Particle image velocimetry (PIV) measurements were made on a highly loaded low-pressure turbine blade in a linear cascade. The Pack B blade has a design Zweifel coefficient of 1.15 and a peak Cp at 63% axial chord on the suction surface. Data were taken at Rec = 20K with 3% inlet freestream turbulence and a wake passing flow coefficient of 0.8. Without unsteady wakes, a non-reattaching separation bubble exists on the suction surface of the blade beginning at 68% axial chord. The time averaged separation zone is reduced in size by approximately 35% in the presence of unsteady wakes. Phase-locked hot-wire and PIV measurements were used to document the dynamics of this separation zone when subjected to synchronized, unsteady forcing from a spanwise row of vortex generator jets (VGJs) in addition to the unsteady wakes. The phase difference between VGJ actuation and the wake passing was optimized. Both steady state Cp and phase-locked velocity measurements confirm that the optimal combination of wakes and jets yields the smallest separation.

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Particle image velocimetry (PIV) measurements were made on a highly loaded low-pressure turbine blade in a linear cascade. The Pack B blade has a design Zweifel coefficient of 1.15 and a peak Cp at 63% axial chord on the suction surface. Data were taken at Rec=20K with 3% inlet freestream turbulence and a wake-passing flow coefficient of 0.8. Without unsteady wakes, a nonreattaching separation bubble exists on the suction surface of the blade beginning at 68% axial chord. The time-averaged separation zone is reduced in size by approximately 35% in the presence of unsteady wakes. Phase-locked hot-wire and PIV measurements were used to document the dynamics of this separation zone when subjected to synchronized, unsteady forcing from a spanwise row of vortex generator jets (VGJs) in addition to the unsteady wakes. The phase difference between VGJ actuation and the wake passing was optimized. Both steady state Cp and phase-locked velocity measurements confirm that the optimal combination of wakes and jets yields the smallest separation.

Flow measurements were made on two highly loaded low-pressure turbine blade configurations in a linear cascade facility with and without the application of flow control. The L1M blade has a design Zweifel coefficient of 1.34 with a peak c p near 47% c x (midloaded) and the Pack B blade has a design Zweifel coefficient of 1.15 with a peak c p at 63% c x (aft-loaded). Flow and surface pressure data were taken for Re c 20; 000 with 3% inlet freestream turbulence. For these operating conditions, a large separation bubble forms on the blade suction surface, beginning at 59% c x and reattaching at 86% c x on the L1M blade, with a nonreattaching bubble beginning at 68% c x on the Pack B blade. Data were taken using a single-element hot-film anemometer. Higher-order turbulence statistics were used to identify transition and separation zones. Similar measurements were also made in the presence of unsteady forcing using pulsed vortex generator jets located 9% c x upstream of the separation location. For the uncontrolled case, it was found that the separated laminar shear layer on the L1M blade started turbulent transition earlier than the Pack B but took longer to fully transition. This earlier transition appears to be a significant contributor to boundary-layer reattachment for the L1M blade. With the application of pulsed vortex-generating jets, the separation bubble was convected entirely off the blade for both blade configurations, but the Pack B bubble responded more slowly to the jet pulse due to a lower convection speed for the jet disturbance. Once the bubble was swept off of the L1M blade, a new bubble began to grow immediately. However, on the Pack B blade, there was a significant phase lag before bubble regrowth occurred. Nomenclature B = vortex-generating-jet blowing ratio, U jet =U local c p = pressure coefficient, P Tin P local =P Tin P Sin c x = blade axial chord Re c = Reynolds number based on cascade inlet conditions, c x U in = T = forcing period, 200 ms t = time U = velocity u = instantaneous streamwise-velocity component u = ensemble-averaged streamwise velocitỹ u max = maximum ensemble-averaged velocity in the measurement domain u mean = mean streamwise-velocity component u rms = root mean square streamwise-velocity component x = axial coordinate from the cascade inlet face y = local surface-normal coordinate z = spanwise coordinate = flow angle relative to the cascade axial direction = intermittency distribution = intermittency = kinematic viscosity Subscripts ex = cascade exit in = cascade inlet jet = vortex-generating jet local = local freestream conditions max = maximum value in the full cycle S = static T = total