Author(s): Saiyu Yuan; Hongwu Tang; Xuehan Qiu
Keywords: Shear layer; Distortion; Turbulence; Spectrum analysis
Abstract: The distortion of the shear layer caused by the penetration of the tributary flow into the mainstream flow is a common phenomenon in city channel confluence with small width-to-depth ratios. The mean and turbulent flow structures at the confluent zone with the distorted shear layer in a dischargeadjustable circulating flume with two different discharge ratios are addressed. There are some important characteristics for the case with the distorted shear layer: (1) A much stronger helical cell develops and continues for a longer distance, which is helpful for sufficient mixing and exchange of matters between the two flows. (2) The turbulent kinetic energy, Reynolds shear stress and occurrence probabilities of ejection and sweep events mainly reach their maximum values within the middle zone of water depth, rather than near the water surface. (3) There is no obvious energy concentration observed by the spectrum analysis. The power law relations for individual velocity components all have exponents which are slightly larger than-5/3, and the spectral densities of lateral velocity have the larger values than the other two in the low frequency. (4) The distortion of the shear layer brings about an increase in the occurrence probabilities of ejection and sweep events within the shear layer, and a decrease outside the shear layer which are related to the turbulence presenting vortices induced by wall. All these results suggested that sediment transport, bed morphology and contaminant transport in the distorted shear layer at city channel confluences could significantly differ from that at natural river confluences.