An investigation of the dynamics of coherent structures in a turbulent channel flow with a vertical sidewall obstruction

Abstract

The physics of the flow around a vertical-wall obstruction attached to one of the sidewalls of a straight channel is numerically investigated using detached eddy simulation (DES) at a high channel Reynolds number, Re=5×105 (case HR). In particular, the study investigates the role played by the large-scale coherent structures in the sediment entrainment processes at the bed for conditions close to the initiation of scour (flat bed) in a loose-bed channel. Scale effects are investigated by comparing the results of the present DES simulation with results from a large eddy simulation performed at a much lower Reynolds number, Re=18 000 (case LR). Similar to laboratory flume studies of flow and scour around in-stream obstructions, the incoming flow in the simulations was fully turbulent and contained unsteady velocity fluctuations. The main necklace vortex of the horseshoe vortex (HV) system forming near the upstream base of the flow obstruction was subject to bimodal large-scale oscillations. The intensity of the bimodal oscillations peaked at vertical sections cutting through the tip of the obstruction. Present results show the size of the region of high turbulence amplification within the HV system decreases with the increase in the Reynolds number. Patches of vorticity were observed to detach from the leg of the main necklace vortex and to be convected at a small distance from the bed. Before dissipating, these patches could induce relatively large values of the bed shear stress beneath them. In case HR, the formation of these patches was primarily determined by the interaction of the main necklace vortex with the leg of the secondary necklace vortex rather than the interaction of the main necklace vortex with the tip of the obstruction, as was the case in the LR simulation. The degree of deformation of the cores of the vortex tubes shed in the upstream part of the separated shear layer (SSL) originating at the tip of the obstruction was much larger in the HR simulation. This provided an additional mechanism for the amplification of the horizontal vorticity in the near-bed region. Large-scale hairpinlike structures formed in the downstream part of the SSL. The legs of these vortices were oriented parallel to the interface between the high-speed outer flow and the recirculating flow past the obstruction. The lower leg of some of these hairpin vortices was situated, at times, at a small distance from the bed and was able to strongly amplify the local bed shear stress values. Consequently, the bed shear stress distribution in the instantaneous flow fields displayed a streaky structure over part of the SSL region. The changes in the relative position and size of the regions of high turbulence amplification inside the HV system and the SSL induced noticeable scale effects on the distributions of the shear stress and pressure root-mean-square (rms) fluctuations at the bed which control the sediment entrainment processes. In particular, due to the richer eddy content of the SSL, the intensity of the nondimensional pressure rms fluctuations beneath the SSL was found to be about two times larger in the HR simulation.