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Interdisciplinary Engineering | Fluidisation and Fluid Mechanics
Publisher: AIP Publishing
Date: 08-2023
DOI: 10.1063/5.0169672
Publisher: American Society of Mechanical Engineers
Date: 09-06-2019
Abstract: Experimental investigations on the hydrodynamic forces on an intermittently spanning pipeline exposed to steady currents were carried out. The effect of intermittent local spanning sections on the global hydrodynamic behavior was studied by changing the ratio between the non-spanning length (B) and the total length (L), namely the blocking ratio B / L. A range of gap height (G) to diameter (D) ratios, i.e. gap ratio G / D, and 4 different Reynolds numbers (Re) in the subcritical region were tested in the experiments. The results show: i) for a certain gap ratio, the mean drag increases gently with the decreasing blocking ratio at Re = 5.5 × 104, whereas the mean lift decreases significantly with the decreasing blocking ratio at all values of Re tested and ii) for a certain blocking ratio, increasing the gap ratio leads to an increase in mean drag and decrease in mean lift. Further, simple approaches are proposed based on the present dataset for estimating the global effects on hydrodynamic drag and lift forces due to local spanning geometry.
Publisher: American Society of Mechanical Engineers
Date: 05-06-2022
Abstract: This paper investigates the moment loading and heading response of a turret-moored vessel subjected to surface currents using two-dimensional Computational Fluid Dynamics. To investigate the hydrodynamic loads on the vessel, the moment load was first calculated when the vessel was held fixed at different headings such that the angle between the long axis of the vessel and the current ranged from 0° to 360°. At any fixed angle of attack, the moment load showed nonlinear time-dependent behaviour, where the litude of the load is larger at angles when the ship is aligned normal to the current (approx. 90° to 120°). The instantaneous moment is dependent on the vortex flows around the structure. Following this, the vessel was modelled with one rotational degree-of-freedom and forced to rotate at a fixed angular speed. Several angular speeds were simulated, indicating that the moment loads correlate with the ratio of the steady current velocity to the linear velocity of the vessel at its far end. These comparisons illustrate the complex dynamics of forces with and without vessel rotation. Ongoing work will compare these results to predictions based on a simplified dynamic model for the vessel heading and extends the results to a three-dimensional vessel, where the dynamics remain complicated, but the role of vortex shedding is less significant.
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: Cambridge University Press (CUP)
Date: 16-01-2015
DOI: 10.1017/JFM.2014.708
Abstract: This paper presents a numerical study on steady flow around two identical circular cylinders of various arrangements at a low subcritical Reynolds number ( $\\mathit{Re}=10^{3}$ ). The ratio of centre-to-centre pitch distance ( $P$ ) to the diameter of the cylinder ( $D$ ) ranges from 1.5 to 4, and the alignment angle $({\\it\\alpha})$ between the two cylinders and the direction of the cross-flow varies from 0 to 90°. The detailed flow information obtained from direct numerical simulation allows a comprehensive interpretation of the underlying physics responsible for some interesting flow features observed around two staggered cylinders. Four distinct vortex shedding regimes are identified and it is demonstrated that accurate classification of vortex shedding regimes around two staggered cylinders should consider the combination of the flow visualization with the analyses of lift forces and velocity signal in the wake. It is revealed that the change in pressure distribution, as a result of different vortex shedding mechanisms, leads to a variety of characteristics of hydrodynamic forces on both cylinders, including negative drag force, attractive and repulsive lift forces. Two distinct vortex shedding frequencies are identified and are attributed to the space differences based on the flow structures observed in the wake of the cylinders. It is also found that the three-dimensionality of flow in the gap and the shared wake region is significantly weakened in almost two of the classified flow regimes however, compared with the flow around a single cylinder, active wake interaction at large ${\\it\\alpha}$ does not clearly increase the three-dimensionality.
