CUP)
Date: 20-04-2015
DOI: 10.1017/JFM.2015.175
Abstract: Amplifications of flow past a backward-facing step with respect to optimal inflow and initial perturbations are investigated at Reynolds number 500. Two mechanisms of receptivity to inflow noise are identified: the bubble-induced inflectional point instability and the misalignment effect downstream of the secondary bubble. Further development of the misalignment results in decay of perturbations from $x=28$ onwards (the step is located at $x=0$ ), as has been observed in previous non-normality studies (Blackburn et al. , J. Fluid Mech. , vol. 603, 2008, pp. 271–304), and eventually limits the receptivity. The receptivity is found to be maximized at an inflow perturbation frequency of ${\\it\\omega}=0.50$ and a spanwise wavenumber of ${\\it\\beta}=0$ , where the inflow noise takes full advantage of both mechanisms and is lified over two orders of magnitude in terms of the velocity magnitude. In direct numerical simulations (DNS) of the flow perturbed by optimal or random inflow noise, vortex shedding, flapping of bubbles, three-dimensionality and turbulence are observed in succession as the magnitude of the inflow noise increases. Similar features of linear and nonlinear receptivity are observed at higher Reynolds numbers. The Strouhal number of the bubble flapping is 0.08, at which the receptivity to inflow noise reaches a maximum. This Strouhal number is close to reported values extracted from DNS or large eddy simulations (LES) at larger Reynolds numbers (Le et al. , J. Fluid Mech. , vol. 330, 1997, pp. 349–374 Kaiktsis et al. , J. Fluid Mech. , vol. 321, 1996, pp. 157–187 Métais, New Trends in Turbulence , 2001, Springer Wee et al. , Phys. Fluids , vol. 16, 2004, pp. 3361–3373). Methods to further clarify the mechanisms of receptivity and to suppress the noise lifications by modifying the base flow using a linearly optimal body force are proposed. It is observed that the mechanisms of optimal noise lification are fully revealed by the distribution of the base flow modification, which weakens the bubble instabilities and misalignment effects and subsequently reduces the receptivity significantly. Comparing the base flow modifications with respect to lifications of inflow and initial perturbations, it is found that the maximum receptivity to initial perturbations is highly correlated with the receptivity to inflow noise at the optimal frequency ${\\it\\omega}=0.50$ , and the correlation reduces as the inflow frequency deviates from this optimal value.
Publisher: American Physical Society (APS)
Date: 14-09-2016
Publisher: AIP Publishing
Date: 05-2022
DOI: 10.1063/5.0089997
Abstract: The aim of this paper is to investigate the linear and weakly nonlinear dynamics in flow over a flat-plate with leading edge. Linear optimal and suboptimal inflow perturbations are obtained using a Lagrangian multiplier technique. In particular, the suboptimal inflow conditions and the corresponding downstream responses are investigated in detail for the first time. Unlike the suboptimal dynamics reported in other canonical cases such as the backward-facing step flow, the growth rate of the suboptimal perturbation is in the same order as the optimal one, and both of them depend on the lift-up mechanism even though they are orthogonal. The suboptimal mode has an additional layer of vorticity that penetrates into the boundary layer farther downstream, generating a second patch of high- and low-speed streaks. The farther suboptimal ones spread to the free-stream without entering the boundary layer. The weakly nonlinear dynamics are examined by decomposing the flow field into multiple orders of perturbations using the Volterra series. Small structures in the higher order perturbations mainly concentrate in the region farther away from wall, suggesting a mechanism of outward perturbation developments, which is opposite with the well reported inward development of perturbations, i.e., from free-stream to boundary layer. The significance of these modes is then demonstrated through a prediction of flow field from the inflow condition by exploiting the orthogonality of the modes.
Publisher: Cambridge University Press (CUP)
Date: 16-06-2022
DOI: 10.1017/JFM.2022.451
Abstract: The vortical structures over a thin rectangular wing with a very low aspect ratio of 0.277 were investigated in a wind tunnel at an effective Reynolds number of $3 \\times 10^6$ . When applying pitch-up motion pivoted at mid-chord, the maximum lift angle was increased with an increase in the pitch rate, but the maximum lift coefficient was reduced. The pitching motion also caused delay in the vortical development over the wing, which was increased with an increase in the pitch rate. The delay in the leading-edge vortex development due to the pitching motion was nearly identical to that in the tip vortex development, indicating that the dynamics of the leading-edge vortex was strongly influenced by the tip vortex. This was confirmed by particle image velocimetry measurements, which demonstrated that the tip vortex over a very low aspect-ratio wing induced strong downwash to influence the development of the leading-edge vortex during the pitching motion, which led to a delay in flow separation.
