AIAA Paper 95-2304,  26th Fluid Dynamics Conference, June 1995


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.