77 results found
Xie Z, 2022, An implicit Cartesian cut-cell method for incompressible viscous flows with complex geometries, Computer Methods in Applied Mechanics and Engineering, Vol: 399, ISSN: 0045-7825
A versatile conservative three-dimensional Cartesian cut-cell method for simulation of incompressible viscous flows over complex geometries is presented in this paper. The present method is based on the finite volume method on a non-uniform staggered grid together with a consistent mass and momentum flux computation. Contrary to the commonly cut-cell methods, an implicit time integration scheme is employed in the present method, which avoids numerical instability without any additional small cut-cell treatment. Strict conservation of the mass and momentum for both fluid and cut cells is enforced through the PISO algorithm for the pressure–velocity coupling. The versatility and robustness of the present cut-cell method are demonstrated by simulating various two- and three-dimensional canonical benchmarks (flow over a circular cylinder, airfoil, sphere, pipe, and heart sculpture) and the computed results agree well with previous experimental measurements and various numerical results obtained from the boundary-fitted, immersed boundary/interface, and other cut-cell methods, verifying the accuracy of the proposed method.
Xie Z, Lin P, Stoesser T, 2022, A conservative and consistent implicit Cartesian cut-cell method for moving geometries with reduced spurious pressure oscillations, Journal of Computational Physics, Vol: 459, ISSN: 0021-9991
A conservative and consistent three-dimensional Cartesian cut-cell method is presented for reducing the spurious pressure oscillations often observed in moving body simulations in sharp-interface Cartesian grid methods. By analysing the potential sources of the oscillation in the cut-cell framework, an improved moving body algorithm is proposed for the cut-cell method for the temporal discontinuity of the solid volume change. Strict conservation of mass and momentum for both fluid and cut cells is enforced through pressure-velocity coupling to reduce local mass conservation errors. A consistent mass and momentum flux computation is employed in the finite volume method. In contrary to the commonly cut-cell methods, an implicit time integration scheme is employed in the present method, which prevents numerical instability without any additional small cut-cell treatment. The effectiveness of the present cut-cell method for reducing spurious pressure oscillations is demonstrated by simulating various two- and three-dimensional benchmark cases (in-line and transversely oscillating cylinder, oscillating and free-falling sphere), with good agreement with previous experimental measurements and other numerical methods available in the literature.
Brocchini M, Marini F, Postacchini M, et al., 2022, Interaction between breaking-induced vortices and near-bed structures. Part 1. Experimental and theoretical investigation, JOURNAL OF FLUID MECHANICS, Vol: 940, ISSN: 0022-1120
Ai C, Ma Y, Yuan C, et al., 2022, An efficient 3D non-hydrostatic model for predicting nonlinear wave interactions with fixed floating structures, Ocean Engineering, Vol: 248, ISSN: 0029-8018
This paper presents a three-dimensional (3D) non-hydrostatic model for the prediction of the interaction between nonlinear waves and fixed floating structures. The model solves the incompressible Euler equation by use of a semi-implicit, fractional step algorithm. The water surface elevation is treated as a single-valued function of horizontal position. In order to deal with floating structures, a new numerical algorithm is proposed which combines the immersed boundary method and the global continuity equation in the pressurized region (flow region under the structure). This new algorithm holds the symmetry of the Poisson equation and therefore results in an efficient model. The developed model is validated with the data of two test cases involving 3D nonlinear wave interactions with a floating structure. The model results are compared with experimental data or results of other models. The proposed model exhibits generally good agreement with experimental data and/or other model results, demonstrating its accuracy in resolving 3D nonlinear wave interaction with floating structures.
Xie Z, Lin P, 2022, Eulerian and Lagrangian transport by shallow-water breaking waves, Physics of Fluids, Vol: 34, ISSN: 1070-6631
This study examines the mass and Lagrangian transport, kinematic and dynamic characteristics of shallow-water breaking waves, focusing on the wave breaking, and jet impingement processes. A multiphase Navier-Stokes flow model has been developed to track the origin and trajectory for the jet and the splash-up using both a geometric piece-wise linear interface calculation volume-of-fluid (PLIC-VOF) and the Lagrangian particle tracking approaches. The model is first validated both quantitatively and qualitatively against the experimental data for the plunging jet and the splash-up during wave breaking, in which a good agreement is obtained. The mass transport and the origin of the jet and splash-up are studied using the new multi-component PLIC-VOF approach, and the different regions in the interior of the wave are tracked in an Eulerian way. Both horizontal and vertical drifts for the interior and surface particles are shown using the Lagrangian particles. The location and origin of the plunging jet can be clearly seen from the simulations. Various wave steepness and beach slopes have been investigated for different types of breakers. Furthermore, the detailed jet impingement, velocity, pressure, vorticity, and turbulence fields during wave breaking are discussed and presented, providing more detailed flow fields to gain further insight into the plunging jet and splash-up in shallow-water breaking waves.
