EXPLORING PHYSICS OF ACOUSTIC FLOW CONTROL OVER AIRFOILS TOWARDS POTENTIAL APPLICATION TO HIGH WORK AND LIFT TURBINES

In this study, acoustic actuation was applied experimentally to massively separated flows on simplified hump geometries which mimic the pressure distribution over high-work-and-lift low Reynolds airfoils. The acoustic excitation demonstrated significant control over flow separation, resulting in higher relative lift enhancement than standard, localized actuation techniques with similar momentum coefficients. Full field velocity measurements were used to examine the transient behavior of the actuated flow in order to explain the physical mechanism of separation control. The velocity measurements revealed the presence of a viscous wall-mode that organized the vorticity upstream of the separation point. A spatio-temporal correlation analysis found that the generation of these wall modes in the attached flow was the dominant cause of the subsequent reorganization of the separating shear layer and the change in separation dynamics. The importance of wall-modes to acoustic flow control mechanism has important implications for the design of new acoustic control strategies for high-speed turbomachinery. Along these lines, the ramifications of this phenomena are explored over geometries which are designed to approximate flow fields in high-speed turbomachinery. At the conducive Strouhal number, which scale linearly with the square root of Reynolds numbers, up to 22% lift enhancement is observed for excitation amplitudes in the range of ~128dB, typical to the engine environment. Of the many diverse flow-control techniques, acoustics can be effectively employed in low Reynolds turbine blades, which are prone to flow separation in the off-design conditions with the ever-increasing demand for higher flow turning.