Towards Density Measurements In A Supersonic Turbulent Boundary Layer

Quantitative mean density measurements using calibrated schlieren technique are demonstrated in a Mach 2 compressible turbulent boundary layer. A calibration procedure, where schlieren and particle image velocimetry measurements are conducted on a symmetric double wedge model, is utilised. By assuming that the shock and expansion fan generated by the double wedge model can be idealised, the density gradient field is calculated based on the measured velocity field. Thus, based on the calibration data, a linear relation from schlieren intensity to density gradient is constructed. Moreover, to resolve a large range of density gradients expected within the boundary layer, schlieren measurements with five different knife edge positions are performed and the results are stitched together. The combined density gradient and the integrated density profiles showed good agreement with the literature, and therefore the current technique may provide a promising avenue to measure the mean density fields in compressible boundary layer flows.