Micro-PIV Measurements and Challenges Towards Characterizing Active Flow Control Actuators for Rotor Applications

The aerodynamic interaction between an unsteady, inclined synthetic jet and a boundary layer was studied as a precursor to the forthcoming experiment on dynamic stall control using active flow control (AFC) concepts at the US Army AFDD 7- by 10-ft wind tunnel. Because the flow field offered numerous challenges from a measurement perspective, several experiments were carried out using phase-locked, 2-D microscopic-PIV technique in a building block approach, by adding one complexity after another. The procedure began with boundary layer measurements made on a simple flat plate using the micro- PIV technique. Velocity measurements were made deep in the viscous sub-layer, as close as 20-μm from the surface. Following this, the synthetic jet actuator (SJA) that is used in the wind tunnel experiment was characterized while operating in quiescent air, as well as in cross flow. The results showed that the evolution of the synthetic jet in cross flow was substantially different from its evolution in quiescent air, suggesting that any flow physics or performance prediction (for e.g., the depth of penetration of the jet into the boundary layer) made based on the quiescent flow conditions may not be applicable in cross flow. All the momentum added to the boundary layer had its source from the SJA, and the penetration of the jet was limited to the viscous sub-layer and log-layer; the outer layer was unaffected, despite using a jet to freestream velocity ratio of four. Significant effort was also made to validate the micro-PIV technique and evaluate its capability to accurately resolve such a complex flow field. To this end, micro-PIV measurements were compared to hot-wire measurements made on a simple steady jet, as well as an unsteady, periodic synthetic jet. Excellent correlation was found between the two techniques, validating micro-PIV measurements.