Publisher: Cambridge University Press (CUP)
Date: 29-09-2023
DOI: 10.1017/JFM.2023.699
Publisher: Springer Science and Business Media LLC
Date: 12-2010
Publisher: Elsevier BV
Date: 08-2020
Publisher: Elsevier BV
Date: 10-2021
Publisher: AIP Publishing
Date: 10-2016
DOI: 10.1063/1.4964379
Abstract: A stable three-dimensional (3D) wake structure for flow past a circular cylinder has been discovered through 3D direct numerical simulations (DNS). The stable 3D wake structure occurs over a small range of Reynolds number (Re) below the critical Re for Mode A∗ (i.e., Mode A with large-scale vortex dislocations, where Mode A is the first 3D instability mode which will evolve into Mode A∗) instability. It is believed that the stable 3D wake structure discovered in this study is the stable state of Mode A wake structure inferred by Williamson [“Three-dimensional wake transition,” J. Fluid Mech. 328, 345–407 (1996)]. This confirms the wake transition sequence of 2D → A → A∗ → B suggested by Williamson. Compared with conventional Mode A structure, the stable state of Mode A structure has much weaker litude and does not evolve into large-scale vortex dislocations. The stable state of Mode A structure is triggered by small-scale spanwise disturbance introduced upstream of the cylinder, due to energy lification through convective instability of the flow. The stable state of Mode A is transient and is d ed out eventually under a transient initial disturbance condition, but is sustained throughout under a persistent disturbance condition. The emergence of the stable state of Mode A structure is correlated with both Re and the disturbance level. With the decrease of Re, the stable state of Mode A structure gradually becomes less well-defined and eventually disappears. With the decrease of the disturbance level, the stable state of Mode A structure emerges at a higher Re.
Publisher: American Society of Mechanical Engineers
Date: 09-06-2019
Abstract: Vortex shedding and eddy generation can provide a substantial contribution to the hydrodynamic roll d ing of vessels. While sharp-edge bilge geometries generally result in greater d ing, a more rounded bilge may be beneficial for the other ship motions and mooring design. To this end, the effect of the bilge rounding on the vortex shedding dynamics and the hydrodynamic d ing of hulls is investigated numerically. A series of new computations of the response of hulls with differing bilge radii and subject to forced oscillatory motion are reported. The severity of the vortex shedding and the magnitude of the d ing is shown to depend strongly on the bilge geometry. Computations using an efficient isolated edge discrete vortex model are also found to be generally in qualitative agreement with the CFD results. The results extend the relatively scarce literature on the effect of bilge rounding and can assist in informing hull design and concept selection.
Publisher: Cambridge University Press (CUP)
Date: 21-01-2020
Publisher: Cambridge University Press (CUP)
Date: 06-05-2022
DOI: 10.1017/JFM.2022.314
Abstract: This paper presents the response and the wake modes of a freely vibrating D-section prism with varying angles of attack ( $\\alpha = 0^\\circ \\text {--}180^\\circ$ ) and reduced velocity ( $U^* = 2\\text {--}20$ ) by a numerical investigation. The Reynolds number, based on the effective diameter, is fixed at 100. The vibration of the prism is allowed only in the transverse direction. We found six types of response with increasing angle of attack: typical vortex-induced vibration (VIV) at $\\alpha = 0^\\circ \\text {--}35^\\circ$ extended VIV at $\\alpha = 40^\\circ \\text {--}65^\\circ$ combined VIV and galloping at $\\alpha = 70^\\circ \\text {--}80^\\circ$ narrowed VIV at $\\alpha = 85^\\circ \\text {--}150^\\circ$ transition response, from narrowed VIV to pure galloping, at $\\alpha = 155^\\circ \\text {--}160^\\circ$ and pure galloping at $\\alpha = 165^\\circ \\text {--}180^\\circ$ . The typical and narrowed VIVs are characterized by linearly increasing normalized vibration frequency with increasing $U^*$ , which is attributed to the stationary separation points of the boundary layer. On the other hand, in the extended VIV, the vortex shedding frequency matches the natural frequency in a large $U^*$ range with increasing $\\alpha$ generally. The galloping is characterized by monotonically increasing litude with enlarging $U^*$ , with the largest litude being $A^* = 3.2$ . For the combined VIV and galloping, the vibration litude is marginal in the VIV branch while it significantly increases with $U^*$ in the galloping branch. In the transition from narrowed VIV to pure galloping, the vibration frequency shows a galloping-like feature, but the litude does not monotonically increase with increasing $U^*$ . Moreover, a partition of the wake modes in the $U^*$ – $\\alpha$ parametric plane is presented, and the flow physics is elucidated through time variations of the displacement, drag and lift coefficients and vortex dynamics. The angle-of-attack range of galloping is largely predicted by performing a quasi-steady analysis of the galloping instability. Finally, the effects of $m^*$ and ${\\textit {Re}}$ , the roles of afterbody and the roles of separation point in determining vibration responses and vortex shedding frequency are further discussed.
