Turbulence structure in sharp open-channel bends

In spite of its practical relevance, little is known about the turbulence characteristics in sharp open-channel bends, which may largely be attributed to a lack of accurate experimental data. This paper reports an experimental investigation of the turbulence structure in one cross-section of an open-channel bend. The flow pattern in this section is characterized by a bicellular pattern of cross-stream circulation (secondary flow) and, in the outer part, a strongly reduced turbulence activity as compared with straight uniform shear flow. The turbulence structure differs fundamentally from that in straight uniform shear flow. The velocity fluctuations are atypically coherent over the channel width, whence the measured signal is decomposed into slow width-coherent fluctuations and a fast background signal. The width-coherent fluctuations reflect a bulk spatio-temporal oscillation of the pattern of circulation cells whereas the background signal represents developed turbulence. A spectral analysis shows that the width-coherent fluctuations have the characteristics of a wavelike motion, i.e. they contribute significantly to the turbulent normal stresses but only weakly to the shear stress, whereas the background turbulence is characterized by efficient shear stress generation. The reduced turbulence activity and the tendency of the secondary-flow pattern to oscillate are both effects of the streamline curvature. Similar observations on reduced turbulence activity and the tendency to wavelike motion have been reported in literature for flows in curved wind tunnels and density-stratified flows. Our experimental results indicate that these phenomena are potentially important in curved open-channel flows, where they affect the mixing and transport capacity of the flow.