65 results found
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Xie Z, Lin B, Falconer RA, et al., 2021, Large-eddy simulation of turbulent free surface flow over a gravel bed, Journal of Hydraulic Research, 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.
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.
Xie Z, Stoesser T, 2020, A three-dimensional Cartesian cut-cell/volume-of-fluid method for two-phase flows with moving bodies, Journal of Computational Physics, Vol: 416, ISSN: 0021-9991
A three-dimensional Cartesian cut-cell method for the large-eddy simulation of two-phase flows with moving bodies is presented in this study, which combines a volume-of-fluid method to capture the air-water interface and a moving body algorithm on a stationary, non-uniform, staggered, Cartesian grid. The filtered Navier–Stokes equations are discretised using the finite volume method with the PISO algorithm for velocity-pressure coupling and the dynamic Smagorinsky subgrid-scale model is employed to compute the effect of the unresolved (subgrid) scales of turbulence on the large scales. In the present study, the small cut-cells are unmodified and due to the use of an implicit time integration no instabilities occur during the computations. The versatility and robustness of the present two-phase flow model is illustrated via various two- and three-dimensional flow problems with fixed/moving bodies, such as dambreak flows with and without a square cylinder, a moving cylinder in a quiescent fluid, dambreak flow over a wet bed with a moving gate, water entry and exist of a circular cylinder, and landside-generated waves. Good agreement is obtained between the numerical results and the corresponding experimental measurements.
Wang J, Yan S, Ma Q, et al., 2020, Modelling of focused wave interaction with wave energy converter models using qaleFOAM Junxian Wang MSc, Proceedings of the Institution of Civil Engineers: Engineering and Computational Mechanics, Vol: 173, Pages: 100-118, ISSN: 1755-0777
This paper presents a numerical investigation on the interaction between focused waves and wave energy converter (WEC) models using a hybrid solver, qaleFOAM, which couples a two-phase incompressible Navier-Stokes (NS) solver OpenFOAM/InterDyMFoam with the quasi Lagrangian-Eulerian finite element method (QALE-FEM) based on the fully non-linear potential theory (FNPT) using the domain-decomposition approach. In the qaleFOAM, the NS solver deals with a small region near the structures (NS domain), where the viscous effect may be significant; the QALE-FEM covers the remaining computational domain (FNPT domain); an overlap (transitional) zone is applied between two domains. The WEC models, mooring system and the wave conditions are given by the Collaborative Computational Project in Wave-Structure Interaction (CCP-WSI) Blind Test Series 2. In the numerical simulation, the incident wave is generated in the FNPT domain using a self-correction wavemaker and propagates into the NS domain through the coupling boundaries and attached transitional zones. An improved passive wave absorber is imposed at the outlet of the NS domain for wave absorption. The practical performance of the qaleFOAM is demonstrated by comparing its prediction with the experimental data, including the wave elevation, motion responses (surge, heave and pitch) and mooring load.
Sun C, Wu F, Wallis DJ, et al., 2020, Gallium Nitride: A Versatile Compound Semiconductor as Novel Piezoelectric Film for Acoustic Tweezer in Manipulation of Cancer Cells, IEEE Transactions on Electron Devices, Vol: 67, Pages: 3355-3361, ISSN: 0018-9383
Gallium nitride (GaN) is a compound semiconductor which has advantages to generate new functionalities and applications due to its piezoelectric, pyroelectric, and piezo-resistive properties. Recently, surface acoustic wave (SAW)-based acoustic tweezers were developed as an efficient and versatile tool to manipulate nano- and microparticles aiming for patterning, separating, and mixing biological and chemical components. Conventional piezoelectric materials to fabricate SAW devices such as lithium niobate suffer from its low thermal conductivity and incapability of fabricating multiphysical and integrated devices. This article piloted the development of a GaN-based acoustic tweezer (GaNAT) and its application in manipulating microparticles and biological cells. For the first time, the GaN SAW device was integrated with a microfluidic channel to form an acoustofluidic chip for biological applications. The GaNAT demonstrated its ability to work on high power (up to 10 W) with minimal cooling requirement while maintaining the device temperature below 32°C. Acoustofluidic modeling was successfully applied to numerically study and predict acoustic pressure field and particle trajectories within the GaNAT, which agree well with the experimental results on patterning polystyrene microspheres and two types of biological cells including fibroblast and renal tumor cells. The GaNAT allowed both cell types to maintain high viabilities of 84.5% and 92.1%, respectively.