Publisher: Cambridge University Press (CUP)
Date: 17-03-2015
DOI: 10.1017/JFM.2015.107
Abstract: Sinusoidally oscillatory flow around four circular cylinders in an in-line square arrangement is numerically investigated at Keulegan–Carpenter numbers ( $\\mathit{KC}$ ) ranging from 1 to 12 and at Reynolds numbers ( $\\mathit{Re}$ ) from 20 to 200. A set of flow patterns is observed and classified based on known oscillatory flow regimes around a single cylinder. These include six types of reflection symmetry regimes to the axis of flow oscillation, two types of spatio-temporal symmetry regimes and a series of symmetry-breaking flow patterns. In general, at small gap distances, the four structures behave more like a single body, and the flow fields therefore resemble those around a single cylinder with a large effective cylinder diameter. With increasing gap distance, flow structures around each in idual cylinder in the array start to influence the overall flow patterns, and the flow field shows a variety of symmetry and asymmetry patterns as a result of vortex and shear layer interactions. The characteristics of hydrodynamic forces on in idual cylinders as well as on the cylinder group are also examined. It is found that the hydrodynamic forces respond in a similar manner to the flow field to the cylinder proximity and wake interference.
Publisher: Cambridge University Press (CUP)
Date: 02-09-2019
DOI: 10.1017/JFM.2019.609
Abstract: Oscillatory flow around a cluster of four circular cylinders in a diamond arrangement is investigated using two-dimensional direct numerical simulation over Keulegan–Carpenter numbers ( KC ) ranging from 4 to 12 and Reynolds numbers ( Re ) from 40 to 230 at four gap-to-diameter ratios ( G ) of 0.5, 1, 2 and 4. Three types of flows, namely synchronous, quasi-periodic and desynchronized flows (along with 14 flow regimes) are mapped out in the ( G , KC , Re )-parameter space. The observed flow characteristics around four cylinders in a diamond arrangement show a few unique features that are absent in the flow around four cylinders in a square arrangement reported by Tong et al. ( J. Fluid Mech. , vol. 769, 2015, pp. 298–336). These include (i) the dominance of flow around the cluster-scale structure at $G=0.5$ and 1, (ii) a substantial reduction of regime D flows in the regime maps, (iii) new quasi-periodic (phase trapping) $\\text{D}^{\\prime }$ (at $G=0.5$ and 1) and period-doubling $\\text{A}^{\\prime }$ flows (at $G=1$ ) and most noteworthily (iv) abnormal behaviours at ( $G\\leqslant 2$ ) (referred to as holes hereafter) such as the appearance of spatio-temporal synchronized flows in an area surrounded by a single type of synchronized flow in the regime map ( $G=0.5$ ). The mode competition between the cluster-scale and cylinder-scale flows is identified as the key flow mechanism responsible for those unique flow features, with the support of evidence derived from quantitative analysis. Phase dynamics is introduced for the first time in bluff-body flows, to the best knowledge of the authors, to quantitatively interpret the flow response (e.g. quasi-periodic flow features) around the cluster. It is instrumental in revealing the nature of regime $\\text{D}^{\\prime }$ flows where the cluster-scale flow features are largely synchronized with the forcing of incoming oscillatory flow (phase trapping) but are modulated by localized flow features.
