Interaction of a dynamic vortex generator with a laminar boundary layer

Abstract

An experimental investigation was performed to study the fundamental interaction between a static and dynamic vortex generator with a laminar boundary layer. The effectiveness of static vortex generators (VGs) on delaying boundary layer separation is well established. However, as a passive flow control device, static VGs are associated with a drag penalty since they are always present in the flow. In the current study a piezoelectric-based dynamic vortex generator (DVG) was developed with the goal of mitigating the drag experienced when using a VG as a flow control device and exploring whether or not a DVG was more effective in flow mixing within the boundary layer. Experiments were conducted in a small wind tunnel, where the VG was flush mounted to the floor. The VG was rectangular in shape and erected into the flow with a mean height of the local boundary layer thickness, δ, or h_m = 3 mm. The skew angle of the VG was θ = 18° with respect to the incoming flow, oscillated at a driving frequency of f = 40 Hz with a peak to peak displacement (or amplitude) of 0.5∙δ, or h_a = 1.5 mm. During the experiments, the free stream velocity was held constant at U_∞ = 10 m/s. This corresponded to a Reynolds number of 〖Re〗_δ ≈ 2000, which was based on the local boundary layer thickness at the center of the VG.

Surface oil flow visualization experiments were performed to obtain qualitative information on the structures present in the flow, while Stereoscopic particle image velocimetry (SPIV) was used to provide quantitative measurements of the 3-D flow field at multiple spanwise planes downstream of the VG under both static and dynamic conditions. Several flow features were detected in the oil flow visualization experiments, including two vortical structures—the main vortex and primary horseshoe vortex—which were confirmed in the SPIV results. The time-averaged flow field showed similar results, though the strength of the vortices appeared less when the VG was actuated. However, phase-averaged data revealed the size, strength, and location of the vortices varied as a function of the actuation cycle, with peaks of vorticity magnitude being greater at certain phases as compared to the static case. The varying flow field associated with the dynamic motion of the DVG showed higher levels of turbulent kinetic energy, therefore confirming enhanced mixing in contrast to the static case.