Velocity and Turbulence Distribution in Combined Oscillatory and Steady Flow Boundary Layer Over a Rippled Bed

Abstract

This paper describes an experimental study carried out in a laboratory water tunnel to examine the mean, periodic and turbulent components of the velocities in combined oscillatory and steady flow boundary layer over a fixed bed of vortex ripples. The fluid velocities, both the horizontal and the vertical components, were measured simultaneously with a laser Doppler anemometer. The measurements indicate that the addition of a steady current causes a significant effect on the near-bed ensemble velocity field of the oscillatory motion, owing primarily to the asymmetry of the lee-vortices that are formed during each half cycle of the oscillation. On the other hand, the effect of adding an oscillatory motion on the time-mean velocity field of a steady current is also significant in that the current experiences a considerably enhanced roughness due to the additional turbulence produced by the wave motion. The measurements also indicate that, depending on the spatial position over a ripple wave length, the turbulent intensities in combined flows are either increased or decreased with respect to the same in oscillatory flow alone. However, peak values of the spatially averaged turbulence quantities in the near-bed interaction layer of combined flows do not show a significant change with respect to the pure oscillatory flow components. The decay of turbulence as well as the near-bed momentum transfer in combined flows have also been examined.