Flow around impulsively started square prisms

The flow around square and diamond prisms and a circular cylinder impulsively set into motion was studied experimentally using the particle image velocimetry (PIV) technique. The experiments were conducted in water in an X–Y towing tank for Reynolds numbers from Re=200–1000. The temporal development of the near-wake recirculation zone, and its pair of primary eddies, was examined from the initial start until the wake became asymmetric, at a dimensionless elapsed time of t⁎=4 or 5. For both bodies, the length of the recirculation zone, the streamwise location of the primary eddies, and the strength of the primary eddies increased with time following the impulsive start, while the cross-stream spacing of the eddy centres remained nearly constant. The recirculation zones of the square and diamond prisms were longer than that of the impulsively started circular cylinder. For t⁎>2, the primary eddy strength, maximum vorticity, and cross-stream spacing of the primary eddies, were the same for both the square prism and circular cylinder. The diamond prism had the strongest primary eddies and highest maximum values of vorticity. A comparison of recirculation zone length data for impulsively started bluff bodies of six different cross-sections illustrated the effects of afterbody and forebody shape, with the normal flat plate (no afterbody and no forebody) having the longest recirculation zone and the circular cylinder (rounded afterbody and rounded forebody) having the shortest recirculation zone.