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.