Gokhan Kirkil

Detached Eddy Simulation Investigation of Turbulence at a Circular Pier with Scour Hole

This paper uses results from detached eddy simulation to reveal the dynamics of large-scale coherent eddies in the flow around a circular pier with an equilibrium scour hole. This is important for the sediment transport because the local scour process is controlled to a large extent by the large-scale coherent structures present in the near-bed region. The present paper investigates the dynamics of these coherent structures, their interactions and their role in entraining sediment in the later stages of the scour process when the horseshoe vortex system is stabilized by the presence of a large scour hole. The pier Reynolds number was 2.06× 105 , outside the range of well-resolved large-eddy simulation (LES). Additionally, scale effects are investigated based on comparison with LES results obtained at a much lower Reynolds number of 16,000 in a previous investigation. The paper provides a detailed study of the dynamics of the main necklace vortices of the horseshoe vortex system, including an investigation...

Effects of cylinder Reynolds number on the turbulent horseshoe vortex system and near wake of a surface-mounted circular cylinder

The turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel are investigated based on the results of eddy-resolving simulations and supporting flow visualizations. Of particular interest are the changes in the mean flow and turbulence statistics within the HV region as the necklace vortices wrap around the cylinder’s base and the variation of the mean flow and turbulence statistics in the near wake, in between the channel bed and the free surface. While it is well known that the drag crisis induces important changes in the flow past infinitely long circular cylinders, the changes are less understood and more complex for the case of flow past a surface-mounted cylinder. This is because even at very high cylinder Reynolds numbers, ReD, the flow regime remains subcritical in the vicinity of the bed surface due to the reduction of the incoming flow velocity within the bottom boundary layer. The paper provides a detailed discussion of the ...

The Horseshoe Vortex System Around a Circular Bridge Pier on Equilibrium Scoured Bed

Large eddy simulation (LES) is used to investigate the horseshoe vortex (HV) system around a circular bridge pier located on scoured bed corresponding to equilibrium conditions. The deformed bed bathymetry is obtained from experiments. Simulation is performed for two different inflow boundary conditions, one in which the upstream flow is fully turbulent including the turbulent fluctuations obtained from a precalculated LES simulation in a periodic channel (Re=18,000) and one in which the mean streamwise velocity profile corresponding to the first simulation is used (Re=5000, no turbulent fluctuations). The former case corresponds exactly to the experimental conditions, the latter serves to highlight the effect of incoming turbulence on the HV system. The dynamics of the instantaneous coherent structures associated with the HV system around the cylinder and the spectral content of the flow in this region are analyzed. Special attention is given to the interaction between the legs of the horseshoe vortices and the flow behind the cylinder and to the distribution of the bed shear stress. The statistics of the flow are calculated. It is found that the bed shear stress fluctuations around the local mean values can be very high, especially in the scour and near wake regions. The mean flow fields show that for the given scour bathymetry and flow conditions, the mean HV system contains one primary eddy situated fairly close to the cylinder. Inside this eddy the mean pressure fluctuations and resolved kinetic energy levels are very high. Maximum mean bed shear stress levels are observed beneath this eddy as well as close to the small corner vortex at the base of the cylinder.