Very-low frequency wake dynamics of an axisymmetric body

Abstract

The so-called very-low-frequency (VLF) azimuthal meandering of the reversed-flow region has been shown to contribute significantly to the unsteadiness of the turbulent wake flow past a bluff body of revolution (Rigas et al. 2014; Grandemange et al. 2014). Such an erratic behavior causes a continuous change in the wake topology and, particularly due to its slow nature, has been linked with the pronounced sensitivity that turbulent wake flows typically display with respect to the boundary conditions (Klei 2012; Wolf et al. 2013; Grandemange et al. 2012). Currently, the existence of such an instability is attributed to the persistence at high Reynolds numbers of the reflectional symmetry breaking mode (RSB) at laminar regime (Fabre et al. 2008; Bury and Jardin 2012). Despite the numerous investigations, some even attempting its theoretical modeling (Rigas et al. 2015), the relation of such an instability with the main vortex shedding process has not been characterized yet. Moreover, the backflow meandering reflects a condition of indifferent equilibrium in the azimuthalradial plane, which is ultimately dictated by the axial symmetry of the flow. Such a very-low frequency dynamics however, still needs to be examined under the influence of off-nominal (i.e. asymmetric) inflow conditions. Scope of the present work is to examine how the backflow unsteadiness evolves moving away from separation and additionally, to assess how it is affected by asymmetric inflow conditions. For this purpose time-resolved stereoscopic Particle Image Velocimetry (PIV) measurements are carried in the turbulent near-wake of an ogive-cylinder at different stations downstream of the base and for varying pitch angles, whereas the velocity fluctuations are examined using a snapshot POD approach.