"Rotor Wake Interaction with Separated Flow".
Komerath, N.M., Kim, J.M., Liou, S.G..
Invited Paper, Mathematical and Computer Modelling, Special Issue on Rotorcraft,
Vol. 18, No. 3/4, pp. 73-87, Pergamon Press, October 1993.
Abstract: Aerodynamic interactions represent one of the toughest challenges
to computational methods for the prediction of rotorcraft performance, loads,
and noise. Efforts over the past ten years using a simple hemisphere/cylinder
airframe under a teetering two-bladed rotor have shown that most of the
dominant effects can be predicted using potential-flow concepts, in the
absence of massive flow separation and strong vortex-surface collision.
Here the prospects for modeling the separated flow interaction are studied.
The rotor wake interacts with the separated flow over a pressure-instrumented
boom downstream of a backstep cut into the original cylinder. The undisturbed
backstep flow is characterized using spectral analysis of the velocity and
surface pressure fields. The interaction between the tip vortex and the
backstep free shear layer is visualized using laser sheet videography, and
measured in detail using laser velocimetry and pressure sensing. The tip
vortex dominates the interaction, causing periodic destruction and reconstruction
of the shear layer, and large-amplitude longitudinal motion of the reattachment
zone. However characteristics of the undisturbed shear layer are still detected
in the surface pressure spectra. The tip vortex and its collision with the
boom surface appear to be unaffected by shear layer interaction. It is argued
that this experiment is a conservative representation of the interaction,
so that occurrences on real rotorcraft should be less complex. Thus, while
the separated flow interaction appears to be extremely complex at first
sight, the dominant effects are surprisingly simple, and may be easily modeled.