A fully developed Mach 3 turbulent boundary layer subjected to
four expansion
regions (centred and gradual expansions of 7° and 14°) was investigated
with laser
Doppler velocimetry. Measurements were acquired in the incoming flat-plate
boundary
layer and to s/δ≃20 downstream of the
expansions. While mean velocity profiles
exhibit significant progress towards recovery by the most downstream measurements,
the turbulence structure remains far from equilibrium. Comparisons of computed
(method of characteristics) and measured velocity profiles indicate that
the post-expansion
flow evolution is largely inviscid for approximately 10δ. Turbulence
levels
decrease across the expansion, and the reductions increase in severity
as the wall is
approached. Downstream of the 14° expansions, the reductions are more
severe and
reverse transition is indicated by sharp reductions in turbulent kinetic
energy levels and
a change in sign of the Reynolds shear stress. Dimensionless parameters
such as
anisotropy and shear stress correlation coefficient highlight the complex
evolution of
the post-expansion boundary layer. An examination of the compressible vorticity
transport equation and estimates of the perturbation impulses attributable
to
streamline curvature, acceleration, and dilatation both confirm dilatation
to be the
primary stabilizer. However, the dilatation impulse increases only slightly
for the 14°
expansions, so the dramatic differences downstream of the 7° and 14°
expansions
indicate nonlinear boundary layer response. Differences attributable to
the varied radii
of surface curvature are fleeting for the 7° expansions, but persist
through the spatial
extent of the measurements for the 14° expansions.
Comparison of the Exact and Approximate Values of Certain Parameters in Laminar Boundary Layer Flow Using Some Velocity Profiles