Identification and Control of
Quasi-Periodicity in Vortex Flows
Air Force Office of Scientific Research
An example of basic research growing
out of a pressing problem, this project studies why steady vortex flows
over swept wings develop quasi-periodic fluctuations which drive structural
vibrations of downstream components. The phenomenon has been shown to be
present on all swept-wing configurations. Using asynchronous laser imaging
techniques coupled with laser velocimetry and spectral analysis of thermal
anemometry, the fluctuations have been traced to their origin near the
wing surface. Empirical predictions have been developed for existing configurations.
The origin of the fluctuations has been traced to counter-rotating vortical
structures in the flow over the wing at high angles of attack; these correspond
to structures formed by centrifugal instability. This matches the
observation that they amplify and focus in an otherwise chaotic flow,
and that they were independent of precise leading edge shapes and Reynolds
and Mach numbers. We have demonstrated over 70% reduction in spectral peak
intensity at the vertical tail, using the surface layer hypothesis. This
means that the spectral peak has essentially been removed, and the fluctuation
level reduced to the broadband turbulence level. This has been shown to
work on scale-models of present aircraft, as well as on isolated delta
Klein, M.A. and Komerath, N.M., "Reduction
of Narrow-Band Velocity Fluctuations Over an Aircraft Model," AIAA
Paper 97-2266, 15th AIAA Applied Aerodynamics Conference, Atlanta,
GA, June, 1997
Klein, M.A., Lentz, W. K. and Komerath, N.M.,
"Reduction of Fluctuations over Swept Wings Using Passive and Active Mini-Fences,"
AIAA Paper 97-0548, 35th Aerospace
Sciences Meeting, Reno, NV, Jan. 1997.
Hubner, J.P., Komerath, N.M., "Counter-Rotating
Structures Over a Delta Wing". AIAA
Journal, Vol.34, No. 9, 1996, p. 1958-1960 .
Hubner, J.P., Komerath, N.M., "Modification of
Spectral Characteristics in a Vortex Flow Field". AIAA
95-1795, Applied Aerodynamics Conference, San Diego, CA, June 1995.
Komerath, N.M., Hubner, J.P., "Development of
Narrow-Band Velocity Fluctuations in Vortex Flows". Invited
paper AIAA 95-2304, 26th Fluid Dynamics Conference, San Diego, CA, June
Hubner, J.P., Komerath, N.M., "Spectral Mapping
of Quasiperiodic Structures in a Vortex Flow." Journal of Aircraft,
Vol.32, No. 3, May-June 1995, pp. 493-500.
Klein, M.A., Hubner, J.P., Komerath, N.M., "Spectral
Measurements in Vortex Flow over Swept-Winged Configurations at High Angle
of Attack". AIAA Paper 94-1804,
12th Applied Aerodynamics Conference, Colorado Springs, Co., June 1994.