Flow around a tall finite cylinder explored by large eddy simulation

Artikel i vetenskaplig tidskrift, 2011

The flow around a finite circular cylinder with a height-to-diameter ratio of 6 was studied using large eddy simulation (LES). The cylinder was mounted on a ground plane where a thin boundary layer with a thickness of only 7% of the cylinder's diameter was formed. The flow resulting from the present LES was used to present a detailed picture of both the instantaneous and the time-averaged flow. Among the results of the present study is the explanation for the formation of two time-averaged tip vortices from two pairs of instantaneous vortices. The main recirculation region on the cylinder's top was found to be one vortex with both legs attached to the top surface, in agreement with some previous investigations. The mean flow in the near wake region was found to contain two horseshoe vortices, one large arch vortex stretching from the cylinder's top and another vortex resulting from the flow moving down below the rear stagnation point on the cylinder. The instantaneous horseshoe vortex near the ground was found to be unsteady and changed shape over time. The time-averaged flow showed three complete horseshoe vortices, in agreement with previous knowledge. The downwash process above the free end into the near wake occurs at a rather constant angle with respect to the plane normal to the streamwise direction. The same angle was preserved in the inclination of the arch vortex in the time-averaged flow.