Publisher: IOP Publishing
Date: 07-06-2019
Abstract: Flying fish is a family of unique aerial-aquatic animals, which can both swim in the water and glide over the sea surface. Most previous studies on their aerodynamic characteristics were based on field observations or measurements of their morphometric parameters. In the present study, we consider three different flying fish models, of which the preliminary one mimics the Cypselurus hiraii in the pectoral fin morphology, following a previous wind tunnel experiment (Park and Choi 2010 J. Exp. Biol. 213 3269-79). Their aerodynamic performances are numerically studied by the computational fluid dynamics (CFD) method. The maximum lift force coefficient of 1.03 is reached at the angle of attack [Formula: see text], and the maximum lift-to-drag ratio of 4.7 is achieved at [Formula: see text]. By choosing appropriately the center of gravity, the flying fish model is proved to be longitudinally stable, according to the negative slope of pitching moment profile. Furthermore, we build a three-degrees-of-freedom (3-DOF) dynamic model in the longitudinal plane based on the aerodynamic coefficients obtained in our simulations, to predict its gliding performance. The results show that the flying fish can achieve a distance up to 45.4 m, and reach a height of 13.2 m, indicating an extraordinary gliding performance. Our numerical simulations are consistent with previous experimental results and theoretical prediction, which can be taken as the basis of further research on robotic flying fish.
Publisher: Elsevier BV
Date: 02-2013
Publisher: American Physical Society (APS)
Date: 25-11-2019
Publisher: Elsevier BV
Date: 03-2009
Publisher: Springer Science and Business Media LLC
Date: 23-10-2009
Publisher: Elsevier BV
Date: 09-2015
Publisher: Cambridge University Press (CUP)
Date: 04-05-2018
DOI: 10.1017/JFM.2018.275
Abstract: A novel algorithm is developed to calculate the nonlinear optimal boundary perturbations in three-dimensional incompressible flow. An optimal step length in the optimization loop is calculated without any additional calls to the Navier–Stokes equations. The algorithm is applied to compute the optimal inflow eddies for the flow around a wind turbine to clarify the mechanisms behind wake meandering, a phenomenon usually observed in wind farms. The turbine is modelled as an actuator disc using an immersed boundary method with the loading prescribed as a body force. At Reynolds number (based on free-stream velocity and turbine radius) $Re=1000$ , the most energetic inflow perturbation has a frequency $\\unicode[STIX]{x1D714}=0.8$ –2, and is in the form of an azimuthal wave with wavenumber $m=1$ and the same radius as the actuator disc. The inflow perturbation is lified by the strong shear downstream of the edge of the disc and then tilts the rolling-up vortex rings to induce wake meandering. This mechanism is verified by studying randomly perturbed flow at $Re\\leqslant 8000$ . At five turbine diameters downstream of the disc, the axial velocity oscillates at a magnitude of more than 60 % of the free-stream velocity when the magnitude of the inflow perturbation is 6 % of the free-stream wind speed. The dominant Strouhal number of the wake oscillation is 0.16 at $Re=3000$ and keeps approximately constant at higher $Re$ . This Strouhal number agrees well with previous experimental findings. Overall the observations indicate that the well-observed stochastic wake meandering phenomenon appearing far downstream of wind turbines is induced by large-scale (the same order as the turbine rotor) and low-frequency free-stream eddies.
