Transient Grating Spectroscopy in a Hot Turbulent Compressible Free Jet

Speed of sound profiles of an unseeded hot, supersonic, axisymmetric free air jet at atmospheric pressure are measured with 2% uncertainty using laser-induced electrostrictive gratings. Signal level and lifetime decay with increasing temperature, and no data are obtained for static temperatures above 540 K. The influence of temperature, turbulence, beam steering, and mechanical vibrations on the signal amplitude and lifetime is examined quantitatively. It is argued that electrostrictive gratings are applicable for thermometry and velocimetry in supersonic combustors by employing a powerful pulsed infrared probe beam. The significantly larger beam power raises the signal level by three orders of magnitude, whereas the longer wavelength reduces the effects of beam steering by increasing the angular separation between probe and signal beam.