Investigation of free-surface dynamics in an open-channel flow

Abstract

ABSTRACT Space–time variation of the free-surface roughness pattern, due to sub-surface turbulence, is one of the most important features of turbulent open-channel flows. Despite its importance, the free-surface dynamics is not fully understood, especially regarding the advection and propagation features of surface variations. In this study, a direct numerical simulation of a smooth-wall turbulent open-channel flow allowing free-surface deformation was performed to investigate the dependence of the free-surface features on Reynolds and Froude numbers. The results show that the key features of the free surface are due to surface waves and turbulence, and one or another may prevail depending on the Froude number. While the turbulence is dominant in subcritical flow at low Froude number, the surface waves propagating upstream become more dominant at high Froude number when flow becomes supercritical. Furthermore, it was found that the advection velocity of the roughness patterns becomes slower than the mean surface velocity in the flow where the surface waves become dominant.