Optical technique for direct measurement of power spectra in compressible turbulence

We present exploratory tests of a new optical method which measures directly two components of the 3-D power spectrum of the refractive-index fluctuation. The method was applied to a Mach 2 turbulent shear layer. Length scales of 0.1 mm to 1.0 mm were resolved within a shear-layer thickness ranging from 5 mm to 10 mm. At low-to-moderate wavenumbers, the spectral behavior is roughly κ^(−3), while at higher wavenumbers it is κ^(−3.7). These spectral slopes are in agreement with prevailing theoretical predictions for anisotropic (low-wavenumber) and isotropic (high-wavenumber) turbulence.