查看更多>>摘要：Bow wave breaking is a common phenomenon during ship navigation,especially at a high speed,involving complex physical mechanism such as interface mixing,air entrainment,and jet splashing.This study uses the delayed detached eddy simulation(DDES)turbulence model on the OpenFOAM platform to simulate flow around a KRISO Container Ship(KCS)model for a Froude number of 0.35,examining trim angles of 0°,0.5°,1°.This paper analyzes the statistical and power spectral density(PSD)characteristics of bow wave heights.The analysis shows root mean square(rms)and mean difference between top and bottom views indicate wave breaking.As the trim angle increases,peaks of rms in the bottom view become much higher than that in the top view,reaching 38％at 1°.PSD analysis reveals that resistance and wave height periods differ by no more than 5％,with small-scale structures like jetting and splashing causing non-dominant periodic and high-frequency wave height variations.
查看更多>>摘要：This study conducts a comparative analysis between detached eddy simulation(DES)and Unsteady Reynolds-averaged Navier-Stokes(URANS)models for simulating pressure fluctuations in a stilling basin,aiming to assess the URANS mode's performance in modeling pressure fluctuation.The URANS model predicts accurately a smoother flow field and its time-average pressure,yet it underestimates the root mean square of pressure(RMSP)fluctuation,achieving approximately 70％of the results predicted by DES model on the bottom floor of the stilling basin.Compared with DES model's results,which are in alignment with the Kolmogorov-5/3 law,the URANS model significantly overestimates low-frequency pulsations,particularly those below 0.1 Hz.We further propose a novel method for estimating the RMSP in the stilling basin using URANS model results,based on the establishment of a quantitative relationship between the RMSP,time-averaged pressure,and turbulent kinetic energy in the boundary layer.The proposed method closely aligns with DES results,showing a mere 15％error level.These findings offer vital insights for selecting appropriate turbulence models in hydraulic engineering and provide a valuable tool for engineers to estimate pressure fluctuation in stilling basins.
查看更多>>摘要：In this work,computational fluid dynamics(CFD)-based simulations and linear diffraction analysis are carried out to investigate the interaction between water waves and metamaterials composed of an array of C-shaped cylinders.The flow visualization by CFD-based simulations reveals that local resonance is a result of constructive interference between the incident wave and the wave radiated from the cavity of the C-shaped cylinder.The wave-induced water motion inside the cavity acts as a source of generating this radiated wave,which has the same angular wave frequency and wavenumber but opposite propagation direction as the incident wave.In addition,it is found from the CFD-based simulations that the energy dissipation increases as the opening of the C-shaped cylinder becomes shorter and sharper,along with an increase in its outer radius,and the variation trend of energy dissipation is only affected by the outer radius.Meanwhile,except for very small opening lengths,variations in opening length,width,and outer radius do not significantly impact the wave attenuation effect of the C-shaped cylinder array.Moreover,the results obtained by CFD and the linear potential flow model are compared.The linear potential flow theory is proven to be a reliable approach for accurately predicting the local resonant frequency and transmission coefficients within the local resonant band across a range of geometric parameters.However,it is noted that this theory may have limitations when applied to cases with extremely small opening lengths,where it struggles to accurately predict the local resonant frequency and the intensity of local resonance.
查看更多>>摘要：This study delved into the acoustic spectrum of bubble clusters,each consisting of 352 vapor bubbles across volume fractions ranging from 0.005％to 40％.The clusters,organized in five distinct layers,were modeled using the volume of fluid(VOF)method to capture the bubble interfaces,and the Ffowcs Williams-Hawkings(FW-H)methodology to compute the far-field acoustic pressure from bubble collapse.Further analysis revealed distinct sound pressure behaviors across different volume fractions:For 25％-40％,time-domain analysis shows that the peak acoustic pressure pulses from the two innermost layers of bubbles are significantly higher than those from the outer layers.In the frequency domain,the octave decay rate of the acoustic pressure levels is relatively low,around-3dB/octave.For 0.5％-25％,four acoustic pressure pulses with similar widths and peak values were observed in the time domain.In the frequency domain,there are three distinct peaks in sound pressure levels(SPL),directly linked to the difference in collapse times of bubbles within the cluster,and the octave decay rate accelerates as the volume fraction decreases,stabilizing at-6dB/octave when the volume fraction is reduced to 17.5％.For 0.005％-0.5％,as the volume fraction decreases from 0.5％to 0.1％,the number of acoustic pressure pulses significantly reduces.Below 0.1％volume fraction,only a single wider pulse is observed.In the frequency domain,the octave decay rate gradually increases with decreasing volume fraction,significantly exceeding-l0dB/octave when it drops below 0.1％,reaching up to-11.7dB/octave.
查看更多>>摘要：Metal contaminants from surface water pollution events often enter hyporheic zones,under certain conditions,they may be released back into streams,causing secondary pollution to the water quality.The present study investigated the effects of adsorption,permeability,and anisotropy of sediment beds on the release of zinc ions(Zn2+)from the hyporheic zone into overlying turbulent flows using large-eddy simulations(LES).The volume-averaged Navier-Stokes equations and advection-diffusion equation with adsorption term were used to describe the sediment in-flow,adsorption,and convective diffusion of Zn2+within the sediment layer.The effects of sediment permeability on the Zn2+concentration distribution and mass transfer processes were investigated by time-averaged statistics of flow and concentration fields.The results show that adsorption becomes stronger as the pH value increases,leading to a slow increase in Zn2+concentration in the overlying water layer and reaching a lower steady-state concentration.Higher overall permeability of the sediment layer can enhance mass and momentum exchange near the sediment-water interface(SWI),and intensify the release of Zn2+from the sediment layer into the overlying water.As the wall-normal permeability of the sediment layer increases,the normal turbulent intensity strengthens,momentum transport enhances,the wall-normal Zn2+concentration flux increases,the effective diffusion coefficient increases,and the concentration in the overlying water increases.
