Abstract
This paper focuses on the turbulence structure in a non-uniform, gradually varied, sub-critical open channel flow (OCF) on a rough bed. The flow field is analysed under accelerating, near-uniform and decelerating conditions. Information for the flow and turbulence parameters was obtained at multiple sections and planes using two different techniques: two-component laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Different outer region velocity scaling methods were explored for evaluation of the local friction velocity. Analysis of the mean velocity profiles showed that the overlap layer exists for all flow cases. The outer layer of the decelerated velocity profile was strongly affected by the pressure gradient, where a large wake was noted. Due to the prevailing nature of the experimental setup it was found that the time-averaged flow quantities do not attained equilibrium conditions and the flow is spatially heterogeneous. The roughness generally increases the friction velocity and its effect was stronger than the effect of the pressure gradient. It was found that for the decelerated flow section over a rough bed, the mean flow and turbulence intensities were affected throughout the flow depth. The flow features presented in this study can be used to develop a model for simulating flow over a block ramp. The effect of the non-uniformity and roughness on turbulence intensities and Reynolds shear stresses was further investigated.
Highlights
The turbulence structure of a steady uniform open channel flow has been a subject of numerous experimental and numerical studies since the 1950s [1]
Due to change in the channel shape, size and slope, uniform flow conditions rarely occur in rivers and man-made channels
The changes of the bed topology coupled with the heterogeneous roughness create a complex non-uniform flow conditions which affect the flow resistance, sediment and pollution transport
Summary
The turbulence structure of a steady uniform open channel flow has been a subject of numerous experimental and numerical studies since the 1950s [1]. Due to change in the channel shape, size and slope, uniform flow conditions rarely occur in rivers and man-made channels. The changes of the bed topology coupled with the heterogeneous roughness create a complex non-uniform flow conditions which affect the flow resistance, sediment and pollution transport. Even in the simplest case of gradually varying open channel flow, the turbulence structure is significantly different from that in a uniform open channel flow [2]. The effect of non-uniformity in gradually varied flow is associated with the difference between the free surface and bed slope, S. For S > 0 the flow is accelerating, while for S < 0, the flow is decelerating and at S = 0, the flow is uniform.
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