Drag reduction of bluff bodies by passive control of boundary layer transition and separation

Abstract

In many sporting activities the athletes and their associated equipment often operate in a range of Reynolds number, Re, that is close to the critical regime where the boundary layer flow undergoes transition from a laminar to a turbulent state. It is well known from numerous studies on circular cylinder and sphere flows that boundary layer transition can be forced to occur at lower Re, and hence drag reduced by delaying flow separation, through the application of, for example, roughness to the surface. This thesis is aimed at increasing understanding of how passive flow control methods might be employed to influence boundary layer flow in order to reduce the drag of bluff bodies. A wind tunnel based research programme was undertaken to study these aspects, including a review of a selection of commonly studied boundary layer tripping methods. The main body of the thesis is devoted to the investigation of two novel passive flow control concepts, developed for this research, which were found to significantly reduce the drag coefficient in the sub-critical Re flow regime of a plain cylinder. Of these two concepts, the main research focus was on identifying the drag reducing mechanisms of a system of passive, continuously blowing jets. It was found that the interaction of the jets and cross-flow induced a very high frequency instability which leads to the downstream formation of tornado-like vortices which are shed into, and identified in the near-wake. It is postulated that the introduction of stream-wise vorticity into the separating shear layer develops favourable drag reducing mechanisms. Discrete cylindrical surface protrusions were additionally found to develop similar effects on flow topology and drag reduction, and were more effective than the passive jets at low Reynolds numbers.