DEVELOPMENT OF NARROW-BAND VELOCITY FLUCTUATIONS IN VORTEX FLOWS
Narayanan M. Komerath and J. Paul Hubner
School of Aerospace Engineering
Georgia Institute of Technology
Atlanta, GA 30332-0150
The velocity field in the vicinity of the twin-tails of combat aircraft at high angle of attack exhibits small-amplitude fluctuations which are nearly periodic. Their frequency increases in direct proportion to freestream velocity, and inversely with model size, the relationship holding over a wide range of Reynolds number. The Strouhal number and spectral shape vary with angle of attack and wing leading edge sweep but appear to be relatively insensitive to leading-edge shape. The phenomenon is general to leading-edge vortex flows for angles of attack ranging from 15 to 40 degrees depending on the geometry. It is a probable driver of tail fatigue. The search for the origin and mechanism of this phenomenon is summarized. Empirical correlations are developed for various configuration shapes and isolated wing planforms. A 1/32-scale model of an F-15 and a 59.3-deg. cropped delta wing are used for detailed studies. Cross-spectral analysis of hot-film anemometer signals traces the fluctuations upstream along a helical path. Surface streaklines visualized in two orthogonal planes show nearly-spanwise vortical structures amplifying and propagating downstream, suggestive of cross-flow instability. Spectra obtained using laser velocimetry (LV) confirm the hot-film data. LV data phase-synchronized with a surface hot-film signal capture the size, partial shape, and convective speed of cross-flow vortical structures as they move downstream. Counter-rotation is observed. Remaining hypotheses for the origin of these fluctuations are based on centrifugal instability of the flow beneath the vortex center, cross-flow shear layer instability and unsteady phenomena associated with the interaction of the secondary vortex with the surface. Preliminary attempts to modify the spectra based on the surface-origin hypotheses are successful.