Flow around a circular cylinder “impulsively” set into motion was investigated experimentally using the particle image velocimetry (PIV) technique. The experiments were conducted in water in an X-Y towing tank at a Reynolds number of Re = 500. Since a true impulsive start can only be approximated in physical experiments, the objective was to determine the experimental conditions for which the cylinder could be considered impulsively started. The dimensionless acceleration parameter was varied from a* = 0.5 to 10, and the temporal development of the near-wake recirculation zone was examined from the initial start until the wake became asymmetric. With increasing acceleration parameter, at a given elapsed time from the start of motion, there was an increase in the length of the recirculation zone, the maximum vorticity within the recirculation zone, the streamwise location of the primary attached eddies from the base of the cylinder, and the strength (circulation) of the primary eddies. The results showed that the spatial development of the recirculation zone was strongly dependent on the distance traveled by the cylinder following the start of motion, rather than the method of acceleration, and the flow around the circular cylinder could be considered impulsively started for a* ≥ 0.5.

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The flow around a square prism impulsively set into motion was studied experimentally using particle image velocimetry (PIV). The experiments were conducted in an X-Y towing tank for Reynolds numbers from Re = 200 to 1000 and dimensionless acceleration parameters from a* = 0.5 to 10. 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. When considering the time elapsed from the start of motion, the temporal development of the wake was sensitive the initial acceleration. “Impulsively started” conditions were effectively attained for a* ≥ 3. However, when considering the distance traveled from the start of motion, the wake parameters were sensibly independent of a* for a* ≥ 0.5. Concerning the temporal development of the recirculation zone, 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 zone of the square prism was longer than that of the impulsively started circular cylinder but shorter than an impulsively started flat plate. 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.

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