Development of Narrow-band Velocity Fluctuations in Vortex Flows

1 The velocity field in the vicinity of the twin-tails of combat aircraft at high angle of attack exhibits smallamplitude 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.