"Measurement of the Velocity Field of a Cylinder in the Wake of a Rotor
in Forward Flight"
Mavris, D.M., Liou, S-G., Komerath, N.M., and McMahon, H.M.
AIAA Paper 89-1844, 20th Fluid Dynamics, PlasmaDynamics, and Lasers Conference,
Buffalo, NY, June 1989.
Abstract: The problem of predicting the flowfield around a rotorcraft in
low-speed forward flight is studied using a potential-flow code, whose results
are compared with surface pressure measurements and flow velocity measurements.
The test case used is a 2-bladed teetering rotor above a hemisphere-cylinder
airframe in a wind tunnel. The dominant features of this problem are modeled
by a lifting line/lifting surface rotor model with a free wake distorting
in the presence of the airframe. The airframe flowfield is modeled using
a source/doublet panel method. The instantaneous flowfield is computed at
specified intervals of rotor azimuth, with the effects of blade motion added
to the formulation. Modeling the energy addition at the rotor using actuator
segments leads to successful prediction of the time-averaged pressure field.
The periodic velocity variations along the spine of the airframe are predicted
successfully. When a fully unsteady potential formulation is used, however,
large differences appear between measured and computed periodic velocity
at the sides of the airframe. These are attributed to the inadequate understanding
of interaction of the rotor tip vortices with the airframe surface, as well
as to inadequate modeling of the flowfield around the rotor hub.