Xie Z, Pavlidis D, Salinas P, et al., 2020, A control volume finite element method for three‐dimensional three‐phase flows, International Journal for Numerical Methods in Fluids, Vol: 92, Pages: 765-784, ISSN: 0271-2091
A novel control volume finite element method with adaptive anisotropic unstructured meshes is presented for three‐dimensional three‐phase flows with interfacial tension. The numerical framework consists of a mixed control volume and finite element formulation with a new P1DG‐P2 elements (linear discontinuous velocity between elements and quadratic continuous pressure between elements). A “volume of fluid” type method is used for the interface capturing, which is based on compressive control volume advection and second‐order finite element methods. A force‐balanced continuum surface force model is employed for the interfacial tension on unstructured meshes. The interfacial tension coefficient decomposition method is also used to deal with interfacial tension pairings between different phases. Numerical examples of benchmark tests and the dynamics of three‐dimensional three‐phase rising bubble, and droplet impact are presented. The results are compared with the analytical solutions and previously published experimental data, demonstrating the capability of the present method.
Mikhaylov R, Wu F, Wang H, et al., 2020, Development and characterisation of acoustofluidic devices using detachable electrodes made from PCB., Lab Chip, Vol: 20, Pages: 1807-1814
Acoustofluidics has been increasingly applied in biology, medicine and chemistry due to its versatility in manipulating fluids, cells and nano-/micro-particles. In this paper, we develop a novel and simple technology to fabricate a surface acoustic wave (SAW)-based acoustofluidic device by clamping electrodes made using a printed circuit board (PCB) with a piezoelectric substrate. The PCB-based SAW (PCB-SAW) device is systematically characterised and benchmarked with a SAW device made using the conventional photolithography process with the same specifications. Microparticle manipulations such as streaming in droplets and patterning in microchannels were demonstrated in the PCB-SAW device. In addition, the PCB-SAW device was applied as an acoustic tweezer to pattern lung cancer cells to form three or four traces inside the microchannel in a controllable manner. Cell viability of ∼97% was achieved after acoustic manipulation using the PCB-SAW device, which proved its ability as a suitable tool for acoustophoretic applications.
Yan S, Wang J, Wang J, et al., 2020, CCP-WSI blind test using qaleFOAM with an improved passive wave absorber, International Journal of Offshore and Polar Engineering, Vol: 30, Pages: 43-52, ISSN: 1053-5381
The paper presents the contribution to the CCP-WSI Blind Test, in which the responses of wave energy converters subjected to extreme waves are considered, by a hybrid model, qaleFOAM, coupling a two-phase Navier–Stokes (NS) model and the fully nonlinear potential theory (FNPT) using the spatially hierarchical approach. The former governs a limited computational domain (NS domain) around the structures and is solved by the OpenFOAM/InterDyMFoam. The latter covers the rest of the domain (FNPT domain) and is solved by using the quasi Lagrangian-Eulerian finite element method. Two numerical techniques have been developed to tackle the challenges and maximizing the computational efficiency of the qaleFOAM, including a modified solver for the six-degree-of-freedom motions of rigid bodies in the NS model and an improved passive wave absorber imposed at the outlet of the NS domain. With these developments, the accuracy and the computational efficiency of the qaleFOAM are analyzed for the cases considered in the blind test.
Ransley E, Yan S, Brown S, et al., 2020, A blind comparative study of focused wave interactions with floating structures (CCP-WSI blind test series 3), International Journal of Offshore and Polar Engineering, Vol: 30, Pages: 1-10, ISSN: 1053-5381
Results from the CCP-WSI Blind Test Series 3 are presented. Participants, with numerical methods, ranging from low-fidelity linear models to high-fidelity Navier–Stokes (NS) solvers, simulate the interaction between focused waves and floating structures without prior access to the physical data. The waves are crest-focused NewWaves with various crest heights. Two structures are considered: a hemispherical-bottomed buoy and a truncated cylinder with a moon-pool; both are taut-moored with one linear spring mooring. To assess the predictive capability of each method, numerical results for heave, surge, pitch, and mooring load are compared against corresponding physical data. In general, the NS solvers appear to predict the behaviour of the structures better than the linearised methods, but there is considerable variation in the results (even between similar methods). Recommendations are made for future comparative studies and development of numerical modelling standards.
Salinas P, Pain C, Osman H, et al., 2020, Vanishing artifficial diffusion as a mechanism to accelerate convergence for multiphase porous media flow, Computer Methods in Applied Mechanics and Engineering, Vol: 359, Pages: 1-15, ISSN: 0045-7825
Numerical solution of the equations governing multiphase porous media flow is challenging. A common approach to improve the performance of iterative non-linear solvers for these problems is to introduce artificial diffusion. Here, we present a mass conservative artificial diffusion that accelerates the non-linear solver but vanishes when the solution is converged. The vanishing artificial diffusion term is saturation dependent and is larger in regions of the solution domain where there are steep saturation gradients. The non-linear solver converges more slowly in these regions because of the highly non-linear nature of the solution. The new method provides accurate results while significantly reducing the number of iterations required by the non-linear solver. It is particularly valuable in reducing the computational cost of highly challenging numerical simulations, such as those where physical capillary pressure effects are dominant. Moreover, the method allows converged solutions to be obtained for Courant numbers that are at least two orders of magnitude larger than would otherwise be possible.
