A model for the prediction of bed shear stress fluctuations in Oscillatory Boundary Layer flows

Abstract:

Oscillatory boundary layer flows play an important role on coastal processes, and sediment transport and morphodynamics. While ensemble-averaged quantities are usually used for sediment transport analysis, turbulent events can play a significant role in sediment entrainment and transport. Direct Numerical Simulations (DNS) are used to examine the flow characteristics under purely oscillatory conditions. DNS results for mean flow and turbulence statistics are compared with data from the literature to evaluate the accuracy of the applied model. Bed-shear stress variation is examined, and the data are explained in terms of transition to a fully turbulent state. Quadrant analysis shows the dominance of sweeps and ejections over the turbulent part of the period. The statistical distribution of the resulting bed-shear stress fluctuations is examined over the period and an empirical probability distribution that fits well the data is proposed. In addition, empirical equations for the variation of root-mean-square values of both the streamwise and spanwise component of the bed-shear stress are evaluated and compared against previously published data. The proposed models can be used in practical coastal applications, to include the effect of bed-shear stress extreme values on sediment entrainment.