"Measurement Of The Interaction Between A Rotor Tip Vortex And A Cylinder".
S.G. Liou , N.M. Komerath, and H.M. McMahon.
AIAA Journal, Vol, 28, No. 6, June 1990, p. 975-981.
Abstract: The transient interaction between a cylinder and the trailing
vortex from a rotor in forward flight is studied. Phase-averaged laser velocimetry
and surface pressure measurements made with flush-mounted microphones are
used to study the velocity and pressure variations during such an interaction.
Vorticity contours constructed from the velocity measurements exhibit the
presence of a secondary structure with vorticity opposite in sense to that
of the primary tip vortex. This structure moves rapidly around the tip vortex
from upstream to downstream. The pressure variations caused by the tip vortex
on the surface of the cylinder are smooth as the vortex core passes by,
and no evidence is found of fine structure inside the vortex core region.
After vortex interaction, the secondary structure causes large variations
in the surface pressure before being dissipated. Calculations using measured
vortex strength and speed data indicate that the distortions and deflections
of the vortex immediately prior to impingement on the surface differ significantly
from those computed using 2-D potential flow concepts.