Publisher: IOP Publishing
Date: 09-2016
Publisher: Cambridge University Press (CUP)
Date: 19-10-2021
DOI: 10.1017/JFM.2021.835
Abstract: Bypass transition in flow over a flat plate triggered by a pair of dielectric-barrier-discharge plasma actuators mounted on the plate surface and aligned in the streamwise direction is investigated. A four-species plasma–fluid model is used to model the electrohydrodynamic force generated by the plasma actuation. A pair of counter-rotating streamwise vortices is created downstream of the actuators, leading to the formation of a high-speed streak in the centre and two low-speed streaks on each side. As the length of actuators increases, more momentum is added to the boundary layer and eventually a turbulent wedge is generated at an almost fixed location. With large spanwise distance between the actuators (wide layout), direct numerical simulations indicate that the low-speed streaks on both sides lose secondary stability via an inclined varicose-like mode simultaneously, leaving a symmetric perturbation pattern with respect to the centre of the actuators. Further downstream, the perturbations are tilted by the mean shear of the high- and low-speed streaks and consequently a ‘W’-shaped pattern is observed. When the pair of plasma actuators is placed closer (narrow layout) in the spanwise direction, the mean shear in the centre becomes stronger and secondary instability first occurs on the high-speed streak with an asymmetric pattern. Inclined varicose-like and sinuous-like instabilities coexist in the following breakdown of the negative streaks on the side and the perturbations remain asymmetric with respect to the centre. Here the tilting of disturbances is dominated by the mean shear in the centre and the perturbations display a ‘V’ shape. Linear analysis techniques, including biglobal stability and transient growth, are performed to further examine the fluid physics the aforementioned phenomena at narrow and wide layouts, such as the secondary instabilities, the ‘V’ and ‘W’ shapes, and the symmetric and asymmetric breakdown, are all observed.
Publisher: American Physical Society (APS)
Date: 17-08-2021
Publisher: AIP Publishing
Date: 07-2014
DOI: 10.1063/1.4887518
Abstract: Flows past symmetric two-dimensional bluff bodies such as the circular cylinder, the square cylinder, and the normal flat plate produce large separation bubbles which (in the steady restriction) may become very elongated at comparatively low Reynolds numbers. The present work focuses on the steady/symmetric wake of a square cylinder at Re ⩽ 300, which loses stability to anti-symmetric two-dimensional global modes at Rec = 45. It is observed that while there is a continuous evolution in the parameters of the leading instability mode with increasing Re, a change in spatial structure of the modes begins near Re = 175, where the peak growth rate is found. At lower Re, instability modes are generally associated with the wake downstream of the separation bubble, but as Re increases the origin of the modes migrates into the separated shear layers. At the optimal Reynolds number Re = 175, the base flow streamlines around the end of the bubble are most parallel. The observed relation between structures of the bubble and global instabilities is explained on the basis of local stability analyses.
Publisher: IOP Publishing
Date: 31-01-2012
Publisher: American Institute of Aeronautics and Astronautics (AIAA)
Date: 06-2016
DOI: 10.2514/1.J054484
Publisher: Elsevier BV
Date: 10-2015
Publisher: Springer Science and Business Media LLC
Date: 26-03-2018
Publisher: Cambridge University Press (CUP)
Date: 16-05-2012
DOI: 10.1017/JFM.2012.171
Abstract: Transient energy growth of disturbances to co-rotating pairs of vortices with axial core flows is investigated in an analysis where vortex core expansion and vortex merging are included by adopting a time-evolving base flow. The dynamics of pairs are compared with those of in idual vortices in order to highlight the effect of vortex interaction. Three typical vortex pair cases are studied, with the pairs comprised respectively of in idually inviscidly unstable vortices at the streamwise wavenumber that maximizes the in idual instabilities, viscously unstable vortices also at the streamwise wavenumber maximizing the in idual instabilities and asymptotically stable vortices at streamwise wavenumber zero. For the inviscidly unstable case, the optimal perturbation takes the form of a superposition of two in idual helical unstable modes and the optimal energy growth is similar to that predicted for an in idual inviscid unstable vortex, while where the in idual vortices are viscously unstable, the optimal disturbances within each core have similar spatial distributions to the in idually stable case. For both of these cases, time horizons considered are much lower than those required for the merger of the undisturbed vortices. However, for the asymptotically stable case, large linear transient energy growth of optimal perturbations occurs for time horizons corresponding to vortex merging. Linear transient disturbance energy growth exhibited by pairs in this stable case is two to three orders of magnitude larger than that for a corresponding in idual vortex. The superposition of the perturbation and the base flow shows that the perturbation has a displacement effect on the vortices in the base flow. Direct numerical simulations of stable pairs seeded by optimal initial perturbations have been carried out and acceleration/delay of vortex merging associated with a dual vortex meandering and vortex breakup related to axially periodic acceleration and delay of vortex merging are observed. For axially invariant cases, the sign of perturbation has an effect, as well as magnitude the sign dependence relates to whether or not the perturbation adds to or subtracts from the swirl of the base flow. For a two-dimensional perturbation that adds to the swirl of the base flow, seeding with the linear optimal disturbance at a relative energy level $1\\ensuremath{\\times} 1{0}^{\\ensuremath{-} 4} $ induces the pair to move towards each other and approximately halves the time required for merger. Direct numerical simulation shows that the optimal three-dimensional perturbation can induce the vortex system to break up before merging occurs, since the two-dimensional nature of vortex merging is broken by the development of axially periodic perturbations.
