A numerical investigation of acceleration-skewed oscillatory flows

Numerical simulations of wall-bounded acceleration-skewed oscillatory flows are here presented. The relevance of this type of boundary layer arises in connection with coastal hydrodynamics and sediment transport, as it is generated at the bottom of sea waves in shallow water. Because of the acceleration skewness, the bed shear stressduring the onshore half-cycle is larger than in the offshore half-cycle. The asymmetry in the bed shear stress increases with increasing acceleration skewness, while anincrease of the Reynolds number from the laminar regime causes the asymmetry first to decrease and then increase. Low- and high-speed streaks of fluid elongated inthe streamwise direction emerge near the wall, shortly after the beginning of eachhalf-cycle, at a phase that depends on the flow parameters. Such flow structures strengthen during the first part of the accelerating phase, without causing a significantdeviation of the streamwise wall shear stress from the laminar values. Before theoccurrence of the peak of the free stream velocity, the low-speed streaks break downinto small turbulent structures causing a large increase in wall shear stress. The ratio of the root-mean-square (r.m.s.) of the fluctuations to the mean value (relative intensity) of the wall shear stress is approximately 0.4 throughout a relatively wide interval of the flow cycle that begins when breaking down of the streaks has occurred in the entire fluid domain. The acceleration skewness and the Reynolds number determine the phase at which this time interval begins. Both the skewness and the flatness coefficients of the streamwise wall shear stress are large when elongatedstreaks are present, while values of approximately 1.1 and 5.4 respectively occurjust after breaking has occurred. The trend of both the relative intensity and theflatness of the spanwise wall shear stress are qualitatively similar to those of thewall shear in the streamwise direction. As a result of the acceleration skewness, theperiod-averaged Reynolds stress does not vanish. Consequently, an offshore directed steady streaming is generated which persists into the irrotational region.