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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