Visible light scattering to measure small scale turbulence

A laser scattering optical bench using ionized argon laser was set up to study the small scales of turbulence. Scales between 200 and 10 mu m are resolved. The signal being proportional to the Fourier transform of the density fluctuations, heterodyne detection is used to recover its real and imaginary parts. The optical and electronic facilities are described in detail. The minimum density fluctuations for a signal-to-noise ratio greater than 1 is assessed. Using an acoustic wave, the spatial bandpass filter of the Fourier transform is demonstrated. A sample of recorded signals on a turbulent air jet is shown and the dependence of their intensity on the analyzing wave vector is discussed. The frequency spectra are shown to reflect the temperature and the pressure fluctuations. One can thus measure the flow and sound speed. (C) 2000 American Institute of Physics. [S0034-6748(00)01601-4].