Effect of a Roughness Transition on Turbulent Structures in the Outer Layer

Abstract

The physical mechanisms that need to be involved in sediment transport modelling are indissolubly associated to the evolution of coherent structures in a rough boundary layer. In literature many attempt have been found to relate burst frequency and intensity with sediment entrainment or saltation phenomena. Several questions remained partially unanswered concerning the feed back effect of sediment particles on hydrodynamics, but even a deep knowledge of roughness effects on coherent structures has to be involved. The question that may still rise is related to the undisturbed spatial scales of the structures responsible for sediment entrainment. Those structures are the very same structures triggered by the vast regime of roughness elements (from sand grains to cobbles or even stones) lying in the bottom of a river. Turbulent flows in a smooth wall boundary layer have been studied extensively over the past decades. Since the early work of Kline (1967), experimental investigations on the detailed structure of the turbulent field in boundary layers have revealed the existence of distinctly organized motions. The present research and the recent investigation of Tomkins (2001) are focused mainly on the effect of a roughness transition over turbulent structures, with the purpose of shedding more light on the variation that may occur on bursting intensity, pattern of dominant structures and representative length scales. In literature, Grass (1993) outlined intense ejection event able to disrupt vortices organization with the consequent reduction of the streamwise integral length scale.