查看更多>>摘要：This study evaluates computational fluid dynamics(CFD)turbulence closures for Reynolds-averaged Navier-Stokes(RANS)equations against experimental data to model complex open channel flows,like those occurring over dune-shaped salmon spawning nests called"redds".Open channel flow complexity,characterized by near-bed turbulence,adverse pressure,and free surfaces,requires suitable turbulence closure capable of capturing the flow structure between streambed and water surface.We evaluated three RANS models:Standard k-ω,shear-stress transport(SST)k-ω and realizable k-ε,along with four wall treatments for the realizable k-ε:Standard,and scalable wall functions,enhanced wall treatment,and an unconventional closure combining standard wall function with near-wall mesh resolving the viscous sublayer.Despite all models generally capturing the bulk flow characteristics,considerable discrepancies were evident in their ability to predict specific flow features,such as flow detachments.The realizable k-ε model,with standard wall function and mesh resolving viscous sublayer,outperformed other closures in predicting near-wall flow separations,velocity fields,and free surface elevation.This realizable k-ε model with a log-layer resolved mesh predicted the free surface elevation equally well but lacked precision for near-wall flows.The SST k-ω model outperformed in predicting turbulent kinetic energy and provided better predictions of the near-boundary velocity distributions than realizable k-ε closure with any of the conventional wall treatments but overestimated the separation vortex magnitude.The standard k-ω model also overestimated near-wall separation.This study highlights the variability in accuracy among turbulence models,underlining the need for careful model selection based on specific prediction regions.
查看更多>>摘要：Large wood in rivers can lead to accumulations in the river channel,affecting local flow structures,aquatic habitats,and the river's topography.This plays a crucial role in the ecological restoration of the river.This paper presents flow field measurements downstream of six types of logjams at different flow velocities using acoustic Doppler velocimetry(ADV)for artificially designed engineered logjams.The results indicate that the presence of logjams reduces the flow velocity and increases the turbulent kinetic energy in the wake region,and as the distance downstream increases,the flow velocity and turbulence intensity in the wake region gradually return to the upstream level.The minimum values of normalized flow velocity under different conditions are located in the region of the bottommost logs.The differences in normalized flow velocity at various flow rates are not significant.Jets are less likely to be generated in logjams with larger and more concentrated projection areas,but the strength of the jet is influenced by the physical structure of the logjam(projection area,gap ratio).The flow distribution behind the logjam is primarily influenced by the proportion of the projected area in different regions.Changes in the vertical physical structure of the logjam have minimal effect on the lateral flow distribution.Flow velocity in the gap area(b0)at the bottom of different logjams is influenced by their physical structure.The larger the overall blockage area of the logjams,the larger the flow velocity in the bottom gap area will be.The flow velocity in the bottom gap area of a densely placed logjam is mainly influenced by the gap ratio.The velocity of flow in the gap area can impact the initiation and deposition of sediment near the logjam.However,the internal structure complexity of the logjam does not significantly affect river energy dissipation and flow attenuation.This study broadens the applicability of certain theoretical models and explores the impact of logjams on river ecology and channel geomorphology.The findings can serve as a theoretical foundation for ecological restoration,timber management,and logjam construction in rivers.
查看更多>>摘要：Through direct numerical simulations,we investigated the flow structure and heat transfer of the centrally confined 2-D Rayleigh-Bénard(RB)convection over the Rayleigh number range 9×105 ≤ Ra ≤ 109 at a fixed Prandtl number Pr=4.3.It is found that with increasing Ra,the number of convection rolls in the central vertical channel increases from zero to three.When there is no rolls in the vertical channel,the convective flow in central region is significantly influenced by the boundary layer,whereas when the convection rolls is generated in the vertical channel,the convective flows in central regions is free from the boundary layer limitation,and by defining the characteristic length,one obtains the heat transfer scaling law relation in vertical channel,i.e.,Nuvc～Ra0.476±0005,which could be the evidence of"ultimate regime".
查看更多>>摘要：The flow field structure within the clearances of turbomachinery is complex and diverse,exhibiting high-dimensional nonlinearity.How to accurately extract the main structures that affect the internal flow within the turbine from the complex clearance flow has always been a key issue.To explore the impact of the dynamic structure of the clearance flow on the mainstream region in a centrifugal pump,this study combines the dynamic mode decomposition(DMD)method to conduct a thorough analysis of the velocity and pressure pulsation frequencies in the multi-physics fields within the clearance.The study has identified the main characteristic structures under different physical conditions in the clearance and has established the relationship between the characteristic structure frequencies in different physical fields and the impeller frequency.The research indicates that the internal flow within the clearance affects the occurrence of vortices in the volute.Under design conditions,the velocity field within the clearance is primarily influenced by high-order harmonic frequencies of the impeller,and the pressure field is mainly affected by low-order harmonic frequencies of the impeller.This reflects the crucial influence of impeller frequency and inlet flow on the coherent structures within the clearance flow.The research results offer new insights and methods for analyzing complex internal flows in large turbomachinery.
查看更多>>摘要：In simulating vegetated flows using the porous approach,the reasonableness of the drag coefficient significantly impacts the calculation results.This study employs large eddy simulation(LES)to quantitatively investigate the effect of drag parameters on key flow characteristics in submerged vegetated flows.The results indicate that changes in the drag coefficient significantly alter the velocity in the middle of the vegetation layer and near the water surface in the free-flow layer.Compared with longitudinal velocity,the drag coefficient has a more pronounced effect on the vertical distribution of Reynolds stress,especially its peak at the top of the vegetation layer.The porous approach can accurately reproduce the vertical distribution of longitudinal velocity and Reynolds stress,consistent with experimental measurements,only when shear-scale flow dominates.Due to the high-intensity secondary flow under moderate vegetation density,fluctuations in the drag coefficient have a more significant impact on the numerical results than in very dense vegetation.Therefore,selecting the drag coefficient value should be done cautiously,especially in the absence of experimental measurements for validation.
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