Abstract
The effect of aspect ratio on the higher order statistics of velocity fluctuations in a hydraulically rough turbulent open channel flow is investigated. In this regard, an experiment was conducted in a rough bed narrow open channel flow of aspect ratio equal to three and the instantaneous flow velocities were measured using a Nortek Vectrino+ Acoustic Doppler Velocimeter. To understand the effect of aspect ratio, the results obtained from the present study are compared with the literature data of approximately same Reynolds number and bed roughness in a wide open channel flow for turbulence intensities and higher order statistics of velocity fluctuations. Comparison of turbulence intensities between Narrow OCF and Wide OCF shows occurrence of higher streamwise and vertical turbulence intensities in the outer region of Narrow OCF. The results of third order moments of velocity fluctuations are sensitive to aspect ratio in the outer region.
Highlights
An open channel flow can be classified as either narrow or wide channel based on the aspect ratio of flow, which is defined as the ratio of the width of the channel to the depth of the flow
As the momentum is transported from the sidewalls to the channel center, a strong downward motion takes place at the channel center and the velocity retardation causes velocity dip in the narrow open channel flow
The higher order correlations of velocity fluctuations contain important information like coherent structures which can be described by diffusion and flux of turbulence generated Reynolds stresses [4]
Summary
An open channel flow can be classified as either narrow or wide channel based on the aspect ratio of flow, which is defined as the ratio of the width of the channel to the depth of the flow. In a narrow channel where the ratio of width (b) of the channel to the depth of the flow (h) is less than five, strong secondary circulations such as stronger free surface vortex and a comparably weaker bottom vortex were observed while in a wide channel(bbbb⁄b > 10) secondary currents were observed only near the sidewalls but its effect is totally absent in the central region of the channel. The secondary currents have important effects on mean velocity, bed shear stresses, shear velocity, and higher order statistics of turbulent components which are important parameters for the sediment transport. The higher order correlations of velocity fluctuations contain important information like coherent structures which can be described by diffusion and flux of turbulence generated Reynolds stresses [4]. The crossover from sweep events to eject events can be identified from the point of change of sign on the third order correlations profiles [5]
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