Experimental analysis of cross-sectional flow motion in a large amplitude meandering bend

Flow in meandering bends is characterized by the formation of a large cross-sectional central-region circulation cell. The width-to-depth ratio is one of the most important parameters affecting the entity of the cross-circulation motion. In steep outside bends, beside the central-region cell, a counter-rotating circulation cell often forms in the upper part of the outer-bank. In spite of its practical importance, the evolving mechanisms of both the circulation cells and their role on boundary shear stress distribution in bends are not yet fully understood. The aim of the present paper is to gain some insight into how cross-sectional flow motion evolves along meandering bends. Experiments have been carried out in a laboratory meandering channel of large amplitude, over a deformed-rigid bed, for two values of the width-to-depth ratio. The three-dimensional flow velocity field has been measured in detail at five cross-sections, almost equally spaced along the channel reach between two consecutive apex sections. The measurements have been carried out on a fine grid by an acoustic Doppler velocity profiler. The distributions of the cross-sectional flow (e.g. cross-sectional flow velocity, net transversal flux) and turbulent kinetic energy are analyzed in each investigated section. Measurements show that the counter-rotating circulation cell is evident only in the case of 'small' width-to-depth ratio. Such circulation cell begins at the bend entrance and it is fully developed at the bend apex; then it decays. At the bend apex, the core of maximum velocity is found near the bed at about the separation between the central and the outer-bank circulation cells. Moreover, the presence of the counter-rotating circulation cell allows the bank shear stress to maintain low values in the outer-side of the bend. Copyright (C) 2010 John Wiley & Sons, Ltd.