Bluff body boundary-layer modification and its effect on the near-wake topology

Abstract

Boundary-layer modification over a bullet-shaped body has been accomplished using wall suction and its effect on the near-wake topology is reported. Two-dimensional velocity fields in the Reynolds number range 6.7×103−5.5×104 are acquired by means of particle image velocimetry and the mean, periodic, and random velocity components are analyzed. Wall suction reduces the boundary-layer thickness and increases the wall-normal velocity gradient. When the boundary layer is laminar up to the point of separation, the modification of the boundary layer leads to a vortex formation region that is up to 19% shorter compared with the case without wall suction. The modification of the boundary layer is accompanied by an increase of the Reynolds normal and shear stresses, which corresponds to larger forces acting on a fictive control area enclosing the mean recirculation region. The increase in Reynolds normal and shear stresses is due to an increase of the random fluctuation component, while the contribution of the periodic fluctuation component remains constant. The altered topology of the vortex formation region is accompanied by a significant change of the vortex shedding frequency and the base pressure. Empirical relations between the wake inlet conditions and the wake characteristics are reported, which hold for both laminar and turbulent boundary layers. The curvature of the streamlines enclosing the rear part of the body was also quantified and found to be proportional to the base pressure.