Publisher: ASME International
Date: 28-09-2012
DOI: 10.1115/1.4007426
Abstract: Vortex-induced vibration (VIV) of a circular cylinder at a low mass ratio of 1.5 between two lateral walls is investigated numerically. The focus of the study is to examine the effects of the two lateral walls on the VIV. Numerical simulations are carried out for w/D = 4, 6, 10, and 20 with D and w being the cylinder diameter and the distance between the two walls, respectively. It is found that the effects of the two walls on the VIV are obvious as w/D ≤ 6 and negligibly small as w/D = 10. The VIV litudes in both x- and y-directions increase with the increasing w/D in the lock-in regime.
Publisher: Cambridge University Press (CUP)
Date: 26-10-2017
DOI: 10.1017/JFM.2017.655
Abstract: A numerical analysis of flow around a circular cylinder oscillating in-line with a steady flow is carried out over a range of driving frequencies $(f_{d})$ at relatively low litudes $(A)$ and a constant Reynolds number of 175 (based on the free-stream velocity). The vortex shedding is investigated, especially when the shedding frequency $(f_{s})$ synchronises with the driving frequency. A series of modes of synchronisation are presented, which are referred to as the $p/q$ modes, where $p$ and $q$ are natural numbers. When a $p/q$ mode occurs, $f_{s}$ is detuned to $(p/q)f_{d}$ , representing the shedding of $p$ pairs of vortices over $q$ cycles of cylinder oscillation. The $p/q$ modes are further characterised by the periodicity of the transverse force over every $q$ cycles of oscillation and a spatial–temporal symmetry possessed by the global wake. The synchronisation modes $(p/q)$ with relatively small natural numbers are less sensitive to the change of external control parameters than those with large natural numbers, while the latter is featured with a narrow space of occurrence. Although the mode of synchronisation can be almost any rational ratio (as shown for $p$ and $q$ smaller than 10), the probability of occurrence of synchronisation modes with $q$ being an even number is much higher than $q$ being an odd number, which is believed to be influenced by the natural even distribution of vortices in the wake of a stationary cylinder.
Publisher: American Society of Mechanical Engineers
Date: 11-06-2023
Publisher: Cambridge University Press (CUP)
Date: 26-06-2018
DOI: 10.1017/JFM.2018.436
Abstract: This paper focuses on the characteristics of the regime C flow (Tatsuno & Bearman, J. Fluid Mech. , vol. 211, 1990, pp. 157–182) around an oscillating circular cylinder in still water. The regime C flow is characterised by the formation of large-scale vortex cores arranged as opposed von Kármán vortex streets, resulting from a regular switching of vortex shedding directions with respect to the axis of oscillation. Both Floquet analysis and direct numerical simulations (DNS) are performed to investigate the two- (2-D) and three-dimensional (3-D) instabilities. The present study reveals that the low-wavenumber 3-D instability can emerge slightly before the 2-D instability in regime C. In total, five spanwise vortex modes were identified: (i) standing-wave pattern, S-mode (ii) travelling-wave pattern, T-mode (iii) mixed ST-mode (iv) X-type vortex pattern, X-mode and (v) U-type vortex pattern, U-mode. The modal analysis conducted in this study demonstrates that the vortex patterns and the corresponding spatial and temporal modulations of the dynamic loads of the S-, T- and mixed ST-modes are mainly induced by the 3-D instability of a single wavenumber. The characteristics of the X-mode are due to the superposition of the 3-D instabilities of multiple wavenumbers. The U-mode is dominated by a 2-D instability and its interaction with 3-D instabilities. The domain size dependence study demonstrates that the regime C flow is very sensitive to the spanwise length of the computational domain. The subcritical nature of the regime C flow is responsible for the discrepancy in the marginal stability curves obtained by independent Floquet stability analysis, DNS and physical experiments.
