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 wings.

Related Publications

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 1995.

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.