Li Q, Xia J, Xie Z, et al., 2022, Hazard and vulnerability in urban inundated underground space: Hydrodynamic analysis of human instability for stairway evacuation, International Journal of Disaster Risk Reduction, Vol: 70, ISSN: 2212-4209
Underground flooding events are being exacerbated due to the rapid expanding of underground space in urban and the extreme precipitation events by climate change. It is increasingly necessary to study hydrodynamics and instabilities of human on staircases in the flood-prone underground space for risk identification and disaster reduction. However, the turbulent complexity and complicated fluid-human interaction still challenge the study of flow structure and the calibration of human instability model. In this work, a hydrodynamic model coupled with the mechanics-based method was proposed to study fluid-human interactions and hazard risks on flooding stairways. Numerical validations show that the model can obtain reliable solutions of flow characteristics on staircases. It is found that there exists a jet flow downstream the rest platform and the critical region after the 3rd step downstream the platform is identified as a high risk area to cause sliding instability. The risk of sliding instability for a child is higher than that for an adult in jet flow region. In addition, results show that the downstream vortical flow structure and turbulent effect are obviously enhanced because of the interdict of jet flow by the human obstacle.
Ai C, Ma Y, Ding W, et al., 2022, Three-dimensional non-hydrostatic model for dam-break flows, Physics of Fluids, Vol: 34, ISSN: 1070-6631
A three-dimensional (3D) non-hydrostatic model is presented for the simulation of dam-break flows. The model solves the Reynolds-averaged Navier-Stokes equations using the projection method. 3D computational grids are constructed from a two-dimensional horizontal unstructured mesh by adding horizontal layers in the vertical direction. Based on the horizontal unstructured grid system, horizontal advection terms are discretized by a momentum conservative scheme. The proposed model is validated with several physical experiments. The agreement between the model results and experimental data is generally good, which demonstrates the capability of the proposed model to resolve dam-break flows over flat and uneven bottoms with complex geometries. Moreover, the efficiency of the model is evaluated with 3D dam-break flow experiments. Comparisons between the non-hydrostatic model and the corresponding quasi-3D shallow water model are also performed, which confirm the role of non-hydrostatic effects in dam-break flows.
Ai C, Ma Y, Yuan C, et al., 2022, Vortex shedding and evolution induced by the interactions between a solitary wave and a submerged horizontal plate, Journal of Hydraulic Research, Vol: 60, Pages: 311-325, ISSN: 0022-1686
This paper presents results of a solitary wave propagating over a submerged horizontal plate based on a non-hydrostatic model. A high-resolution advection scheme is employed based on a second-order flux-limiter method. The model is first validated by comparing numerical results with recently obtained measured data. Then, a series of simulations are carried out to investigate the effect of the plate length on the characteristics of vortex shedding and evolution. With the increase in the length of the plate, more dominant vortical structures induced by the solitary wave with the same wave height can be identified near the lee side of the plate. There are two separate positive vortex shedding from the lower corner of the plate near the lee side, which are not presented in experiments. The secondary positive vortex leads to the water surface deformation for the case that is close to the limit when wave breaking occurs.
Xie Z, Lin B, Falconer RA, et al., 2022, Large-eddy simulation of turbulent free surface flow over a gravel bed, Journal of Hydraulic Research, Vol: 60, Pages: 205-219, ISSN: 0022-1686
In this paper, a large-eddy simulation study of turbulent free surface flow over a natural rough bed is presented. A three-dimensional multiphase flow model is employed to study the roughness effects on the turbulence properties and free surface dynamics. The governing equations have been discretized using the finite volume method, with the Cartesian cut-cell method being implemented to deal with the precise scanned gravel bed topography and the deformable free surface being captured by a volume of fluid method. The predicted mean flow velocities and turbulence statistics have been compared with experimental data. A close agreement has been obtained between the two sets of results, providing confirmation that this complementary approach to experimental investigations gives further insight into the turbulent free surface flow dynamics over rough beds. It is found that small waves are generated on the free surface due to the roughness effect for the relative low submergence case.