查看更多>>摘要：As one of the most common river patterns in nature,meandering river has very complex flow structures in its curved channel bends,including secondary flow structure and primary flow velocity redistributions.To date,most of the studies have been carried out on the flow structures in channel bends with unavoidable influences from inlet and outlet boundaries,while a streamwise periodic boundary can overcome this shortcoming elegantly.In this paper,large eddy simulations(LES)are employed to investigate the complex flow structures in periodically continuous sharp sine-generated bends.The influence of width-to-depth ratios and dimensionless curvature radiuses are studied.The results highlight two additional vortex structures beyond the commonly known secondary currents:The recirculation zone(RZ)and the inner bank cell(IBC).The width-to-depth ratio shows the determining effect on the recirculation zone.The size of recirculation zone is usually bigger in sine-generated-curve(SGC)channel with large width-to-depth ratios.The biggest recirculation zones appear between the zero-curvature section and the apex section.The inner bank cell only forms in SGC channels with small width-to-depth ratios and low curvature.For SGC channel with large width-to-depth ratios,only one circulation cell is observed near the inner bank.The spatial variations of turbulent features are also revealed by statistical analysis based on the LES sampling data.Results highlight remarkable effect of width-to-depth ratio and dimensionless curvature radius on the turbulent kinetic energy(TKE)and bed shear stress in SGC channels.
查看更多>>摘要：Ship bow wave breaking contains complex flow mechanism,which is very important for ship performance.In this study,a practical numerical simulation scheme for bow wave breaking is proposed and the scheme is applied to the simulation of bow wave breaking of KCS ship model with Fr=0.26,0.30,0.35,0.40,analyzing the impact of speed on the bow wave breaking.The results indicate that an increase in speed leads to a significant rise in viscous pressure resistance and more pronounced bow wave breaking.Moreover,it is found that the traditional wave height function in OpenFOAM is not suitable for detailed studies of bow wave breaking.This study extracts different free surfaces through top and bottom views to further analyze the free surface overturning,droplet splashing,and cavity entrainment in bow wave breaking.Additionally,the spatial and temporal distribution of cavities at Fr=0.40 is analyzed,revealing that cavity distribution is closely related to vortex structures and exhibits a periodic pulsation characteristic of approximately 12 s.
查看更多>>摘要：Marine turbines have been extensively utilized to harness tidal stream energy from free-flowing tides and currents.However,the assessment of the influences of these marine structures on the surrounding environment is still in its early stage.In this study,a numerical model that couples hydrodynamics and sediment transport is developed to simulate the scour processes around a monopile-supported horizontal axial tidal stream turbine under steady currents.The flow characteristics are calculated by solving the 3-D Navier-Stokes equations with the k-ω shear stress transport(SST)turbulence model for closure.The simulation of sediment bed elevation is achieved by solving the Exner equation.The turbine rotor is parameterized using the actuator line method.The developed model is validated against wake velocity and scour depth measurement obtained from previous literature,showing a good agreement.Subsequently,the effects of tip clearance on the flow characteristics around the turbine model on a rigid flatbed are examined.Finally,the scour processes of the turbine model are presented,along with the vortex system within the scour hole.The numerical model proposed in this study has the potential to contribute to the understanding of the scour mechanism of the tidal stream turbines.
查看更多>>摘要：Visual and pressurized pipeline systems with single-and multi-undulation layouts were used to study experimentally and analyze theoretically the transient characteristics of water-air two-phase flow during water fillings in undulation pipelines based on the combination action analyses of both the communicating pipe and the gravity of the water-air two-phase flows in the descending pipe.For the single undulation pipeline,the complex two-phase flow-pattern evolutions including full pipe flow and stratified flow for low,medium,high water-filling velocity cases,lead to a great difference in transient pressure,flow pattern and the water-filling duration.Especially for low and medium water-filling velocity cases,the hydraulic theories related to hydraulic drop and hydraulic jump were employed to investigate the entrapped air pocket evolutions in the descending pipe,and the mechanism of negative pressure at the top of the undulation pipes was analyzed.For the same multi-undulation pipeline,due to the different elevations of the three undulation points along flow direction,namely three different types of pipeline layout,high-medium-low case(high elevation undulation point,medium one,and low one),low-medium-high and high-low-medium ones,their water-filling durations are significantly different,i.e.,approximately 80.02 s,227.34 s,617.78 s.Meanwhile,there are significant differences in flow patterns in water filling,namely larger entrapped air pockets in three descending pipes for the high-medium-low case,entrapped air pockets in the first two descending pipes and open channel stratified flow in the last one for low-medium-high case,some bubbles in three descending pipes for the high-low-medium case.
查看更多>>摘要：Since 1970s,several experimental works revealed that the cavitation sheet inception does not occur at the minimum pressure location but further downstream at the location of a laminar/turbulent transition.Most of the cavitation models use the saturation vapour pressure as a threshold to initiate the production of vapour and therefore,are not able to capture such flows.In this paper,three modifications of the Schnerr and Sauer cavitation model are proposed and coupled with an algebraic laminar/turbulent transition model.Application to a NACA 16 012 profile shows the ability of the modifications to move the cavitation inception at the right location compared with the experiment.One of them,based on the multiplication of the evaporation term by the square of the turbulent intensity seems promising.
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