Publisher: Elsevier BV
Date: 11-2021
Publisher: Cambridge University Press (CUP)
Date: 29-03-2021
DOI: 10.1017/JFM.2021.154
Publisher: Elsevier BV
Date: 12-2021
Publisher: Cambridge University Press (CUP)
Date: 11-04-2022
DOI: 10.1017/JFM.2022.266
Abstract: The effects of wall-normal vibration on bypass transition in a boundary-layer flow over a flat plate with roughness elements in the form of circular cylinders are investigated using direct numerical simulations (DNS), linear Floquet analyses and dynamic mode decompositions (DMD). The vibration of the plate strengthens the streamwise vorticity, consequently enhancing the velocity streaks and reducing the critical Reynolds number for transition. A map is constructed to identify the coupling effect of the vibration litude and Reynolds number on transition. Among all investigated combinations of height and diameter of roughness, the critical Reynolds numbers at different vibration litude $A$ can be unified by a scaling function of $(1-10A)$ . Two instability modes are identified in the vibration-induced transition process: a wake mode immediately downstream of the roughness, related to the inviscid Kelvin–Helmholtz instability of the wall-normal shear in the wake and a streak mode, in response to the spanwise shear of the streaky flow, occurring further downstream. The first one results in the generation of central hairpin vortices which then feed the second one. Further development of the streak instability leads to two arrays of hairpin vortices. Results from linear Floquet analyses and DMD further confirm the two modes of instability observed in DNS. A quantitative study suggests that the lification of vibration-induced disturbance by the base shear dominates the production of streamwise vorticity and subsequently the hairpin vortices.
Publisher: Cambridge University Press (CUP)
Date: 03-2012
DOI: 10.1017/JFM.2012.58
Abstract: We determine optimal inflow boundary perturbations to steady flow through a straight inflexible tube with a smooth axisymmetric stenosis at a bulk-flow Reynolds number $\\mathit{Re}= 400$ , for which the flow is asymptotically stable. The perturbations computed produce an optimal gain, i.e. kinetic energy in the domain at a given time horizon normalized by a measure of time-integrated energy on the inflow boundary segment. We demonstrate that similarly to the optimal initial condition problem, the gain can be interpreted as the leading singular value of the forward linearized operator that evolves the boundary conditions to the final state at a fixed time. In this investigation we restrict our attention to problems where the temporal profile of the perturbations examined is a product of a Gaussian bell and a sinusoid, whose frequency is selected to excite axial wavelengths similar to those of the optimal initial perturbations in the same geometry. Comparison of the final state induced by the optimal boundary perturbation with that induced by the optimal initial condition demonstrates a close agreement for the selected problem. Previous works dealing with optimal boundary perturbation considered a prescribed spatial structure and computed an optimal temporal variation of a wall-normal velocity component, whereas in this paper we consider the problem of a prescribed temporal structure and compute the optimal spatial variation of velocity boundary conditions over a one-dimensional inflow boundary segment. The methodology is capable of optimizing boundary perturbations in general non-parallel two- and three-dimensional flows.
Publisher: AIP Publishing
Date: 06-2020
DOI: 10.1063/5.0009415
Abstract: A novel approach based on the local entropy generation rate, also known as the second law analysis (SLA), is proposed to compute and visualize the flow resistance in mass transfer through a pipe/channel with a sudden contraction component (SCC) at low Reynolds number (Re) featuring velocity slip. The linear Navier velocity slip boundary condition is implemented using the explicit scheme. At small Reynolds number, i.e., Re ≤ 10.0, the flow resistance coefficient of the SCC, KSCC, is found to be a function of the dimensionless velocity slip length Lslip* and Re−1, and gradually increase to a constant value at contraction ratio Rarea ≥ 8, reaching a formula KSCC=(0.4454Lslip* 3−1.894Lslip* 2+2.917Lslip*+8.909)/Re. Over this range of Re, the equivalent length of the flow resistance is almost independent of Re, while out of this range, the equivalent length increases monotonically with Re. Moreover, the dimensionless drag force work around the SCC is negative and reaches a minimum at a critical Lslip*. The SLA reveals that the regions affected by the SCC mainly concentrate around the end section of the upstream pipe/channel rather than the initial partition of the downstream section reported in large Re turbulent flow, and this non-dimensional affected upstream length increases with Lslip*. The fluid physics are further examined using SLA to evaluate the energy loss over the entire domain, decomposed as the viscous dissipation inside the domain and the drag work on the wall boundary.