Adzic F, Stoesser T, Liu Y, et al., 2022, Large-eddy simulation of supercritical free-surface flow in an open-channel contraction, Journal of Hydraulic Research, Vol: 60, Pages: 628-644, ISSN: 0022-1686
Large-eddy simulations (LES) of supercritical flow in a straight-wall, open-channel contraction of 6° and contraction ratio of 2:1 are performed. The LES code solves the filtered Navier-Stokes equations for two-phase flows (water-air) and employs the level-set method. The simulation was validated by replicating a previously reported experiment. Contours of the time-averaged velocities indicate that the flow loses energy and momentum in the contracting channel. Further, secondary currents in the contraction are redistributing momentum and are responsible for local up-and down-flows. The turbulent kinetic energy reaches very high values at the entrance of the contraction, mainly contributed by the streamwise normal stress. The flow contains coherent turbulence structures which are responsible for carrying low-momentum from the bed and the water surface towards the channel centre. Flow deceleration results in significant turbulence anisotropy in the contracted section. It is shown that mainly pressure drag contributes to the energy loss in the contraction.
Li Q, Xia J, Zhou M, et al., 2021, Physical modelling of energy losses at surcharged three-way junction manholes in drainage system, WATER SCIENCE AND TECHNOLOGY, Vol: 85, Pages: 1011-1026, ISSN: 0273-1223
Christou A, Stoesser T, Xie Z, 2021, A large-eddy-simulation-based numerical wave tank for three-dimensional wave-structure interaction, Computers and Fluids, Vol: 231, ISSN: 0045-7930
A three-dimensional numerical wave tank (NWT) based on the open-source large eddy simulation (LES) code Hydro3D is introduced. The code employs the level set and immersed boundary methods to enable accurate computations of the deformation of the water surface and to account for solid structures in the fluid domain, respectively. The spatially-filtered Navier–Stokes (N–S) equations are solved on a staggered Cartesian grid using the finite difference method while time advancement is achieved using the fractional-step method based with a three-step Runge–Kutta scheme. Velocities and pressure are coupled with the Poisson equation and its solution is obtained via a multi-grid technique. The code is then applied to predict the progression and damping of monochromatic waves and the interaction of non-linear waves with various submerged obstacles. The accuracy of Hydro3D is confirmed by comparing numerical results with data of previously reported laboratory experiments. Comparisons of numerically predicted and measured water-levels, local velocity and pressure fields and forces acting on structures under the influence of incoming waves with laboratory data are convincing and confirm that the code is able to predict accurately three-dimensional wave-structure interaction.
Ai C, Ma Y, Ding W, et al., 2021, An efficient three-dimensional non-hydrostatic model for undular bores in open channels, Physics of Fluids, Vol: 33, ISSN: 1070-6631
A three-dimensional (3D) non-hydrostatic model is presented to simulate open-channel free-surface flows involving undular bores. The 3D unsteady mass conservation and momentum equations are solved using an explicit projection method in a nonstandard staggered grid. The grid system is built from a two-dimensional horizontal structured grid by adding horizontal layers. The model is validated using four typical benchmark problems, including undular bore development, an undular bore generated by a sudden discharge, and two test cases involving undular hydraulic jumps. The proposed model results are compared with experimental data and results from other models. Overall, the agreement between the proposed model results and experimental data is generally good, demonstrating the capability of the model to resolve undular bores. In addition, the non-hydrostatic pressure field under the undular free surface is revealed, and the efficiency of the proposed model is presented. It is shown that the proposed model behaves better than a volume of fluid model in terms of efficiency, because the proposed model can use fewer computational grid cells to resolve undular bores in open channels.
