Filtered rayleigh scattering based measurements in compressible mixing layers

Experimental techniques based on filtered Rayleigh scattering have much potential for the planar measurement of density and velocity in high speed flows. Utilizing filtered Rayleigh scattering, visualizations were obtained earlier to investigate the presence and characteristics of large scale structures in M_c = 0.51 and 0.86 shear layers. The visualizations indicated that a significant decrease in large scale organization is sustained in going to the higher convective Mach number. In addition, it appears that there is a strong tendency for the large scale structures to be oriented oblique to the spanwise direction in the M_c = 0.86 case, while large scale structures of the M_c = 0.51 mixing layer appear to be aligned in the spanwise direction similar to incompressible cases. This work presents the initial results of an effort to utilize Rayleigh scattering to make quantitative planar measurements in the compressible free shear layers. In order to make the jump to quantitative measurements, the absorption characteristics of the iodine molecular filter have been investigated. Initial experiments have been performed in an attempt to obtain planar density measurements in the M_c = 0.51 shear layer. The thermally broadened spectral width of the Rayleigh signal from the molecules in the flow makes such measurements difficult in the particular flow investigated here. Nonetheless, the validity of the measurement technique is demonstrated in the preliminary results presented here. Efforts are underway to both improve the capability to measure planar density and to extend the technique to planar velocity measurements.