Publisher: MDPI AG
Date: 05-10-2020
DOI: 10.3390/EN13195197
Abstract: A light detection and ranging (LIDAR) wind profiler was used to estimate the wind speed in the southern coast of Santa Catarina State, Brazil. This profiler was installed on a coastal platform 250 m from the beach, and recorded wind speed and direction from January 2017 to December 2018. The power generation from three wind turbines was simulated, to obtain estimations of the average power, energy generation and capacity factor, as well as to assess the performance of a hypothetical wind farm. The scale and shape parameters of the Weibull distribution were evaluated and compared with those of other localities in the state. The prevailing winds tend to blow predominantly from the northeast and southwest directions. Wind magnitudes are higher for the NE and SW ocean sectors where the average wind power density can reach 610–820 W m−2. The Vestas 3.0 turbine spent the largest percentage of time in operation ( %). The higher incidence of strong northeasterly winds in 2017 and more frequent passage of cold fronts in 2018 were attributed to the cycle of the South Atlantic subtropical high. The results demonstrate a significant coastal wind power potential, and suggest that there is a significant increase of resources offshore.
Publisher: American Society of Civil Engineers (ASCE)
Date: 03-2015
Publisher: Elsevier BV
Date: 10-2020
Publisher: The Royal Society
Date: 12-2015
Abstract: The adjoint-based sensitivity analyses well explored in hydrodynamic stability studies are extended to calculate the sensitivity of forces acting on an aerofoil with respect to wall transpiration. The magnitude of the sensitivity quantifies the controllability of the force, and the distribution of the sensitivity represents a most effective control when the control magnitude is small enough. Since the sensitivity to streamwise control is one order smaller than that to the surface-normal one, the work is concentrated on the normal control. In direct numerical simulations of flow around a NACA0024 aerofoil, the unsteady controls are far less effective than the steady control owing to the lock-in effect. At a momentum coefficient of 0.0008 and a maximum control velocity of 3.6% of the free-stream velocity, the steady surface-normal control reduces drag by 20% or enhances lift by up to 140% at Re =1000. A suction around the low-pressure region on the upper surface upstream of the separation point is found to reduce drag and enhance lift. At higher Reynolds numbers, the uncontrolled flow becomes three dimensional and the sensitivity erges owing to the chaotic dynamics of the flow. Then the mechanism identified at lower Reynolds numbers is exploited to obtain the control, which is localized and can be generated by a limited number of actuators. The control to reduce drag or enhance lift is found to suppress unsteadiness, e.g. vortex shedding and three-dimensional developments. For ex le, at Re =2000 and α =10°, the control with a momentum coefficient of 0.0001 reduces drag by 20%, enhances lift by up to 200% and leads to a steady controlled flow.
Publisher: Cambridge University Press (CUP)
Date: 12-05-2017
DOI: 10.1017/JFM.2017.240
Abstract: Flow past a NACA 65 blade at chord-based Reynolds number 138 500 is studied using stability analysis, generalized (spatially weighted) transient growth analysis and direct numerical simulations (DNS). The mechanisms of transition on various sections of the blade observed in previous work by Zaki et al. ( J. Fluid Mech. , vol. 665, 2010, pp. 57–98) are examined, with a focus on the pressure side around the leading edge. In this region, the linearly most energetic perturbation has spanwise wavenumber $40\\unicode[STIX]{x03C0}$ (five boundary-layer thicknesses) and is tilted against the mean shear to take advantage of the Orr mechanism. In a DNS, the nonlinear development of this optimal perturbation induces $\\unicode[STIX]{x1D6EC}$ structures, which are further stretched to hairpin vortices before breaking down to turbulence. At higher spanwise wavenumber, e.g. $120\\unicode[STIX]{x03C0}$ , a free-stream optimal perturbation is obtained upstream of the leading edge, in the form of streamwise vortices. During its nonlinear evolution, this optimal perturbation tilts the mean shear and generates spanwise periodic high- and low-speed streaks. Then through a nonlinear lift-up mechanism, the low-speed streaks are lifted above the high-speed ones. This layout of streaks generates a mean shear with two inflectional points and activates secondary instabilities, namely inner and outer instabilities previously reported in the literature.