Xie Z, Stoesser T, Xia J, 2021, Simulation of Three-Dimensional Free-Surface Dam-Break Flows over a Cuboid, Cylinder, and Sphere, Journal of Hydraulic Engineering, Vol: 147, ISSN: 0733-9429
A three-dimensional (3D) numerical study is undertaken to investigate dam-break flows over 3D structures. A two-phase flow model has been developed within the large-eddy simulation (LES) framework. The governing equations have been discretized using the finite-volume method, with the air-water interface being captured using a volume-of-fluid method while the Cartesian cut-cell method deals with complex geometries. The robustness and versatility of the proposed numerical approach are demonstrated first by applying it to a 3D dam-break flow over a cuboid. Good agreement is obtained between the simulation results and the corresponding experimental data and other numerical solutions. Then, a horizontal cylinder and a sphere are subjected to the same dam-break flow. Snapshots of water surface profiles are presented and discussed, and turbulent vortical structures are identified in the flow. In addition, the internal kinematics, hydrodynamic loading on the structure, and energy dissipation during dam-break flow impact are analyzed and discussed, providing more insight into such flows.
Ai C, Ma Y, Yuan C, et al., 2021, A three-dimensional non-hydrostatic model for tsunami waves generated by submarine landslides, Applied Mathematical Modelling, Vol: 96, Pages: 1-19, ISSN: 0307-904X
A three-dimensional (3D) non-hydrostatic model for simulating nonlinear and dispersive waves is extended to compute submarine-landslide-generated waves. The model uses a projection method to solve the 3D Navier-Stokes equations on a 3D grid system built from a two-dimensional (2D) horizontal mesh by adding several horizontal layers in the vertical direction. The free surface is efficiently captured by the so-called free surface equation. The bottom movement is incorporated in the model by specifying the kinematic boundary condition at the impermeable bottom. The extension does not alter the property of the discretized Poisson equation for non-hydrostatic pressure correction terms. Thus, it can also be solved efficiently by the preconditioned conjugate gradient method. A wide range of tests including 2D and 3D landslide waves are simulated. Comparisons between numerical results and experimental data and/or other model results are presented. It is found that a good agreement has been obtained for a range of landslide waves using a very small number of horizontal layers (e.g. three or five layers) and the proposed model can be considered as an attractive alternative to simulating submarine-landslide-generated waves.
Jalalabadi R, Stoesser T, Ouro P, et al., 2021, Free surface flow over square bars at different Reynolds numbers, Journal of Hydro-Environment Research, Vol: 36, Pages: 67-76, ISSN: 1570-6443
Large-eddy simulations of free surface flow over bed-mounted square bars are performed for laminar, transitional and turbulent flows at constant Froude number. Two different bar spacings are selected corresponding to transitional and k-type (reattaching flow) roughness, respectively. The turbulent flow simulations are validated with experimental data and convincing agreement between simulation and measurement is obtained in terms of water surface elevations and streamwise velocity profiles. The water surface deforms in response to the underlying bed roughness ranging from mild undulation for transitional roughness to distinct standing waves for k-type roughness. The instantaneous water surface deformations increase with an increase in Reynolds number. Contours of the mean streamwise and wall-normal velocities, the total shear stress and the streamfunction reveal the presence and extension of recirculation zones in the trough between two consecutive bars. The flow is governed by strong local velocity gradients as a result of the rough bed and the deformed water surface. The local Froude number at the free surface increases for low Reynolds number in the flow over transitional roughness and decreases for low Reynolds number in the flow over k-type roughness. The transitional and turbulent flows exhibit a very similar distribution of the pressure coefficient Cp in both cases, whilst Cp is generally lower for the laminar flow. Regarding the friction coefficient, Cf, it is significantly lower in the turbulent case than in the transitional and laminar cases. The bar spacing does not affect significantly the relative contribution of friction and pressure forces to the total force, neither does the Reynolds number. The friction factor is greater for transitional roughness and decreases with increasing Reynolds number.
