The effect of expansion on large scale structure evolution in a compressible turbulent boundary layer

The effects of four expansion regions (centered and gradual [R/δ = 50] expansions of both 7° and 14°) on the large scale structure of an incoming fiilly-developed Mach 3 turbulent boundary layer were investigated. Filtered Rayleigh scattering and double-pulse visualizations were made possible by the presence of scalar water condensation in the freestream and its absence in the boundary layer. Spatial correlations show the increase in scale of the large scale structures across the 7° centered expansion is commensurate with the increase in boundary layer thickness. Spatial correlations at s/δ₀ “ 10 downstream of the 7° centered expansion show the structure angle is similar to that of the flat plate. Ensembles of plan views were used to study the elongated longitudinal structures found previously in these flows. The elongated structures occupy regions of high spectral density in two-dimensional spectra calculated from the plan views. The visualizations and spectra suggest the elongated structures are less significant across the expansions. Spatial correlations and intensity profiles downstream of the 14° centered expansion bear little resemblance to flat plate or 7° centered expansion results. Further, although convection velocities derived from double-pulse correlations are reasonable in the flat plate and 7° centered expansion boundary layers, velocities are unreasonably low downstream of the 14° centered expansion. These results may be related to excess condensation (probably CO₂) which forms downstream of the 14° expansions. Initial results have been obtained with laser-induced fluorescence of acetone vapor. The large scale structures of the outer layer, which are so clearly rendered in H₂O freestream condensation visualizations, are not evident in PLIF density images. This indicates the structures’ density is close to that of the freestream. When the acetone flow rate is set such that most of the acetone in the freestream condenses, the appearance of the large scale structures is similar to that in H₂O condensation visualizations. Potential for acquiring instantaneous density measurements in compressible boundary layers with PLIF is demonstrated.