Author(s): Gokhan Kirkil; George Constantinescu; Robert Ettema
Keywords: Coherent structures; Large eddy Simulation; Bridge pier; Horseshoe vortex
Abstract: Large eddy simulation (LES) is used to study the horseshoe vortex (HV) system around a cylindrical bridge pier mounted on a flat bed and to investigate the mechanism responsible for the scour initiation. The simulation is performed with fully turbulent inflow boundary conditions at a Reynolds number of ReD=18, 000. The spatial and time variations of the HV system structure are analyzed and its spectral content is investigated. It is observed that the HV system is not formed of structures that wrap more or less uniformly around the cylinder as is the case when the HV system is laminar. Rather the main eddies that are part of the HV system extend over very variable distances in the polar direction around the upstream part of the pier and are continuously and randomly created and destroyed. All this creates very large variations in the structure, position, size and overall intensity of the turbulent HV system in the case of a flat bed when the HV system is not stabilized by the presence of the scour hole. It is also found that the down flow on the upstream face of the cylinder can significantly affect the structure of the HV system. As expected, the regions characterized by large values of the bed shear stress are situated beneath the eddies associated with the HV system and the detached shear layers where very energetic vortex tubes structures are shed at a high frequency. The largest values are recorded for polar angles larger than 300 very close to the junction which explain why the scour is initiated on the sides of the cylinder in the case of a loose flat bed. The instantaneous bed shear stress in these regions is characterized by large variations around the mean. The simulation confirms that, in the mean, the HV system region is characterized by high pressure fluctuations and turbulent kinetic energy.