Abstract
An experiment was conducted to identify and measure the sources of uncertainty that are associated with the application of particle image velocimtery (PIV) to the measurement of the vortical wakes trailing from helicopter rotor blades. Phase-resolved 3-D PIV measurements were performed in the wake of a small-scale rotor operating in hover, and were compared with high-resolution 3-D laser Doppler velocimetry (LDV) measurements obtained with the same rotor under identical operating conditions. This helped formulate the essential experimental conditions that need to be satisfied for PIV to accurately resolve the high velocity gradient, streamline curvature flows that are present inside the rotor wake and the blade tip vortices. Uncertainties associated with the calibration, acquisition, and processing PIV images were analyzed in detail. It was shown that the optimization of laser pulse separation time needed to reduce the errors associated with the acceleration and curvature eects, as well as the interrogation window size for velocity gradient measurements, are critical in accurately mapping out the wake flow using PIV. The correlation between LDV and PIV measurements of the tip vortex characteristics, such as core radius and peak swirl velocity, were found to be excellent.
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