Vortex Shedding in the Near Wake of Rigid and Flexible Bluff Bodies

The velocity profiles and shedding frequencies of three axisymmetric bluff bodies consisting of a disk, a cup, and a rigid canopy model were examined in the near wake, 0.25 ≤ z/D ≤ 9.0. The measurements were conducted at a Reynolds number of 1.93 × 10 5 using a single element hot-film anemometer. The data revealed that the mean velocity deficit profiles became self-similar beyond z/D ≥ 3.0, and are identical in self-similar coordinates. However, the cup and rigid canopy models recover more quickly than the disk. At least two shedding frequencies were observed in the near wake of these axisymmetric models; one corresponding to the dominant downstream mode at a Strouhal number ≈ 0.15 and the other at low Strouhal numbers which disappears after z/D ≥ 3.0. The dominant mode appears at z/D ≥ 1.6 and persists at all locations further downstream. This mode has typically been associated with a helical mode present for axisymmetric bluff bodies with a fixed separation point. A high Strouhal number mode (≈ 0.55) seen in flexible parachute canopies does not appear to be present in the near wake of these rigid models including the rigid canopy.