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
ABSTRACT
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.