The physical reasons for the diffculty in predicting accurately
strong swept-shock-wave/turbulent-boundary-layer interactions are investigated.
A well-documented
sharp-fin/plate flow has been selected as the main test case for analysis. The
selected flow is calculated by applying a version of the Baldwin–Lomax
turbulence
model, which is known to provide reliable results in flows characterized by the
appearance of crossflow vortices. After the validation of the results, by comparison
with appropriate experimental data, the test case flow is studied by means of stream
surfaces which start at the inflow plane, within the undisturbed boundary layer, and
which are initially parallel to the plate. Each of
these surfaces has been represented by
a number of streamlines. Calculation of the spatial evolution
of some selected stream
surfaces revealed that the inner layers of the undisturbed boundary layer, which are
composed of turbulent air, wind around the core of the vortex. However, the outer
layers, which are composed of low-turbulence air, fold over the vortex and at the
reattachment region penetrate into the separation bubble forming a low-turbulence
tongue, which lies along the plate, underneath the vortex. The conical vortex at its
initial stage of development is completely composed of turbulent air, but gradually,
as it grows linearly in the flow direction, the low-turbulence tongue is formed. Also
the tongue grows in the flow direction and penetrates further into the separation
region. When it reaches the expansion region inboard of the primary vortex, the
secondary vortex starts to be formed at its tip. Examination
of additional test cases
indicated that the turbulence level of the elongated tongue
decreases if the interaction
strength increases. The existence of the low-turbulence tongue in strong
swept-shock-wave/turbulent-boundary-layer interactions creates a
mixed-type separation bubble:
turbulent in the region of the separation line and almost laminar between the
secondary vortex and the reattachment line. This type of separation
cannot be simulated
accurately with the currently used algebraic turbulence models, because the basic
relations of these models are based on the physics of two-dimensional flows, whereas
in a separation bubble the whole recirculation region is turbulent. For improving
the accuracy of the existing algebraic turbulence models in predicting
swept-shock-wave/turbulent-boundary-layer interactions, it is necessary
to develop new equations
for the calculation of the eddy viscosity in the separation region,
which will consider
the mixed-flow character of the conical vortex.
Measurements of the Turbulent Boundary Layer in the Diffuser Behind an Axial-Compressor