Flow and turbulence structure around a spur dike in a channel with a large scour hole
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The flow and turbulence structure around a vertical-wall spur dike that extends over the whole depth of a straight channel are investigated using detached eddy simulation (DES). The channel Reynolds number in DES and the corresponding experiment is Re(D) = 2.4 x 10(5), which is typical of applications in small rivers and outside the range where well-resolved large eddy simulation (LES) can be conducted. The bathymetry at equilibrium scour conditions is obtained from a loose bed experiment. This paper discusses the main mechanisms which drive the growth of the scour hole upstream and downstream from the spur dike during the later stages of the scour process, and how these mechanisms change between the initial (flat bed conditions) and later stages of the scour process. Scale effects are investigated by comparing simulation results at Re(D) = 2.4 x 10(5) with those from simulations performed at a much lower Reynolds number, Re(D) = 18,000. Results show that while the structure of the horseshoe vortex (HV) system changes with respect to the case in which the bed is flat, the main necklace vortex of the HV system is still subject to large-scale aperiodic oscillations, similar to the ones observed in flows past in-stream bluff-body obstacles mounted on a flat surface. Present results show that the amplification of the horizontal vorticity within the lower part of the separated shear layer (SSL) and the associated formation of streaks of high-bed shear stress below the region where the SSL eddies are convected in the near-bed region is a general feature of high-Reynolds-number flow past a vertical-wall spur dike placed in a loose-bed channel at all stages of the scour process.