Publisher: American Institute of Aeronautics and Astronautics (AIAA)
Date: 05-2018
DOI: 10.2514/1.J055329
Publisher: AIP Publishing
Date: 09-2022
DOI: 10.1063/5.0105820
Abstract: We propose a spatial-temporal multi-fidelity Gaussian process regression framework for the fusion of flow field data with various availabilities and fidelities but not sufficiently large to train neural networks commonly encountered in fluid mechanics studies. For ex le, fluid experiments lead to data with high fidelity but sparse in time and space, while most of the numerical data are generally regarded as less accurate but are spatially temporally continuous. The proposed framework aims at generating a new set of fused data by combining the merits of those in the spatial-temporal space. Numerical simulations [e.g., direct numerical simulation (DNS), large eddy simulation, Reynolds-averaged Navier–Stokes] of flow around a National Advisory Committee for Aeronautics 0012 airfoil are performed to collect the original raw data with various fidelities, and a fraction of the DNS result is used to mimic the high-fidelity but sparse experimental data. It is found that the accuracy of the fused data increases with the density of high-fidelity points until reaching a threshold, above which the fusion accuracy becomes insensitive. This limit can be overcome by introducing extra dimensions, such as the gradients of the low-fidelity data field. By examining the error fields, it is found that the high-fidelity points can tune low-fidelity fields but only within a limited local region. The accuracy can be firmly improved by introducing more high-fidelity points or higher levels of spatial gradients if the data set captures the temporal development.
Publisher: Elsevier BV
Date: 09-2021
Publisher: Cambridge University Press (CUP)
Date: 13-11-2020
DOI: 10.1017/JFM.2020.817
Publisher: American Physical Society (APS)
Date: 22-03-2021
Publisher: American Society of Mechanical Engineers
Date: 16-06-2014
DOI: 10.1115/GT2014-25416
Abstract: Roughness on the surface of turbine blades induced by icing, dirt, erosion or manufacturing imperfections changes the aerodynamic configurations of wind turbines and reduces the power generation efficiency. In this work, a modified NACA0024 aerofoil is adopted to study effects of surface roughness on lift/drag forces. Three Reynolds numbers, 1000, 2000 and 5000 and a range of angles of attack [0°,20°] are studied. Since the magnitude of the roughness is small, it can be modelled as non-zero velocity boundary conditions imposed on the smooth surface without roughness. The flow with surface roughness can be therefore decomposed as the sum of a flow without roughness and a flow induced by roughness (or the velocity boundary conditions). The first flow can be obtained by solving the Navier-Stokes (NS) equation while the second one is governed by the linearized NS equation. Correspondingly the lift and drag forces acting on the aerofoil can be also decomposed as the sum of a force without considering roughness and a force induced by roughness. Instead of studying a particular type or distribution of roughness, we calculate the optimal roughness, which changes aerodynamic forces most effectively. This optimal roughness is obtained through a sensitivity study by solving an adjoint equation of the linearized NS equation. It is found that the optimal roughness with respect to both drag and lift forces is concentrated around the trailing edge and upper leading edge of the aerofoil and the lift is much more sensitive to roughness than the drag. Then the optimal roughness with a small magnitude is added to the smooth aerofoil geometry and this new geometry is tested through direct numerical simulations (DNS). It is found that the optimal roughness with a small magnitude (e-norm, defined as the square integration of the roughness around the surface, 0.001) induces over 10% change of the lift. Comparing the forces acting on the smooth surface and on the rough surface, it is noticed that the roughness changes the pressure force significantly while has little influence on the viscous forces. The pressure distribution is further inspected to study mechanisms of the effects of roughness on forces.
Publisher: Wiley
Date: 19-03-2019
DOI: 10.1002/WE.2331
Publisher: Cambridge University Press (CUP)
Date: 26-10-2017
DOI: 10.1017/JFM.2017.675
Abstract: Linear and nonlinear transient growths of perturbations on a vortex ring up to Reynolds number ( $\\equiv$ circulation/viscosity) $Re=27\\,000$ are studied. For short time intervals, perturbations around the ring axis undergo the strongest linear transient growth and lead to secondary structures in the form of ringlets, owing to the Orr mechanism and an inviscid vorticity- lification mechanism: in contrast to the well-reported instabilities and lobe structures along the vortex ring core. These secondary ringlet structures induce a tertiary group of ringlets through similar transient perturbation growth. This cascade of ringlets lead to the breakup of the main ring even before activation of the vortex-core instabilities. Such a cascade scenario is also observed in the development of a vortex ring perturbed by random disturbance in the axis region. These new modes and mechanisms for the generation and breakup of vortex ring structures bring insights into the dynamics and control of vortex ring flows.
