Detached Eddy Simulation (DES) of Turbulent Flow in Isolated Pool-Riffle Bedforms

English Abstract:

One of the most common topographical elements of rivers is pools and riffles. The deeper parts of undulations in the bed are called pools, whereas the shallower parts are riffles. Pool-riffle sequences can be shaped naturally, but are also made artificially to control the stream energy, improve fish habitats, or control the sediment transport rate. Many researchers have focused on the hydraulic characteristics of pool-riffle sequences by numerical analyses, experimental investigations, and field work processes. However, in contrast with other types of investigations, a transient numerical analysis is particularly suited to illustrate the complex mechanisms of flow such as vortex creation and dissipation, turbulence structures, and instantaneous bed shear stress. In this research, detached eddy simulation (DES) of turbulent flow in isolated pool-riffle with different width is presented. The numerical results are verified with experimental data. The simulations show a series of vortex generation near the wall at the downstream end of riffles. Based on the width of the channel, it is illustrated that these vortices can attach to each other and form a tube of vortices or they can break into smaller scale vortices and be dissipated at the downstream by energy cascading. It is illustrated that the narrower channels generate stronger vortices; therefore, these vortices need larger pool length for dissipation. The wall and bed shear stress show more critical conditions for narrower channels. The results of this research can help designers to choose a proper geometry and location for pool-riffle sequences, and give them an insight about the mechanism of natural undulation shaping, and the sediment sorting by size of natural bedforms. In addition, a rational design of pool length can let the flow to be restored and decreases the unwanted erosion at the downstream.