Li Q, Yokoi K, Xie Z, et al., 2021, A fifth-order high-resolution shock-capturing scheme based on modified weighted essentially non-oscillatory method and boundary variation diminishing framework for compressible flows and compressible two-phase flows, Physics of Fluids, Vol: 33, ISSN: 1070-6631
First, a new reconstruction strategy is proposed to improve the accuracy of the fifth-order weighted essentially non-oscillatory (WENO) scheme. It has been noted that conventional WENO schemes still suffer from excessive numerical dissipation near-critical regions. One of the reasons is that they tend to under-use all adjacent smooth substencils thus fail to realize optimal interpolation. Hence in this work, a modified WENO (MWENO) strategy is designed to restore the highest possible order interpolation when three target substencils or two target adjacent substencils are smooth. Since the new detector is formulated under the original smoothness indicators, no obvious complexity and cost are added to the simulation. This idea has been successfully implemented into two classical fifth-order WENO schemes, which improve the accuracy near the critical region but without destroying essentially non-oscillatory properties. Second, the tangent of hyperbola for interface capturing (THINC) scheme is introduced as another reconstruction candidate to better represent the discontinuity. Finally, the MWENO and THINC schemes are implemented with the boundary variation diminishing algorithm to further minimize the numerical dissipation across discontinuities. Numerical verifications show that the proposed scheme accurately captures both smooth and discontinuous flow structures simultaneously with high-resolution quality. Meanwhile, the presented scheme effectively reduces numerical dissipation error and suppresses spurious numerical oscillation in the presence of strong shock or discontinuity for compressible flows and compressible two-phase flows.
Wu F, Shen MH, Yang J, et al., 2021, An Enhanced Tilted-Angle Acoustofluidic Chip for Cancer Cell Manipulation, IEEE Electron Device Letters, Vol: 42, Pages: 577-580, ISSN: 0741-3106
In recent years, surface acoustic wave (SAW) devices have demonstrated great potentials and increasing applications in the manipulation of nano- and micro-particles including biological cells with the advantages of label-free, high sensitivity and accuracy. In this letter, we introduce a novel tilted-angle SAW devices to optimise the acoustic pressure inside a microchannel for cancer-cell manipulation. The SAW generation and acoustic radiation force are improved by seamlessly patterning electrodes in the space surrounding the microchannel. Comparisons between this novel SAW device and a conventional device show a 32% enhanced separation efficiency while the input power, manufacturing cost and fabrication effort remain the same. Effective separation of HeLa cancer cells from peripheral blood mononuclear cells is demonstrated. This novel SAW device has the advantages in minimizing device power consumption, lowering component footprint and increasing device density.
Xie Z, Lopez SM, Christou A, et al., 2021, Numerical investigation of steep focused wave interaction with a vertical cylinder using a cartesian cut-cell method, International Journal of Offshore and Polar Engineering, Vol: 31, Pages: 71-77, ISSN: 1053-5381
A three-dimensional numerical study has been undertaken to investigate the interactions of a steep focused wave and a vertical circular cylinder. The large-eddy simulation approach has been adopted in this study, where the model is based on the filtered Navier–Stokes equations, with the dynamic Smagorinsky subgrid model being used for the unresolved scales of turbulence. The governing equations have been discretised using the finite volume method, with the air–water interface being captured using a volume-of-fluid method and the Cartesian cut-cell method being implemented to deal with the cylinder and moving wavemaker in the Cartesian grid. Numerical results are presented and compared with the available experimental measurements in terms of the wave elevations and pressure on the cylinder. Detailed free surface profiles during wave impact are shown and discussed.
Lu L, Zhou Z, Qin J, et al., 2021, Three-dimensional viscous numerical simulations of focused waves on bottom-mounted multiple rigid circular cylinders, International Journal of Offshore and Polar Engineering, Vol: 31, Pages: 61-70, ISSN: 1053-5381
Three-dimensional viscous numerical simulations are conducted in this study concerning the focused wave acting on (1) an isolated bottom-mounted circular cylinder, (2) four identical circular cylinders in a diamond arrangement, and (3) eight cylinders arranged in a chirped array with varied spacing. The numerical model, based on a Navier–Stokes solver of Fluidity, is first validated against available experimental data of the isolated cylinder, and satisfactory agreements are obtained. This work pays attention to the near trapping of a four-cylinder cluster and the rainbow trapping of a chirped cylinder array subjected to transient-focused water waves with a broad frequency spectrum. Viscous numerical results suggest the transient-focused waves result in a rather weak near trapping in the four-cylinder cluster. However, the chirped array creates significant local amplifications through a rainbow trapping mechanism. Wave nonlinearity and viscous effects are also discussed by comparisons with linear potential flow solutions.
