Numerical Analysis of the Hydraulic Characteristics of a Boundary Layer Streaming over Surf-Zone Using LES and Dynamic Smagorinsky Turbulence Model

摘要

Natural shoreline repeats its re-treatment and advance in response to the endlessly varying sea-conditions,and once severely eroded under stormy weather conditions,natural beaches are gradually recovered via a boundary layer streaming when swells are prevailing after storms cease.Our understanding of the boundary layer streaming over surf-zone often falls short despite its great engineering value,and here it should be noted that the most sediments available along the shore are supplied over the surf-zone.In this rationale,numerical simulation was implemented to investigate the hydraulic characteristics of boundary layer streaming over the surf zone in this study.In doing so,comprehensive numerical models made of Spatially filtered Navier-Stokes Eq.,LES (Large Eddy Simulation),Dynamic Smagorinsky turbulence closure were used,and the effects of turbulence closure such as Dynamic Smagorinsky in LES and k-ε on the numerically simulated flow field were also investigated.Numerical results show that due to the intrinsic limits of k-ε turbulence model,numerically simulated flow velocity near the bottom based on k-ε model and wall function are over-predicted than the one using Dynamic Smagorinsky in LES.It is also shown that flow velocities near the bottom are faster than the one above the bottom which are relatively free from the presence of the bottom,complying the typical boundary layer streaming by Longuet-Higgins (1957),the spatial scope where boundary layer streaming are occurring is extended well into the surf zone as incoming waves are getting longer.These tendencies are plausible considering that it is the bottom friction that triggers a boundary layer streaming,and longer waves start to feel the bottom much faster than shorter waves.