Separation control on high lift low-pressure turbine airfoils using pulsed vortex generator jets

Abstract

Boundary layer separation control has been studied using vortex generator jets (VGJs) on a very high lift, low-pressure turbine airfoil. Experiments were done under low (0.6%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Instantaneous velocity profile measurements were acquired in the suction surface boundary layer. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25000 and 50000. Jet pulsing frequency and duty cycle were varied. In cases without flow control, the boundary layer separated and did not reattach. With the VGJs, separation control was achieved. At sufficiently high pulsing frequencies, separation control was possible with low jet velocities and 10% duty cycle. At lower frequencies, a 50% duty cycle helped by separating the disturbances associated with the jets turning on and turning off, thereby doubling the frequency of separation control events above the pulsing frequency. Phase averaged velocity profiles and wavelet spectra of the velocity show the VGJ disturbance causes the boundary layer to reattach, but that it can re-separate between disturbances. When the disturbances occur at high enough frequency, the time available for separation is reduced, and the separation bubble remains closed at all times.