OPTICAL TECHNIQUE FOR DIRECT MEASUREMENT OF POWER SPECTRA IN COMPRESSIBLE TURBULENCE

We present exploratory tests of a new optical method which measures di rectly two components of the 3-D power spectrum of the refractive-inde x fluctuation. The method was applied to a Mach 2 turbulent shear laye r. Length scales of 0.1 mm to 1.0 mm were resolved within a shear-laye r thickness ranging from 5 mm to 10 mm. At low-to-moderate wavenumbers , the spectral behavior is roughly kappa-3, while at higher wavenumber s it is kappa-3.7. These spectral slopes are in agreement with prevail ing theoretical predictions for anisotropic (low-wavenumber) and isotr opic (high-wavenumber) turbulence.