Publisher: American Society of Mechanical Engineers
Date: 11-06-2023
Publisher: Cambridge University Press (CUP)
Date: 19-10-2021
DOI: 10.1017/JFM.2021.832
Abstract: Bistabilities of two equilibrium states discovered in the coupled side-by-side Kármán wakes are investigated through Floquet analysis and direct numerical simulation (DNS) with different initial conditions over a range of gap-to-diameter ratio ( $g^*= 0.2\\text {--}3.5$ ) and Reynolds number ( $Re = 47\\text {--}100$ ). Two bistabilities are found in the transitional $g^*-Re$ regions from in-phase (IP) to anti-phase (AP) vortex shedding states. By initialising the flow in DNS with zero initial conditions, the flow in the first bistable region (i.e. bistable IP/FF $_C$ at $g^*= 1.4 \\text {--} 2.0$ , where FF $_C$ denotes the conditional flip-flop flow) attains flip-flop (FF) flow, it settles into the IP state by initialising the flow with an IP flow. The second bistability is observed between cylinder-scale IP and AP states at large $g^*$ ( $=$ 2.0–3.5). The transition from the FF $_C$ to IP is dependent on initial conditions and irreversible over the parameter space, meaning that the flow cannot revert back to the FF $_C$ state once it jumps to the IP state irrespective of the direction of $Re$ variations. Its counterpart for the bistable IP/AP state is reversible. We also found that the FF $_C$ flow in the first bistable region is primarily bifurcated from synchronised AP with cluster-scale features, possibly because the cluster-scale AP flow is inherently unstable to FF flow instabilities. It is demonstrated that the irreversible bistability exists in other interacting wakes around multiple cylinders. A good understanding of flow bistabilities is pivotal to flow control applications and the interpretation of desynchronised flow features observed at high $Re$ values.
Publisher: Cambridge University Press (CUP)
Date: 17-01-2017
DOI: 10.1017/JFM.2016.829
Abstract: Two-dimensional direct numerical simulation and Floquet stability analysis have been performed at moderate Keulegan–Carpenter number ( $KC$ ) and low Reynolds number ( $Re$ ) for a square cross-section cylinder with its face normal to the oscillatory flow. Based on the numerical simulations a map of flow regimes is formed and compared to the map of flow around an oscillating circular cylinder by Tatsuno & Bearman ( J. Fluid Mech. , vol. 211, 1990, pp. 157–182). Two new flow regimes have been observed, namely A $^{\\prime }$ and F $^{\\prime }$ . The regime A $^{\\prime }$ found at low $KC$ is characterised by the transverse convection of fluid particles perpendicular to the motion and the regime F $^{\\prime }$ found at high $KC$ shows a quasi-periodic feature with a well-defined secondary period, which is larger than the oscillation period. The Floquet analysis demonstrates that when the two-dimensional flow breaks the reflection symmetry about the axis of oscillation, the quasi-periodic instability and the synchronous instability with the imposed oscillation occur alternately for the square cylinder along the curve of marginal stability. This alternate pattern in instabilities leads to four distinct flow regimes. When compared to the vortex shedding in otherwise unidirectional flow, the two quasi-periodic flow regimes are observed when the oscillation frequency is close to the Strouhal frequency (or to half of it). Both the flow regimes and marginal stability curve shift in the $(Re,KC)$ -space compared to the oscillatory flow around a circular cylinder and this shift appears to be consistent with the change in vortex formation time associated with the lower Strouhal frequency of the square cylinder.