Publisher: Cambridge University Press (CUP)
Date: 23-06-2015
DOI: 10.1017/JFM.2015.304
Abstract: This study is focused on two- and three-dimensional incompressible flow past a circular cylinder for Reynolds number $\\mathit{Re}\\leqslant 1000$ . To gain insight into the mechanisms underlying the suppression of unsteadiness for this flow we determine the nonlinear optimal open-loop control driven by surface-normal wall transpiration. The spanwise-constant wall transpiration is allowed to oscillate in time, although steady forcing is determined to be most effective. At low levels of control cost, defined as the square integration of the control, the sensitivity of unsteadiness with respect to wall transpiration is a good approximation of the optimal control. The distribution of this sensitivity suggests that the optimal control at small magnitude is achieved by applying suction upstream of the upper and lower separation points and blowing at the trailing edge. At high levels of wall transpiration, the assumptions underlying the linearized sensitivity calculation become invalid since the base flow is eventually altered by the size of the control forcing. The large-magnitude optimal control is observed to spread downstream of the separation point and draw the shear layer separation towards the rear of the cylinder through suction, while blowing along the centreline eliminates the recirculation bubble in the wake. We further demonstrate that it is possible to completely suppress vortex shedding in two- and three-dimensional flow past a circular cylinder up to $\\mathit{Re}=1000$ , accompanied by 70 % drag reduction when a nonlinear optimal control of moderate magnitude (with root-mean-square value 8 % of the free-stream velocity) is applied. This is confirmed through linearized stability analysis about the steady-state solution when the nonlinear optimal wall transpiration is applied. While continuously distributed wall transpiration is not physically realizable, the study highlights localized regions where discrete control strategies could be further developed. It also highlights the appropriate range of application of linear and nonlinear optimal control to this type of flow problem.
Publisher: Informa UK Limited
Date: 03-07-2015
Publisher: Springer Netherlands
Date: 10-11-2009
Publisher: IOP Publishing
Date: 16-06-2021
Publisher: Cambridge University Press (CUP)
Date: 29-12-2020
DOI: 10.1017/JFM.2020.923
Publisher: Elsevier BV
Date: 05-2021
Publisher: Cambridge University Press (CUP)
Date: 31-05-2011
DOI: 10.1017/JFM.2011.194
Abstract: The spectra of the Batchelor vortex are obtained by discretizing its linearized evolution operator using a modified Chebyshev polynomial approximation at a Reynolds number of 1000 and zero azimuthal wavenumber. Three types of eigenmodes are identified from the spectra: discrete modes, potential modes and free-stream modes. The discrete modes have been extensively documented but the last two modes have received little attention. A convergence study of the spectra and pseudospectra supports the classification that discrete modes correspond to discrete spectra while the other two modes correspond to continuous spectra. Free-stream modes have finite litude in the far field whilst potential modes decay to zero in the far field. The free-stream modes are therefore a limiting form of the potential modes when the radial decay rate of velocity components reduces to zero. The radial form of the free-stream modes with axial and radial wavenumbers is investigated and the penetration of the free-stream mode into the vortex core highlights the possibility of interaction between the potential region and the vortex core. A wavepacket pseudomode study confirms the existence of continuous spectra and predicts the locations and radial wavenumbers of the eigenmodes. The pseudomodes corresponding to the potential modes are observed to be in the form of one or two wavepackets while the free-stream modes are not observed to be in the form of wavepackets.
