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

• Time-resolved PIV measurements in the wake of an ovulate pine cone (P. halepensis).
• Pine cone can be well-represented by prolate spheroid with aspect ratio of ∼2.
• Vortex shedding was observed at all investigated Reynolds numbers, 246 to 955
• Global vortex shedding topology visualized by streamlines is different from sphere.
• Interaction of shed vortices and separated shear layer is tracked in detail.

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 ReD=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≈2L/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 ReD. At all ReD, vortex shedding occurred, but while shedding was highly organized at ReD=246, it became less so at higher ReDD. Strouhal numbers increased from 0.13 (ReD=246) to ∼0.185 at ReD=955. At the highest ReD, 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 ReD. Furthermore, at ReD≥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.