Three-dimensional Lattice Boltzmann Simulation Of Oscillatory Boundary Layer Flow Over Rippled Bed

Sand ripple is a common bedform in coastal area under the action of waves.The existence of ripples increases the resistance of sea bed significantly,and also greatly influences the flow structure of the near-bed wave boundary layer.A complete understanding of the wave boundary layer flow over ripples,including the structure and dynamics of turbulence and the corresponding bed friction characteristics is the basis of research on sediment transport.A 3-dimentional numerical model based on Lattice Boltzmann method combined with large-eddy simulation(LES)model is developed to simulate the oscillatory boundary layer flow over rippled bed.Flow characteristics,evolution of the vortex structure and bed friction characteristics are investigated.The main results are summarized as follows.(1)A 3D Lattice Boltzmann model combined with Smagorinsky model and WALE model is developed.Simulations of oscillatory boundary layer flow over rough plane bed and rippled bed are carried out.Good agreement is shown between the simulated results and experimental data,which indicates that the 3D LB model is feasible to solve the turbulent oscillatory boundary layer flow and bed resistance problems.(2)Simulations of oscillatory boundary layer flow over rough plane bed composed by randomly arrayed spherical particles,which is closer to the rough plane bed in nature,are carried out.Results from 8 cases with different relative roughness indicate that the theoretical bed is located at 0.19 times diameter in average below the crests of spherical particles,and the equivalent roughness is 2.81 times diameter of particles in average.The phase difference of friction velocity and free-stream velocity is 19.8°～35.4°.The relationship between the wave friction factor f_w and the relative roughness a/k_s for rough beds is obtained through data fitting.The oscillatory boundary layer flow over rough bed surface under the action of cnoidal wave is simulated.The amplitude of friction velocity in the half period of wave peak under coidal wave is larger than that in the half period of wave trough.The distance from the theoretical bed to the top of spheres,as well as the equivalent roughness height under coidal wave are essentially in agreement with those of the linear wave.The wave friction coefficient decreases with the increase of wave nonlinearity.The phase difference of friction velocity and free-stream velocity decreases with the increase of wave nonlinearity and the thickness of the boundary layer increases slightly.(3)Numerical simulation results on three different types of equilibrium ripples,including orbital ripples,suborbital ripples and anorbital ripples,are presented.The boundary layer thickness along orbital ripple is unequal,which is thickest above the trough,and thinnest above the crest.The boundary layer thickness of anorbital ripple is over 4 times ripple height,and equal along ripple,which means that anorbital ripples are submerged in boundary layer.The boundary layer of suborbital ripple is in transient state between the former two types.These differences indicate the rationality of Clifton’s classification of ripples.(4)The evolution processes of vortex structures over three different types of ripples are investigated.Large vortex structures are found to form twice on the lee side of the orbital ripple during each half cycle,which is considered to cause the double-peak feature of suspended sediment concentrations in half period reported in literatures.(5)The amplitude of form friction factor and skin friction factor reduces with decreasing ripple steepness.The radio of form friction factor and skin friction factor tends to the constant of 6.8 for orbital ripples,and reduces gradually with decreasing ripple steepness for suborbital and anorbital ripples.Results indicate that there is scarcely any form friction effect after η/λ≤0.07.(6)The equivalent roughness height k_s of equilibrium ripples,calculated by the obtained relationship between f_w and a/k_s,is shown to be related with both the rippled heightηand steepness η/λ.The resulting relationship suitable for equilibrium ripple is:k_s=14.9η~2/λ.