Measurements of the flow in the near wake of a "rough", semi permeable prolate spheroid at intermediate Reynolds numbers

Time resolved, PIV measurements were performed to investigate the flow field dynamics around an ovulate pine cone (P. halepensis) mounted in a windtunnel at Reynolds numbers ranging from Re_D = 246 to 955, based on the equatorial diameter. Scale/bract complexes arranged in helices characterized the cone's surface morphology. The cone could be represented by a low aspect ratio, prolate spheroid (L/D ≈ 2) and time averaged flow patterns were similar as observed around other bluff bodies such as cylinders and spheres. However, the recirculation zone was almost twice as long than that of a sphere at similar Re_D. At all Re_D, vortex shedding occurred, but while shedding was highly organized at Re_D = 246, it became less so at higher Re_D. Strouhal numbers increased from 0.13 (Re_D = 246) to ∼0.185 at Re_D = 955. At the highest Re_D, power spectra of transverse velocity displayed two frequency peaks with corresponding Strouhal numbers up to 0.36. These double-branch frequencies are similar as those observed in the wake of a sphere but occur here at lower Re_D. Furthermore, at Re_D ≥ 526, small-scale vortices were created through break-up of the large-scale vortices that were generated in the high shear layer. Some were convected back upstream towards the cone where they interacted with the upstream vortices in the shear layer extending from the cone.