查看更多>>摘要：A coastal forest combined with a backward-facing step is an efficient facility to reduce tsunami damage to residential areas behind sea embankments.This study establishes a generalized model,and experimentally explores the water level changes upstream of the vegetation-step mitigation model as well as its energy dissipation effect under different initial Froude numbers,step heights,and vegetation conditions.The results show that the relative backwater rise increases with the growth of vegetation density,patch length and initial Froude number,representing a slowing down of the tsunami inundation.As for energy dissipation,it is mainly caused by the additional resistance of the vegetation and the hydraulic jump.And the vegetation condition not only affects the energy dissipation due to stem-scale turbulence within the patch,but also changes the hydraulic jump process of water falling from the step in cooperation with the step height.As a result,the energy dissipation efficiency always increases with the growth of vegetation density,vegetation patch length and step height.With the criterion that the energy dissipation efficiency and its growth rate can hardly change with vegetation parameters,this study innovatively defines the threshold slope and gives the principle of judging the most cost-effective vegetation conditions at different step heights.These results are expected to provide an important reference for the design of composite tsunami mitigation facilities.
查看更多>>摘要：Wave height forecast(WHF)is of great significance to exploit the marine renewables and improve the safety of ship navigation at sea.With the development of machine learning technology,WHF can be realized in an easy-to-operate and reliable way,which improves its engineering practicability.This paper utilizes a data-driven method,Gaussian process regression(GPR),to model and predict the wave height on the basis of the input and output data.With the help of Bayes inference,the prediction results contain the uncertainty quantification naturally.The comparative studies are carried out to evaluate the performance of GPR based on the simulation data generated by high-order spectral method and the experimental data collected in the deep-water towing tank at the Shanghai Ship and Shipping Research Institute.The results demonstrate that GPR is able to model and predict the wave height with acceptable accuracy,making it a potential choice for engineering application.
查看更多>>摘要：The effect of rotation-curvature correction and inviscid spatial discretization scheme on the aerodynamic performance and flow characteristics of Darricus H-type vertical axis wind turbine(VAWT)are investigated based on an in-house solver.This solver is developed on an in-house platform HRAPIF based on the finite volume method(FVM)with the elemental velocity vector transformation(EVVT)approach.The present solver adopts the density-based method with a low Mach preconditioning technique.The turbulence models are the Spalart-Allmaras(SA)model and the k-ω shear stress transport(SST)model.The inviscid spatial discretization schemes are the third-order monotone upstream-centered schemes for conservation laws(MUSCL)scheme and the fifth-order modified weighted essentially non-oscillatory(WENO-Z)scheme.The power coefficient,instantaneous torque of blades,blade wake,and turbine wake are compared and analyzed at different tip speed ratios.The extensive analysis reveals that the density-based method can be applied in VAWT numerical simulation;the SST models perform better than the SA models in power coefficient prediction;the rotation-curvature correction is not necessary and the third-order MUSCL is enough for power coefficient prediction,the high-order WENO-Z scheme can capture more flow field details,the rotation-curvature correction and high-order WENO-Z scheme reduce the length of the velocity deficit region in the turbine wake.
查看更多>>摘要：Tip leakage flow(TLF)trajectory in a pump with gas entrainment is investigated via visualization experiments and numerical simulations.Starting position of tip leakage vortex(TLV)is determined accurately by numerical simulation.Under high liquid flow rate(Q1)and high inlet gas volume fraction(IGVF)conditions,TLF flows from suction surface to pressure surface near the leading edge of blade,and the direction of TLF gradually changes along the chord which flows from pressure surface to suction surface near the tailing edge.The angle between TLF and blade mean camberline increases progressively as either Q1 or IGVF decreases,and starting position of TLV moves towards leading edge direction.As Q1 or IGVF decreases,value of vorticity increases and high vorticity region moves towards leading edge.The entropy production rate at blade tip clearance is high,and entropy diffuses from pressure surface to suction surface due to jet flow in blade tip clearance.The greater the amount of accumulated gas there is,the greater the amount of entropy in the area.In addition,when gas is entrained in pump,there are many low frequency fluctuations generated in blade tip clearance.
查看更多>>摘要：We experimentally investigate the 3-D flow characteristics caused by synthetic jet in the turbulent boundary layer(TBL),with the aim of analyzing the differences and similarities of hairpin vortices generated by jet of different hole diameters.For flow fields with hole diameters of 3 mm,4 mm,5 mm,the 2D time-resolved particle image velocimetry(TR-PIV)is used for preliminary experiment to determine the generation region of these hairpin vortices,and then the three-dimensional instantaneous snapshots of the region are obtained by tomographic PIV(Tomo-PIV).The statistical average results show that the downstream velocity deficit area is positively correlated with the hole diameter,and the drag reduction effect looks better with small hole diameter.The phase average extracts the three-dimensional morphology of the hairpin vortices produced by synthetic jet,and its distribution tends to be dense with the hole diameter,which is related to the velocity deficit.The two-point cross-correlation coefficient represents the scale of the coherent structure,and the three component scales of these hairpin vortices are smaller with large hole diameter,which is due to insufficient space for development.The flow fields are divided into high-energy and low-energy by proper orthogonal decomposition(POD).It is found that the increase of hole diameter can transfer the generated hairpin vortices from low-energy to high-energy,showing that the strength of high-energy hairpin vortices is positively correlated with the hole diameter.
