Mean and Turbulent Flow Characteristics in Supercritical Narrow Open Channel Flows: Effect of Froude Number

Sedimentation threatens the sustainability of many reservoirs worldwide. The need for active sediment handling such as sediment routing using Sediment Bypass Tunnels (SBT) or Sediment Flushing Channels (SFC) will consequently increase considerably in the future to maintain reservoir storage capacities as well as to restore sediment connectivity. However, highly sediment-laden high-speed flows can cause severe hydro-abrasion at such hydraulic structures. To shed more light on the hydro-abrasion phenomenon in high-speed flows, a systematical experimental investigation on the physical processes of (I) turbulent flow characteristics, (II) bed load particle motion, (III) hydro-abrasive wear, and (IV) their interrelations in supercritical narrow open channel flows is conducted in the framework of a PhD thesis at VAW of ETH Zurich. The present study deals with the former (I). Streamwise and vertical flow velocities were measured using 2D Laser Doppler Anemometry (LDA) at a large number of verticals across the flume width over a fixed-concrete bed in a laboratory flume representing a straight SBT section. Approach flow Froude numbers were F_o = 2 and 4. The results show that the log-law applies in the wall region reaching up to z/h = 0.3, and the mean velocity patterns undulate across the flume, indicating the presence of counter-rotating secondary currents occurring at low aspect ratios below the critical value of 4 - 5. These currents cause re-distribution of turbulence intensity and Reynolds shear stress across the flume. The present findings give an insight on where sediment transport, and hence hydro-abrasion, are concentrated and finally contribute to a better understanding of the abrasion mechanics and to the development of a mechanistic abrasion model for prediction and design purposes.

Keywords: Narrow open channel flow, Laser Doppler Anemometry (LDA), supercritical flow, mean and turbulent flow characteristics, sediment bypass tunnels.

Topic: Hydraulic structures

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