Combined oscillatory and steady flow over ripples

Laser-Doppler-anemometer measurements of the fluid velocities in combined oscillatory and steady flow over rippled beds are reported. The direction of the steady current was parallel to the ripple crests and at right angles to the oscillatory flow. Measurements with the steady flow alone showed that the velocity distribution was quite well represented by the traditional logarithmic formula if height y were replaced by a transformed coordinate [Eta]. It was not possible to detect a universal velocity distribution based directly on height above the local bed level. On the other hand, the combined flow measurements did not show good agreement with any existing formula in the immediate vicinity of the bed, although there was reasonable agreement with an existing formula further out. This formula also gave quite good prediction of the shear velocity in the combined flow. The importance of large-scale momentum exchanges produced by vortex formation and ejection was demonstrated with reference to the fluctuation in velocity in the steady-current direction.