Organization of an axisymmetric turbulent wake in presence of a central protrusion

Abstract

The wake past an ogive-cylinder was investigated using Stereoscopic Particle Image Velocimetry (PIV) at a Reynolds number ReD = 6.7 = 104, based on the object diameter. The intensity of the vertical fluctuations was found to reduce from 0.18 U∞, attained in absence of a rear-protrusion, down to 0.1 U∞ for a main-body to after-body diameter ratio d/D = 0.4. The trace of the backflow centroid motion over a time-interval of 2,000 D/U∞ reflected the occurrence of a random motion of the reverse flow region for d/D = 0.2, where no pattern associated with a consistent precession could be identified, whereas a preferred azimuthal direction could be detected for d/D = 0.4. Proper Orthogonal Decomposition of the velocity fluctuations indicated the occurrence of large-scale velocity fluctuations for all configurations examined, the energy contribution of modes k = 1 and k = 2 attaining 20 % and 11 % for d/D = 0.2 and d/D = 0.4 respectively. Both modes were found to feature a dipole organization for d/D = 0.2, whereby their linear combination could be linked to a displacement of the wake occurring in all azimuthal directions. For d/D = 0.4 such an arrangement appeared to be preserved only by the first mode, thus reflecting wake displacement along a preferred azimuthal direction.