Ransley EJ, Brown SA, Hann M, et al., 2021, Focused wave interactions with floating structures: A blind comparative study, Proceedings of the Institution of Civil Engineers: Engineering and Computational Mechanics, Vol: 174, Pages: 46-61, ISSN: 1755-0777
The paper presents results from the Collaborative Computational Project in Wave Structure Interaction (CCP-WSI) Blind Test Series 2. Without prior access to the physical data, participants, with numerical methods ranging from low-fidelity linear models to fully non-linear Navier-Stokes (NS) solvers, simulate the interaction between focused wave events and two separate, taut-moored, floating structures: a hemispherical-bottomed cylinder and a cylinder with a moonpool. The 'blind' numerical predictions for heave, surge, pitch and mooring load, are compared against physical measurements. Dynamic time warping is used to quantify the predictive capability of participating methods. In general, NS solvers and hybrid methods give more accurate predictions; however, heave amplitude is predicted reasonably well by all methods; and a WEC-Sim implementation, with CFD-informed viscous terms, demonstrates comparable predictive capability to even the stronger NS solvers. Large variations in the solutions are observed (even among similar methods), highlighting a need for standardisation in the numerical modelling of WSI problems.
Sriram V, Agarwal S, Yan S, et al., 2021, A comparative study on the nonlinear interaction between a focusing wave and cylinder using state-of-the-art solvers: Part A, International Journal of Offshore and Polar Engineering, Vol: 31, Pages: 1-10, ISSN: 1053-5381
This paper presents ISOPE’s 2020 comparative study on the interaction between focused waves and a fixed cylinder. The paper discusses the qualitative and quantitative comparisons between 20 different numerical solvers from various universities across the world for a fixed cylinder. The moving cylinder cases are reported in a companion paper as part B (Agarwal, Saincher, et al., 2021). The numerical solvers presented in this paper are the recent state of the art in the field, mostly developed in-house by various academic institutes. The majority of the participants used hybrid modeling (i.e., a combination of potential flow and Navier–Stokes solvers). The qualitative comparisons based on the wave probe and pressure probe time histories and spectral components between laminar, turbulent, and potential flow solvers are presented in this paper. Furthermore, the quantitative error analyses based on the overall relative error in peak and phase shifts in the wave probe and pressure probe of all the 20 different solvers are reported. The quantitative errors with respect to different spectral component energy levels (i.e., in primary, sub-, and superharmonic regions) capturing capability are reported. Thus, the paper discusses the maximum, minimum, and median relative errors present in recent solvers as regards application to industrial problems rather than attempting to find the best solver. Furthermore, recommendations are drawn based on the analysis.
Sun C, Mikhaylov R, Fu Y, et al., 2021, Flexible Printed Circuit Board as Novel Electrodes for Acoustofluidic Devices, IEEE Transactions on Electron Devices, Vol: 68, Pages: 393-398, ISSN: 0018-9383
Surface acoustic wave (SAW)-based acoustofluidics shows broad applications in biomedicine and chemistry. Conventional manufacturing process for SAW devices uses photolithography and metal deposition, thus requires accessing cleanroom facilities. This study presents an efficient and versatile technique based on a flexible printed circuit board (FPCB) for developing SAW acoustofluidic devices. By mechanically clamping interdigital electrodes (IDEs) made on the FPCB onto a piezoelectric substrate, SAWs can be effectively generated with an additional matching network. The SAW amplitudes were measured by a laser vibrometer, which increases with the applied input voltage. The FPCB-SAW device has been applied to actuate 10-μm microspheres to form strong streaming vortices inside a droplet, and to drive a sessile droplet for transportation on the substrate surface. The use of the FPCB rather than a rigid printed circuit board (PCB) can help cut down on the overall footprint of the device and save space. The low requirement in assembling the FPCB-SAW device can facilitate versatile acoustofluidic applications by providing fast prototyping devices.