查看更多>>摘要：In this paper,a passive control method based on a porous material is applied to the surface of a hemispherical cylinder to control a cavitating flow,and the control effect of this method at different cavitation numbers(a)is evaluated through the cavity morphology and volume,which is important for the application in engineering.The results indicate that the control effect is improved with a reduction in the cavitation number,for the reduction of vapor volume increases from 22％-50％with a decreasing from 0.50-0.20.Further investigation indicates that the cavity inception at different cavitation numbers is still induced by the Kelvin-Helmholtz instability,while the spatial distribution of the vapor changes significantly.Moreover,the porous material suppresses the cavitating flow in the front region but enhances it downstream at large cavitation numbers.When a=0.20,the cavitating flow is controlled in both the front and rear regions.
查看更多>>摘要：A two-phase flow model accelerated by graphical processing unit(GPU)is developed to solve fluid-solid interaction(FSI)using the sharp interface immersed boundary method(IBM).This model solves the incompressible Navier-Stokes equations using the projection-based fractional step method in a fixed staggered Cartesian grid system.A volume of fluid(VOF)method with second-order accuracy is employed to trace the free surface.To represent the intricate surface geometry,the structure is discretized using the unstructured triangle mesh.Additionally,a ray tracing method is employed to classify fluid and solid points.A high-order stable scheme has been introduced to reconstruct the local velocity at interface points.Three FSI problems,including wave evolution around a breakwater,interaction between a periodic wave train and a moving float,and a 3-D moving object interacting with the free surface,were investigated to validate the accuracy and stability of the proposed model.The numerical results are in good agreement with the experimental data.Additionally,we evaluated the computational performance of the proposed GPU-based model.The GPU-based model achieved a 42.29 times speedup compared with the single-core CPU-based model in the three-dimensional test.Additionally,the results regarding the time cost of each code section indicate that achieving more significant acceleration is associated with solving the turbulence,advection,and diffusion terms,while solving the pressure Poisson equation(PPE)saves the most time.Furthermore,the impact of grid number on computational efficiency indicates that as the number of grids increases,the GPU-based model outperforms the multi-core CPU-based model.
查看更多>>摘要：Unsteady cloud cavitating flow is detrimental to the efficiency of hydraulic machinery like pumps and propellers due to the resulting side-effects of vibration,noise and erosion damage.Modelling such a unsteady and highly turbulent flow remains a challenging issue.In this paper,cloud cavitating flow in a venturi is calculated using the detached eddy simulation(DES)model combined with the Merkle model.The adaptive mesh refinement(AMR)method is employed to speed up the calculation and investigate the mechanisms for vortex development in the venturi.The results indicate the velocity gradients and the generalized fluid element strongly influence the formation of vortices throughout a cavitation cycle.In addition,the cavitation-turbulence coupling is investigated on the local scale by comparing with high-fidelity experimental data and using profile stations.While the AMR calculation is able to predict well the time-averaged velocities and turbulence-related aspects near the throat,it displays discrepancies further downstream owing to a coarser grid refinement downstream and under-performs compared to a traditional grid simulation.Additionally,the AMR calculation is unable to reproduce the cavity width as observed in the experiments.Therefore,while AMR promises to speed the process significantly by refining the grid only in regions of interest,it is comparatively in line with a traditional calculation for cavitating flows.Thus this study intends to provide a reference to employing the AMR as a tool to speed up calculations and be able to simulate turbulence-cavitation interactions accurately.
查看更多>>摘要：We examine the genesis of coherent vortices in submerged vegetated flows by means of a linear stability analysis.The mathematical framework is comprised of the conservation equations of fluid mass and momentum.The problem is tackled by imposing normal mode perturbations over an underlying undisturbed flow.We find that the growth rate of perturbations takes maximum magnitude for a specific wavenumber,termed as the critical wavenumber.The critical wavenumber indicates the most favorable wavenumber of coherent vortices emerging in submerged vegetated flows.The critical wavenumber amplifies as the flow Reynolds number,and vegetation height and density augment.The migration velocity of incipient coherent vortices characterizes minimum magnitude for a selected value of the vegetation height.The unstable zone in the stability diagram embarks beyond a critical Reynolds number.The critical Reynolds number designates the onset of coherent vortex appearance in submerged vegetated flows.The predictions of the present study are congruent with the existing theoretical and experimental works.
查看更多>>摘要：Compared with periodic structures,quasi-periodic structures have superior band gap properties and topological interface states.In this paper,a one-dimensional quasi-periodic Fibonacci water wave metamaterial model that can be used to apply quasi-periodic structures to shallow-water wave systems is presented.The fluctuation characteristics of periodic and quasi-periodic structures are examined using finite element numerical calculations based on the shallow-water wave equation.The research results show that the band characteristics of quasi-periodic structures are complex,enabling flexible control of the propagation of shallow-water waves.Furthermore,the mirror-symmetrical design of Fibonacci quasi-periodic water wave metamaterials was created to engineer the topological interface states in shallow-water wave systems,ultimately achieving successful localization of wave energy.This research will greatly enrich our understanding of topology,expand the potential applications of quasi-periodic structures,and provide new insights for manipulating water waves and harvesting energy.
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