Low-frequency wake instability of an axisymmetric bluff body in pitch

Abstract

The backflow instability in the wake past a cylindrical blunt-based body in pitch is investigated at a Reynolds number ReD = 6.7 · 104 based on the cylinder diameter. Time-resolved stereoscopic particle image velocimetry measurements have been performed in a cross-flow plane located 0.3 D downstream of the model base. An increasing displacement of the backflow region from the body centerline with increasing pitch angles is observed in the long-time average of the velocity field, with the emergence of a preferred orientation of the wake. The time history of the backflow centroid position shows a progressive reduction of both amplitude and time scales of the fluctuations, reflecting the transition from large-scale azimuthal meandering to a more stable confinement at an off-center position. Proper Orthogonal Decomposition of the velocity fluctuations reveals a reduction by approximately 80% in the contribution of the first two modes for angles increasing up to 1°, accompanied by a distortion of the dipolar distribution typically associated with backflow meandering, for misalignments from 0.3° and higher. The frequency spectra of the POD time-coefficients display a very-lowfrequency peak near StD ~ 10-3 only within 0.1° deviations from axisymmetric inflow conditions, thus endorsing the hypothesis that the long-term backflow instability only survives within small deviations from axisymmetric inflow conditions.