Time Resolved Flow Measurements of Unsteady Vortex Generator Jets in a Separating Boundary Layer

Abstract

Vortex generator jets (VGJs) have been found to be an effective method of active separation control on the suction side of a low pressure turbine (LPT) blade at low Reynolds numbers. The flow mechanisms responsible for this control were studied and documented in order to provide a basis for future improvements in LPT design. Data was collected using a stereo PIV system that enabled all three components of velocity to be measured. First, steady VGJs were injected into a laminar boundary layer on a flat plate (non-separating boundary layer) in order to more fully understand the characteristics and behavior of the produced vortices. Normally injected jets created vortices of lesser strength that migrated out farther from the boundary layer. The vortices produced by angled jets (injected at 30˚ pitch and 90˚ skew angles to the freestream) remained closer to the wall and maintained their structure for a longer distance. The angled jets also produce vortices that were more effective at sweeping low momentum fluid up from the boundary layer while transporting high momentum freestream fluid down towards the wall. Second, pulsed VGJs were injected on a flat plate with an applied adverse pressure gradient equivalent to that experienced by an LPT blade. This configuration was used to study the effectiveness of the flow control exhibited by both jet configurations on a separating boundary layer. Time averaged results showed similar boundary layer separation reduction for both normal and angled jets, however individual characteristics of the control differed. Normal jets created vortices that traveled downstream more quickly, while the vortices from the angled jets reduced the boundary layer separation to a greater extent. NOMENCLATURE B jet blowing ratio (Uj/Ue) Cx axial chord (m) F +