Publisher: Cambridge University Press (CUP)
Date: 25-07-2016
DOI: 10.1017/JFM.2016.446
Abstract: This paper presents three-dimensional (3D) direct numerical simulations (DNS) of flow past a circular cylinder over a range of Reynolds number ( $Re$ ) up to 300. The gradual wake transition process from mode A* (i.e. mode A with large-scale vortex dislocations) to mode B is well captured over a range of $Re$ from 230 to 260. The mode swapping process is investigated in detail with the aid of numerical flow visualization. It is found that the mode B structures in the transition process are developed based on the streamwise vortices of mode A or A* which destabilize the braid shear layer region. For each case within the transition range, the transient mode swapping process consists of dislocation and non-dislocation cycles. With the increase of $Re$ , it becomes more difficult to trigger dislocations from the pure mode A structure and form a dislocation cycle, and each dislocation stage becomes shorter in duration, resulting in a continuous decrease in the probability of occurrence of mode A* and a continuous increase in the probability of occurrence of mode B. The occurrence of mode A* results in a relatively strong flow three-dimensionality. A critical condition is confirmed at approximately $Re=265{-}270$ , where the weakest flow three-dimensionality is observed, marking a transition from the disappearance of mode A* to the emergence of increasingly disordered mode B structures.
Publisher: Science China Press., Co. Ltd.
Date: 2011
DOI: 10.1360/132010-586
Publisher: Springer Science and Business Media LLC
Date: 27-08-2011
Publisher: American Society of Mechanical Engineers
Date: 17-06-2018
Abstract: As offshore renewable energy projects progress from concept demonstration to commercial-scale developments there is a need for improved approaches beyond conventional cable engineering design methods that have evolved from larger diameter pipelines for the oil and gas industry. New approaches are needed to capture the relevant physics for small diameter cables on rocky seabeds to reduce the costs and risks of power transmission and increase operational reliability. This paper reports on subsea cables that MeyGen installed for Phase 1a of the Pentland Firth Inner Sound tidal stream energy project. These cables are located on rocky seabeds in an area where severe metocean conditions occur. ROV field observation of these cables shows them to be stable on the seabed with little or no movement occurring over almost all of the cable routes, despite conventional engineering methods predicting significant dynamic movement. We cite recent research undertaken by the University of Western Australia (UWA) to more accurately assess the hydrodynamic forces and geotechnical interaction of cables on rocky seabeds. We quantify the conformity between the cables and the undulating rocky seabed, and the distributions of cable-seabed contact and spanning via simulations of the centimetric-scale seabed bathymetry. This analysis leads to calculated profiles of lift, drag and seabed friction along the cable, which show that all of these load and reaction components are modelled in an over-conservative way by conventional pipeline engineering techniques. Overall, our analysis highlights that current cable stability design can be unnecessarily conservative on rocky seabeds. Our work foreshadows a new design approach that offers more efficient cable design to reduce project capex and enhance through-life integrity management.
Publisher: Cambridge University Press (CUP)
Date: 03-08-2018
DOI: 10.1017/JFM.2018.538
Abstract: This paper presents a numerical investigation of oscillatory flow around a circular cylinder that is placed in proximity to a plane boundary that is parallel to the cylinder axis. The onset and development of the Honji instability are studied over a range of Stokes numbers ( $\\unicode[STIX]{x1D6FD}$ ) and gap-to-diameter ratios ( $e/D$ ) at a fixed Keulegan–Carpenter number ( $KC=2$ ). Four flow regimes are identified in the ( $e/D,\\unicode[STIX]{x1D6FD}$ )-plane: (I) featureless two-dimensional flow, (II) stable Honji vortex, (III) unstable Honji vortex and (IV) chaotic flow. As $e/D$ increases from $-0.5$ (embedment) to $1$ , the critical Stokes number $\\unicode[STIX]{x1D6FD}_{cr}$ for the onset of the Honji instability follows two side-by-side convex functions, peaking at the connection point of $e/D=0.125$ and reaching troughs at $e/D=0$ and 0.375. The Honji instability is always initiated on the gap side of the cylinder surface for $0.375\\leqslant e/D\\leqslant 2$ and occurs only on the top side for $-0.5\\leqslant e/D .125$ . The location for the initiation of the Honji instability switches from the gap side to the top side of the cylinder surface for $0.125 e/D .375$ . No Honji instability is observed at $e/D=0.125$ , where the flow three-dimensionality is developed through a different flow mechanism. Consistently, the three-dimensional kinetic energy of the flow, which represents a measure of the strength of flow three-dimensionality, varies with $e/D$ in a trend opposite to that of $\\unicode[STIX]{x1D6FD}_{cr}$ . Three physical mechanisms are identified as being responsible for the observed variation trend of $\\unicode[STIX]{x1D6FD}_{cr}$ with $e/D$ and for various flow phenomena, which are the blockage effect induced by the geometry setting, the existence of the Stokes layer on the plane boundary and the favourable pressure gradient in the flow direction over the gap between the cylinder and the plane surface.