Fang K, Liu Z, Sun J, et al., 2020, Development and validation of a two-layer Boussinesq model for simulating free surface waves generated by bottom motion, Applied Ocean Research, Vol: 94, ISSN: 0141-1187
In this work, a recently developed two-layer Boussinesq model with high accuracy regarding linear and nonlinear properties and interior kinematic properties from deep to shallow water is extended to include the time-varying bathymetry for modelling earthquake- and landslide-induced waves. The effect of bottom motion is taken into account by simply adding an additional term within the kinematic bottom condition. The moving shoreline is addressed by utilizing linear extrapolation through the wet–dry boundary and into the dry region. A wide range of numerical tests covering the generation, propagation and runup of dispersive waves induced by bottom motion are carried out with the vertical two-dimensional Boussinesq model. The computed results are compared against available theoretical solutions, experimental measurements and previous numerical simulations. Both the consistencies and the discrepancies are discussed.
Yan S, Li Q, Wang J, et al., 2019, Comparative numerical study on focusing wave interaction with FPSO-like structure, International Journal of Offshore and Polar Engineering, Vol: 29, Pages: 149-157, ISSN: 1053-5381
Evaluating the interactions between offshore structures and extreme waves plays an essential role for securing the survivabilityof the structures. For this purpose, various numerical tools—for example, the fully nonlinear potential theory (FNPT),the Navier–Stokes (NS) models, and hybrid approaches combining different numerical models—have been developed andemployed. However, there is still great uncertainty over the required level of model fidelity when being applied to a widerange of wave-structure interaction problems. This paper aims to shed some light on this issue with a specific focus on theoverall error sourced from wave generation/absorbing techniques and resolving the viscous and turbulent effects, by comparingthe performances of three different models, including the quasi-arbitrary Lagrangian Eulerian finite element method(QALE-FEM) based on the FNPT, an in-house two-phase NS model with large-eddy simulation and a hybrid model couplingthe QALE-FEM with the OpenFOAM/InterDymFoam, in the cases with a fixed FPSO-like structure under extreme focusingwaves. The relative errors of numerical models are defined against the experimental data, which are released after thenumerical works have been completed (i.e., a blind test), in terms of the pressure and wave elevations. This paper providesa practical reference for not only choosing an appropriate model in practices but also on developing/optimizing numericaltools for more reliable and robust predications.
Ransley E, Yan S, Brown S, et al., 2019, A blind comparative study of focused wave interactions with a fixed FPSO-like structure (CCP-WSI blind test series 1), International Journal of Offshore and Polar Engineering, Vol: 29, Pages: 113-127, ISSN: 1053-5381
Results from Blind Test Series 1, part of the Collaborative Computational Project in Wave Structure Interaction (CCP-WSI), are presented. Participants, with a range of numerical methods, simulate blindly the interaction between a fixed structure and focused waves ranging in steepness and direction. Numerical results are compared against corresponding physical data. The predictive capability of each method is assessed based on pressure and run-up measurements. In general, all methods perform well in the cases considered, however, there is notable variation in the results (even between similar methods). Recommendations are made for appropriate considerations and analysis in future comparative studies.
Lei Q, Xie Z, Pavlidis D, et al., 2018, The shape and motion of gas bubbles in a liquid flowing through a thin annulus, Journal of Fluid Mechanics, Vol: 285, Pages: 1017-1039, ISSN: 0022-1120
We study the shape and motion of gas bubbles in a liquid flowing through a horizontal or slightly inclined thin annulus. Experimental data show that in the horizontal annulus, bubbles develop a unique ‘tadpole-like’ shape with a semi-circular cap and a highly stretched tail. As the annulus is inclined, the bubble tail tends to vanish, resulting in a significant decrease of bubble length. To model the bubble evolution, the thin annulus is conceptualised as a ‘Hele-Shaw’ cell in a curvilinear space. The three-dimensional flow within the cell is represented by a gap-averaged, two-dimensional model, which achieved a close match to the experimental data. The numerical model is further used to investigate the effects of gap thickness and pipe diameter on the bubble behaviour. The mechanism for the semi-circular cap formation is interpreted based on an analogous irrotational flow field around a circular cylinder, based on which a theoretical solution to the bubble velocity is derived. The bubble motion and cap geometry is mainly controlled by the gravitational component perpendicular to the flow direction. The bubble elongation in the horizontal annulus is caused by the buoyancy that moves the bubble to the top of the annulus. However, as the annulus is inclined, the gravitational component parallel to the flow direction becomes important, causing bubble separation at the tail and reduction in bubble length.
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