Minimizing the Loss Produced by a Turbulent Separation Using Vortex Generator Jets

Abstract

Vortex generator jets have been applied to control a separating turbulent boundary layer on the suction surface of compressor blades in a linear cascade at high incidence. With the jets operating in a steady blowing mode, loss measurements were taken over a range of jet velocities. An increase in the jet blowing ratio yielded a reduction in the loss coefficient up to a blowing ratio of 70%. At this condition, a loss reduction of 61% was measured relative to the case with no control. Higher jet velocities yielded a slight increase in the loss coefficient. In order to explore the behavior of the boundary layer over this range of blowing ratios, four sets of experiments were performed: static pressure measurements, wall shear stress measurements, stereoscopic Particle Image Velocimetry (PIV) measurements and smoke flow visualization. The static pressure measurements showed that the point of separation moves downstream from 60% surface length with no control, to approximately 92% with a blowing ratio of 70%. At higher blowing ratios, the boundary layer remains attached up to the trailing edge. Shear stress measurements were taken on the suction surface using a streamwise array of novel, dual element MEMS hot-film sensors. The mean quasi-wall shear stress measured with the sensors indicated attached flow in the region of the jet holes and separated flow downstream at all blowing ratios including and below 50%. At a blowing ratio of 75%, the quasi-wall shear stress measurements suggest that the flow is attached, but close to separation. These results suggest that minimum loss is obtained at the blowing ratio required to just keep the boundary layer attached. This conclusion is approximate because the flow from the discrete jets is inherently three-dimensional, and so the separation location will vary across the span. The three-dimensionality of the flow produced by the jets was evident in the quasi-wall shear stress and PIV measurements, as well as the smoke flow visualization. A vortex skewed relative to the streamwise direction was identified in the PIV measurements. By correlating the location of this vortex with the shear stress measurements, this vortex was identified with a region of elevated shear stress. A second region of elevated shear stress was, however, identified between the vortices. Turbulent kinetic energy extracted from the PIV measurements allowed the identification of this region with secondary flow between the co-rotating vortices produced by adjacent jets.