Application of passive flow control techniques to attenuate the unsteady near wake of airborne turrets in subsonic flow

Abstract

External airborne installations of cylindrically-shaped objects, such as turrets, can lead to severe degradation of flight performance. This engineering challenge is mainly due to an unsteady and separated vortex flow field that evolves in the wake downstream of the turret. This study focuses on providing an engineering solution to attenuate the wake unsteadiness. Experiments were performed in the Israeli Air Force (IAF) low speed wind tunnel, where we investigated various passive flow control methods on a hemispherical dome attached to a cylindrical body turret with a low aspect ratio (AR) of 1.36 at a diameter-based Reynolds number of ReD=3.75×105. Three modifications of the baseline turret geometry were examined. The configurations include the addition of a grit roughness strip, a turbulator zigzag trip tape, and a straight edge protuberance (or a bump strip). The velocity fields in the near wake downstream of the various turret configurations were acquired using a particle image velocimetry (PIV) technique. Results show that tripping the flow using a bump strip is the most effective for the turret studied herein. The bump strip prompts symmetrical vortex shedding downstream, which can potentially eliminate the fluctuating side force on the turret, while also contributing the least to any drag increase (or penalty). Implementing such a passive flow technique on cylindrical installations may assist in resolving large fluctuating side forces and meeting operational requirements during flight.