The Effect of VGJ Pulsing Frequency on Separation Bubble Dynamics

Abstract

Particle image velocimetry (PIV) measurements were made on a highly loaded low pressure turbine blade in a linear cascade. Dat a were taken at a Re c=20K with 3% inlet freestream turbulence. At these conditions, a sep aration bubble exists between 59% to 86% axial chord on the aft portion of the s uction surface of the turbine blade. The dynamics of this separation zone were studied when subjected to unsteady forcing from a spanwise row of vortex generator j ets (VGJs) located upstream of the separation zone at 50% axial chord. Two different VGJ pulsing signals were selected by varying the frequency of the jets and t he duty cycle. The signals were both approximately step functions with a max imum blowing ratio (Bmax =ujet /u local ) of 2. The first signal consisted of a 5Hz frequency and a duty cycle of 25%. The second signal consisted of a 3Hz frequency an d a duty cycle of approximately 5%. Phase-locked PIV measurements were made at various times (phases) along the pulse history so that comparisons of the separation zone’s responses to the two VGJ pulses could be made. Measurements provide a global depiction of the response of the separation zone to the uns teady VGJ forcing. Results suggest that transition plays a more extensive rol e in the separation bubble reattachment than vortex development and that jet duration influences the minimum residual separation bubble size.