Publisher: Springer Science and Business Media LLC
Date: 28-01-2015
Publisher: Springer Science and Business Media LLC
Date: 20-08-2020
DOI: 10.1007/S00348-020-03023-4
Abstract: Three-dimensional vortical structures and their interaction over a low-aspect-ratio thin wing have been studied via particle image velocimetry at the chord Reynolds number of $$10^5$$ 10 5 . The maximum lift of this thin wing is found at an angle of attack of $$42^\\circ$$ 42 ∘ . The flow separates at the leading-edge and reattaches to the wing surface, forming a strong leading-edge vortex which plays an important role on the total lift. The results show that the induced velocity of the tip vortex increases with the angle of attack, which helps reattach the separated flow and maintains the leading-edge vortex. Turbulent mixing indicated by the high Reynolds stress can be observed near the leading-edge due to an intense interaction between the leading-edge vortex and the tip vortex however, the reattachment point of the leading-edge vortex moves upstream closer to the wing tip.
Publisher: Cambridge University Press (CUP)
Date: 20-06-2022
DOI: 10.1017/JFM.2022.446
Abstract: The essence of sub-critical transition of oscillatory boundary-layer flows is the non-modal growth of finite- litude disturbances. The current understanding of the mechanisms of the orderly and bypass transitions of oscillatory boundary-layer flows is limited. The present study adopts optimisation approaches to predict the maximum energy lification of two- and three-dimensional perturbations in response to the optimal initial disturbance with or without external forcing. A series of direct numerical simulations are also performed to compare with the results obtained from the stability analyses. In particular, the optimal initial perturbation similar to a Tollmien–Schlichting (T–S) wave yields the largest transient growth under the combined effects of the Orr mechanism and inflectional point instability. With a considerable level of two-dimensional disturbance, the vortex tube nonlinearly develops from the T–S-like wave, and then either deforms into a $\\varLambda$ -vortex in the near-wall region or rolls up to the free shear region. The further burst of turbulence can follow the first pathway as K-type transition or the second one as vortex tube breakdown due to the elliptical instability. Additionally, non-modal growth can initiate the inception of streaky structures by favourable three-dimensional initial perturbations and/or forcing. The secondary instabilities responsible for the streak breakdown are classified as the varicose (symmetric) and sinuous (anti-symmetric) modes. Under a sufficiently high level of three-dimensional disturbance, the bypass transition is predominantly characterised by the formation of the sinuous mode and turbulent spots, which leads to the suppression of inflection point instability.
Publisher: Springer Science and Business Media LLC
Date: 06-2007
Publisher: Cambridge University Press (CUP)
Date: 27-02-2012
DOI: 10.1017/JFM.2012.33
Abstract: The spectrum of the Batchelor vortex can be broadly split into a discrete spectrum, a potential spectrum and a free-stream spectrum where, since the last two spectra are both continuous, they can also be considered as one continuous spectrum. The discrete spectrum has been extensively studied but the continuous spectrum has received limited attention in the context of vortex flow. A local transient growth study is conducted and the contribution of the discrete spectrum and the continuous spectrum to the transient growth is separated by constructing optimal perturbations on the discrete or continuous sub-eigenspaces separately. It is found that the significant transient growth is mainly due to the non-normality of the continuous eigenmodes/spectrum whilst the discrete eigenmodes/spectrum have little contribution to the transient energy growth. A matrix-free method, which reduces to the local analysis when appropriate periodic boundary conditions are imposed, is also applied to investigate the transient growth in both a plane of constant azimuthal angle and a plane constant axial location. Previously studies by other authors have demonstrated that at zero azimuthal wavenumber the transient growth reaches infinitely large values over infinite time intervals while the optimal perturbations are located far from the vortex core. Therefore we limited our scope to small values of the time horizon so as to obtain reasonably strong transient effects stemming from physically relevant optimal perturbations. Two mechanisms of transient growth are observed: namely a redistribution of the azimuthal velocity to the azimuthal vorticity and interaction between out-of-vortex-core structures with those within the vortex core. A direct numerical simulation (DNS) of the vortex perturbed by optimal perturbations is conducted to investigate the nonlinear development of the optimal perturbations. In the azimuthally constant decomposed case, it is found that the optimal perturbation induces a string of bubble structures to be generated as a consequence of the non-orthogonality of continuous eigenmodes and the breakdown bubble is induced by viscous diffusion, while in the axially constant decomposition transient growth analysis, it is observed that the optimal perturbations associated with the continuous eigenmodes drive the vortex to vibrate around the initial vortex centre before eventually returning to its original position at larger times. This transient effect provides a mechanism for the ‘vortex meandering’ observed in previous experimental and numerical studies. These optimal perturbations associated with the continuous spectrum with out-of-vortex-core structures are observed to be activated by anisotropic inflow perturbations in the potential region.
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Xuerui Mao.