Publisher: IOP Publishing
Date: 30-06-2015
Publisher: American Society of Mechanical Engineers
Date: 05-06-2022
Abstract: University of Western Australia’s Oceans Graduate School to transform the design methods used to analyse the on-bottom stability of subsea power cables. These cables form the vital connection enabling renewable energy to be reliably and cost-effectively transported from the source to the consumer — whether the collection device at the end of the line is wave, tidal, fixed or floating wind. These novel design methods are equally applicable to umbilicals and other small diameter pipelines. Existing oil and gas pipeline codes overlook much of the physics that is relevant to small diameter pipes and cables. The new research has unlocked significant improvements through new laboratory, numerical and field observational analysis and modelling. The results of this research are presently being incorporated into new design guidance, including the draft British Standards Institute BS 10009 being developed under PEL 114 technical committee guidance. This paper provides a summary of the research together with observations and lessons learnt in the application of these new design methods to over 7.4 GW of new offshore wind and other renewable energy cables. Given that as at 2020 the global grid-connected total offshore wind capacity was 35 GW, this contribution demonstrates strong evidence of field validation of these research outcomes, as well as their relative importance and transformative potential to contribute to global decarbonisation.
Publisher: AIP Publishing
Date: 07-2023
DOI: 10.1063/5.0155066
Abstract: This paper presents a new flow control approach to suppress the vortex shedding in the wake of a circular cylinder through high-frequency oscillation. The circular cylinder is forced to oscillate in the streamwise direction at high-frequency and low litude, corresponding to a high Stokes number (β = 100–1000) and low Keulegan–Carpenter number (KC = 0.001–4). Two-dimensional (2-D) and three-dimensional (3-D) direct numerical simulations of an oscillating circular cylinder in steady current have been carried out in the parameter space of KC, Rec, and β. Our numerical results show that when the flow remains in the two-dimensional vortex shedding regime, the cylinder wake sequentially experiences transitions from the vortex shedding regime to the suppression of the vortex shedding regime and finally to the symmetry breaking regime, with increasing KC. Corresponding wake characteristics and variations of hydrodynamic forces over the three wake regimes are explored. Three quantities that represent shear-layer characteristics, including the length of separating shear layers, the circulation of shear layers and wake recirculation length, reach maxima at the onset of suppression. The physical mechanisms for the suppression of vortex shedding and occurrence of symmetry breaking are also explained. Once the flow becomes 3-D, vortex shedding from the cylinder cannot be suppressed, primarily because the outer shear layers induced by the steady approaching flow are enhanced in 3-D flows. The cylinder oscillation over the frequency range investigated in the present study delays wake transition to 3-D. The cylinder oscillation alters the 3-D vortical structure and its spanwise wavelength significantly.
Publisher: Elsevier BV
Date: 08-2019
Publisher: Cambridge University Press (CUP)
Date: 17-04-2020
DOI: 10.1017/JFM.2020.237
Publisher: Springer Nature Singapore
Date: 2022
Publisher: American Society of Mechanical Engineers
Date: 17-06-2018
Abstract: We present a study on the hydrodynamics of a circular cylinder close to a wall, which is an idealized representative of subsea pipelines and cables used in both offshore oil and gas and marine renewable energy industries. This research utilizes Computational Fluid Dynamics (CFD) modeling method to investigate the influences of the boundary layer turbulence and the cylinder-to-floor distance on hydrodynamic forces. A significant jump in hydrodynamic forces is observed over the range of Reynolds number that coincides with boundary layer transition from laminar to turbulence. This transition gives rise to a drop in the size of the thickness of the boundary layer at the location of the cylinder. The force jump is also characterised by a pressure increase on the upwind surface of the cylinder, while the pressure decreases on the leeward surface.
