Three-Dimensional Flow Interactions in Low Aspect Ratio Finite Span Cylinders

This paper discusses the interaction of a single synthetic jet with the flow over a finite span low aspect ratio cylinder. The synthetic jet was located at the mid-span of the cylinder, and was oriented such that the length of the jet orifice was parallel to the freestream direction. The investigation incorporated experimental techniques such as stereoscopic particle image velocimetry (SPIV) and hot-wire anemometry. The near wake of the cylinder was dominated by a non-negligible spanwise velocity, or downwash, from the cylinder's free-end that interacted with the near wake resulting in a highly three-dimensional flow field with a strong spanwise dependence. SPIV measurements within the near wake showed that, due to the actuation of the synthetic jet, the wake was vectored and narrowed, resulting in a redirection of the downwash behind the cylinder. Furthermore, the synthetic jet actuation resulted in a localized region of inward-directed flow at a spanwise location outboard of the jet orifice leading to a breakdown of the streamwise vorticity field within the near wake. Measurements obtained near the surface of the cylinder demonstrated that the synthetic jet created an elongated vortex ring that redirected the upwash flow away from the cylinder surface inboard of the jet orifice and entrained flow back towards the surface outboard of the orifice. The result was a three-dimensional separation line along the span of the cylinder that gave rise to the spanwise dependency seen in the near wake. Finally, it was shown, using a stability theory analysis that the near surface interaction of the synthetic jet with the separated mixing layer did not result in excitation of the mixing layer instability.