Christou A, Xie Z, Stoesser T, et al., 2021, Propagation of a solitary wave over a finite submerged thin plate, Applied Ocean Research, Vol: 106, ISSN: 0141-1187
For the purpose of this paper, the in-house large-eddy simulation code, Hydro3D, is refined to study wave structure interaction. First of all, the code is used to develop a numerical wave tank capable of simulating accurately the generation, progression and damping of solitary waves in a tank. Then, Hydro3d is employed to simulate a previous laboratory experiment of a wave propagating over an infinitely wide flat plate. The code's accuracy is validated by comparing computed waterlevels and hydrodynamic forces on the plate with measured data for which good agreement is found for a number of conditions (i.e. varying wave steepness or plate submergence, respectively). Then the study is extended to investigate three-dimensional effects for which the infinitely wide plate is replaced by a finite square plate. It is found that the pressure difference between the lower and upper side of the plate drives a span-wise flow and creates unique flow structures and water-surface fluctuations near the plate due to the three-dimensionality of the problem. A further three-dimensional study is conducted for which the finite plate is fixed at an angle of attack in respect to the incident wave and variations in hydrodynamic forces and free-surface elevations are computed. Both vertical and horizontal forces are reduced when the plate is fixed at 45∘ degrees and minor water-level fluctuations appear, reflecting the pattern of the rotational flow near the plate edges. Plots of the velocity vectors, swirl-strength, pressure and wave elevation and acting forces reveal significant differences between an infinitely wide and a finite square plate subjected to a solitary wave.
Agarwal S, Saincher S, Sriram V, et al., 2021, A comparative study on the nonlinear interaction between a focusing wave and cylinder using state-of-the-art solvers: Part B, International Journal of Offshore and Polar Engineering, Vol: 31, Pages: 11-18, ISSN: 1053-5381
In this paper, the comparative study carried out for focused wave interaction with a moving cylinder in ISOPE-2020 is reported. The fixed cylinder cases are reported in the companion paper as Part A (Sriram, Agarwal, Yan et al., 2021). The paper discusses qualitative and quantitative comparison between four different numerical solvers that participated in this comparative study. This is a challenging problem, as the cylinder moves over 40 m and interacts with the focusing waves. The performance of various solvers is compared for two different moving cylinder speeds. Both weakly coupled models and full Navier–Stokes (NS) solvers with different strategies for modeling the cylinder motion were adopted by the participants. In particular, different methods available for numerically simulating the forward speed problem emerge from this paper. The qualitative comparison based on the wave probe and pressure probe time histories between laminar and turbulent solvers is presented. Furthermore, the quantitative error analysis for individual solvers shows deviations up to 30% for moving wave probes and 50% for pressure time history. The reliability of each method is discussed based on all the wave probe and pressure probe discrepancies against experiments. The deviations for higher speed shown by all solvers indicate that further improvements in the modeling capabilities are required.
Xie Z, Stoesser T, Yan S, et al., 2020, A Cartesian cut-cell based multiphase flow model for large-eddy simulation of three-dimensional wave-structure interaction, Computers and Fluids, Vol: 213, ISSN: 0045-7930
A multiphase flow numerical approach for performing large-eddy simulations of three-dimensional (3D) wave-structure interaction is presented in this study. The approach combines a volume-of-fluid method to capture the air-water interface and a Cartesian cut-cell method to deal with complex geometries. The filtered Navier–Stokes equations are discretised by the finite volume method with the PISO algorithm for velocity-pressure coupling and the dynamic Smagorinsky subgrid-scale model is used to compute the unresolved (subgrid) scales of turbulence. The versatility and robustness of the presented numerical approach are illustrated by applying it to solve various three-dimensional wave-structure interaction problems featuring complex geometries, such as a 3D travelling wave in a closed channel, a 3D solitary wave interacting with a vertical circular cylinder, a 3D solitary wave interacting with a horizontal thin plate, and a 3D focusing wave impacting on an FPSO-like structure. For all cases, convincing agreement between the numerical predictions and the corresponding experimental data and/or analytical or numerical solutions is obtained. In addition, for all cases, water surface profiles and turbulent vortical structures are presented and discussed.
ViaEstrem L, Salinas P, Xie Z, et al., 2020, Robust control volume finite element methods for numerical wave tanks using extreme adaptive anisotropic meshes, International Journal for Numerical Methods in Fluids, Vol: 92, Pages: 1707-1722, ISSN: 0271-2091
Multiphase inertia‐dominated flow simulations, and free surface flow models in particular, continue to this day to present many challenges in terms of accuracy and computational cost to industry and research communities. Numerical wave tanks and their use for studying wave‐structure interactions are a good example. Finite element method (FEM) with anisotropic meshes combined with dynamic mesh algorithms has already shown the potential to significantly reduce the number of elements and simulation time with no accuracy loss. However, mesh anisotropy can lead to mesh quality‐related instabilities. This article presents a very robust FEM approach based on a control volume discretization of the pressure field for inertia dominated flows, which can overcome the typically encountered mesh quality limitations associated with extremely anisotropic elements. Highly compressive methods for the water‐air interface are used here. The combination of these methods is validated with multiphase free surface flow benchmark cases, showing very good agreement with experiments even for extremely anisotropic meshes, reducing by up to two orders of magnitude the required number of elements to obtain accurate solutions.
