AIAA 97- 2325 15th AIAA Applied Aerodynamics Conference

WHOLE-FIELD VELOCITY MEASUREMENT IN UNSTEADY PERIODIC FLOWS

Reddy, U.C., Matos, C.A., Mahalingam, R., Komerath, N.M.

ABSTRACT Measurements of the instantaneous and phase-resolved, ensemble-averaged velocity field in unsteady periodic rotorcraft flows are reported. A method is described for extracting the third component of velocity from 2-component planar velocity data in periodic incompressible flows, and is validated using numerical simulations. The differential mass conservation equation is used to solve for the third component using parallel slices of 2-D vector fields. A 2nd-order differencing procedure is shown to work even in the presence of 25% random noise added to simulate experimental data. The flowfield of a 2-bladed teetering rotor above a full-span 21% thick wing in low-speed forward flight in a wind tunnel is explored, with and without trailing edge flap deflection. A pulsed copper vapor laser sheet is used to illuminate the seeded flow. Flow image pairs from several chordwise planes are analyzed using Spatial Correlation Velocimetry (SCV) and resolved as a function of rotor azimuth. Ensemble-averaged velocity fields for discrete rotor azimuths are presented. Ensemble-averaged traces at selected points show the 1-per-rev variation caused by vortex interaction. The pressure data on the wing show the expected decrease in the time averaged pressure coefficient with flap deflection. Flap deflection is seen to cause substantial lateral deflection of the rotor wake, attributed to changes in the spanwise flow downstream of the unsteady 3D separation line. The efficient capture of the multidimensional velocity field is demonstrated in a large wind tunnel on a complex large-volume unsteady flow problem.
 

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