Abstract
Fully developed three-dimensional turbulent flow over sand ribbons in an open-channel is computed to investigate the coherent structure of streamwise vortices by direct numerical simulation (DNS) using a regular grid under a generalized curvilinear coordinate system. Coherent structure are educed from the simulated instantaneous flow field using a conditional sampling technique developed by Hunt et al, Jeong & Hussain which extracts the entire extent of dominant vortical structures. The results showed that stable secondary currents and turbulent characteristics were reasonably reproduced, highly elongated quasi-streamwise vortices were identified and the streamwise vortices of alternating sign were recognized to stagger over the sand ridge.