Sun C, Wu F, Fu Y, et al., 2020, Thin film Gallium nitride (GaN) based acoustofluidic Tweezer: Modelling and microparticle manipulation., Ultrasonics, Vol: 108
Gallium nitride (GaN) is a compound semiconductor which shows advantages in new functionalities and applications due to its piezoelectric, optoelectronic, and piezo-resistive properties. This study develops a thin film GaN-based acoustic tweezer (GaNAT) using surface acoustic waves (SAWs) and demonstrates its acoustofluidic ability to pattern and manipulate microparticles. Although the piezoelectric performance of the GaNAT is compromised compared with conventional lithium niobate-based SAW devices, the inherited properties of GaN allow higher input powers and superior thermal stability. This study shows for the first time that thin film GaN is suitable for the fabrication of the acoustofluidic devices to manipulate microparticles with excellent performance. Numerical modelling of the acoustic pressure fields and the trajectories of mixtures of microparticles driven by the GaNAT was performed and the results were verified from the experimental studies using samples of polystyrene microspheres. The work has proved the robustness of thin film GaN as a candidate material to develop high-power acoustic tweezers, with the potential of monolithical integration with electronics to offer diverse microsystem applications.
Zhang C, Kang Z, Stoesser T, et al., 2020, Experimental investigation on the VIV of a slender body under the combination of uniform flow and top-end surge, Ocean Engineering, Vol: 216, ISSN: 0029-8018
In order to investigate the effect caused by floating bodies on vortex-induced vibration (VIV) of risers, a composite pipe with the aspect ratio of 250 is tested under coupled uniform and shear oscillatory flow, with Reynolds number ranging from 2000 to 24000. The characteristics of VIV amplitude and frequency of the model under uniform flow are compared and analyzed comprehensively, alongside experimental results from literature, validating the reasonability of the test devices and methods. The key characteristics of VIV under coupled top-end surge and uniform flow are then discussed. Some phenomena involving time-varying frequency, mode transition, amplitude modulation and so on are induced by the oscillation of flow, which also promotes the generation of high-order and low-order harmonic components in cross-flow and in-line frequencies. When combined with uniform flow, the identification of time-varying frequency and amplitude modulation is significantly enhanced by uniform flow with lower velocities while the top-end surge mainly affects the vibration instability in higher-velocity uniform flow. The oscillation amplitude and frequency of the top-end surge mainly change the values of cross-flow vibration amplitude, but affect the in-line amplitude both for trend and values, inducing the increase of values in the range of medium and low reduced velocities and the changing of the distribution of reduced velocity corresponding to the local maximums in the higher range. The combination of the top-end surge related to the movement of floating body and the background flow is responsible for the high-order harmonics of VIV frequency and the enhancement of VIV amplitude under certain reduced velocities, which may induce additional fatigue damage of riser. It is suggested that the movement of floating body should be taken into account in the evaluation of VIV of risers.
Xie Z, Stoesser T, 2020, Two-phase flow simulation of breaking solitary waves over surface-piercing and submerged conical structures, Ocean Engineering, Vol: 213, ISSN: 0029-8018
A two-phase flow model is employed to study three-dimensional (3D) breaking of solitary waves over surface-piercing and submerged conical structures. Details of the wave pre-breaking, overturning, and post-breaking processes are included. The governing equations are discretized by the finite volume method and the PISO algorithm is utilized for the pressure-velocity coupling. The air–water interface is captured using a volume of fluid approach and the Cartesian cut-cell method is implemented to deal with the complex topography of the conical structures. The method is validated first using available experimental data of a solitary wave propagating over a surface-piercing conical island and good agreement between the experiment and simulation data is obtained. The model is then applied to study 3D breaking waves over a submerged conical structure, with 3D wave profiles and surface velocities being presented and discussed. The detailed 3D velocity fields, energy dissipation and transformation during the wave breaking process are presented.
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