Direct numerical simulation of turbulence structures in an open channel flow over sand ridges

Fully developed three-dimensional turbulent flow over sand ribbons in an open-channel is computed to clarify the generation mechanism of secondary currents of the second kind by direct numerical simulation (DNS) using a regular grid under a generalized curvilinear coordinate system. Stable secondary currents and turbulent characteristics are reasonably reproduced. Detailed evaluation of each term in a mean vorticity transport equation revealed that the secondary currents of the second land is generated principally by cross-planar turbulent shear stress (-(nu(+) w(+)) over bar) and that the anisotropy of the cross-planar turbulent normal stresses((nu(+2) - w(+2) ) over bar) restrains the generation of the secondary currents, where nu(+) and w(+) represent vertical and spanwise velocity fluctuations normalized by friction velocity u(tau), respectively and overbar denotes the time average.