Nonlinear Growth and Breakdown of the Hypersonic Crossflow Instability

2019-08-02T15:41:24Z (GMT) by Joshua B Edelman
A sharp, circular 7° half-angle cone was tested in the Boeing/AFOSR Mach-6 Quiet Tunnel
at 6° angle of attack, extending several previous experiments on the growth and breakdown of
stationary crossflow instabilities in the boundary layer.

Measurements were made using infrared
imaging and surface pressure sensors. Detailed measurements of the stationary and traveling
crossflow vortices, as well as various secondary instability modes, were collected over a large
region of the cone.

The Rod Insertion Method (RIM) roughness, first developed for use on a flared cone, was
adapted for application to crossflow work. It was demonstrated that the roughness elements were
the primary factor responsible for the appearance of the specific pattern of stationary streaks
downstream, which are the footprints of the stationary crossflow vortices. In addition, a roughness
insert was created with a high RMS level of normally-distributed roughness to excite the naturally
most-amplified stationary mode.

The nonlinear breakdown mechanism induced by each type of roughness appears to be
different. When using the discrete RIM roughness, the dominant mechanism seems to be the
modulated second mode, which is significantly destabilized by the large stationary vortices. This
is consistent with recent computations. There is no evidence of the presence of traveling crossflow
when using the RIM roughness, though surface measurements cannot provide a complete picture.
The modulated second mode shows strong nonlinearity and harmonic development just prior
to breakdown. In addition, pairs of hot streaks merge together within a constant azimuthal
band, leading to a peak in the heating simultaneously with the peak amplitude of the measured
secondary instability. The heating then decays before rising again to turbulent levels. This nonmonotonic
heating pattern is reminiscent of experiments on a flared cone and earlier computations
of crossflow on an elliptic cone.

When using the distributed roughness there are several differences in the nonlinear breakdown
behavior. The hot streaks appear to be much more uniform and form at a higher wavenumber,
which is expected given computational results. Furthermore, the traveling crossflow waves become
very prominent in the surface pressure fluctuations and weakly nonlinear. In addition there
appears in the spectra a higher-frequency peak which is hypothesized to be a type-I secondary instability
under the upwelling of the stationary vortices. The traveling crossflow and the secondary
instability interact nonlinearly prior to breakdown.