Publisher: Cambridge University Press (CUP)
Date: 04-04-2018
DOI: 10.1017/JFM.2018.164
Abstract: Oscillatory flow around a circular cylinder close to a plane boundary is numerically investigated at low-to-intermediate Keulegan–Carpenter ( $KC$ ) and Stokes numbers ( $\\unicode[STIX]{x1D6FD}$ ) for different gap-to-diameter ratios ( $e/D$ ). A set of unique flow regimes is observed and classified based on the established nomenclature in the ( $KC,\\unicode[STIX]{x1D6FD}$ )-space. It is found that the flow is not only influenced by $e/D$ but also by the ratio of the thickness of the Stokes boundary layer ( $\\unicode[STIX]{x1D6FF}$ ) to the gap size ( e ). At relatively large $\\unicode[STIX]{x1D6FF}/e$ values, vortex shedding through the gap is suppressed and vortices are only shed from the top of the cylinder. At intermediate values of $\\unicode[STIX]{x1D6FF}/e$ , flow through the gap is enhanced, resulting in horizontal gap vortex shedding. As $\\unicode[STIX]{x1D6FF}/e$ is further reduced below a critical value, the influence of $\\unicode[STIX]{x1D6FF}/e$ becomes negligible and the flow is largely dependent on $e/D$ . A hysteresis phenomenon is observed for the transitions in the flow regime. The physical mechanisms responsible for the hysteresis and the variation of marginal stability curves with $e/D$ are explored at $KC=6$ through specifically designed numerical simulations. The Stokes boundary layer over the plane boundary is found to be responsible for the relatively large hysteresis range over $0.25 e/D .0$ . Three mechanisms have been identified to the change of the marginal stability curve over $e/D$ , which are the blockage effect due to the geometry setting, the favourable pressure gradient over the gap and the location of the leading eigenmode relative to the cylinder.
Publisher: American Society of Mechanical Engineers
Date: 11-06-2023
Publisher: American Society of Mechanical Engineers
Date: 17-06-2018
Abstract: At OMAE in 2008 the ‘state of the art’ in pipeline on-bottom stability engineering was summarized, providing an overview of the current available knowledge for addressing pipeline stability. The aim of that work was to summarise key aspects of the pipeline stability design process and to include some historical perspective. The paper discusses the advantage and shortfalls of the different design approaches with a view to consolidate understanding, rather than to provide a ready-made solution to a complex design problem [1]. Since that time, a decade of research and further methodology refinement has extended the boundaries of the industry’s knowledge and understanding of the behaviour of subsea pipelines and cables, including geotechnics, hydrodynamics, oceanography and structural response modelling. In particular, progress has been made in: • The response of pipelines to sediment transport and scour • Understanding the behaviour of small diameter pipelines and cables within wave and current boundary layers and • The behaviour of cables on rocky seabeds in high energy marine environments. This paper summarises these innovations to enable the application of new paradigms in engineering practice and improved outcomes for initial project capital cost, reliability and operational integrity, as well as better models to predict the long-term behaviour where pipes are decommissioned in-situ. While a relatively widely studied field of engineering, there remain areas of active ongoing research to improve our understanding and ability to model and predict subsea pipeline on-bottom behavior, with a summary of the anticipated future opportunities proposed.
Publisher: Springer Berlin Heidelberg
Date: 18-12-2016
Publisher: Elsevier BV
Date: 12-2017
Publisher: Elsevier BV
Date: 2021
Start Date: 09-2020
End Date: 08-2025
Amount: $420,000.00
Funder: Australian Research Council
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