Spanwise Wake and Discrete Jet Disturbances on a Separating Turbine Blade

Abstract

Flow velocity and surface pressure measurements were made in a Pack-B low-pressure turbine linear cascade at Re c = 20, 000 and 3% inlet freestream turbulence. Under these conditions, the aft-loaded Pack-B (with a peak c P at 63% c x ) experiences laminar boundary-layer separation on the suction surface near 68% c x . A wake generator was used to simulate the periodic passing of upstream wakes through the blade passage. The wake passing frequency of 4.5 Hz was set to match a typical engine flow coefficient for a low-pressure turbine (Φ = 0.85). A spanwise row of discrete vortex-generating jets located at 59% c x were also used as a separation control device and were pulsed at a frequency of 5 Hz with a duty cycle of 25%. Data were taken using particle image velocimetry and a hot-film anemometer mounted on a blade-following device. Velocity, turbulence, and intermittency measurements were made along the suction surface of the blade to characterize the bubble dynamics and transitional behaviors for both the presence of unsteady wakes and pulsing vortex generator jets. The wakes caused early breakdown of the separated free-shear layer resulting in a thinning of the separation region. The vortex generator jets caused a near-wall disturbance that was convected downstream, temporarily pushing off the separation bubble. Overall, both wakes and vortex generator jets suppress the size of the steady-state separation bubble, though through different mechanisms. Three-dimensional aspects of the jet disturbance are also discussed.