Abstract
The oscillatory boundary layer over a sand ripple model was measured in detail by 2-D laser doppler velocimeter. The measurement was performed in an oscillatory flow tunnel for three cases with different period and amplitude. It is found that the boundary layer is governed by large organized vortices generated alternately from the crests of the ripples. The generation of the Reynolds stress is limited within the vortices, and the associated eddy viscosity shows sinusoidal variation according to the growth and decay of the vortices. Two stationary cells were observed between the crests, the velocity of which takes about 60% of the velocity amplitude of the undisturbed oscillatory flow near the crests. An inner boundary layer was observed, the thickness of which is found to be about √νT
