The heterodyne detection laser-induced electrostrictive gratings technique was employed for characterization of a flow velocities field in the plume of a rocket nozzle. The intention of the work was to validate CFD calculations, that predict the appearance of a recirculation zone inside the flow at a certain ratio of nozzle and ambient pressures. This zone co-exists with the side loads to the nozzle wall that were observed experimentally as transient forces during the start-up or the shut-down of the thrust chamber. Measurements were performed in cold-gas flows from a subscale thrust-optimized, parabolic nozzle. Nitrogen at stagnation pressures up to 40 bar was supplied to the nozzle. The experimental results clearly proved the existence of a recirculation zone in the flow field, in a reasonable agreement with the predictions of CFD calculations.

The signal of heterodyne-detected electrostrictive laser-induced gratings (LIGs) originates from the interference of a reference laser beam with the laser light diffracted at the counterpropagating sound-wave packets, which are generated in the overlap volume of crossed laser beams. The frequencies of the sound waves, which contain the information on the sound velocity and on the motion of the medium, can approximately be extracted from the frequencies at the maxima of the two peaks, which dominate the power spectrum of the heterodyne LIG signal intensity. In free-air jets behind a slot nozzle, flow velocities up to 60 m s −1 were determined by quick fits from the power spectrum and by fitting the time-dependent signal intensity data to model functions. Results agree well, the standard deviations being about one-half in the latter case (1.6-2.6 m s −1 , for positions close to the nozzle). Problems arising from the sampling and from the finite observation time of the signal intensity are discussed. Furthermore, the results of the LIG measurements were compared with data provided by laser Doppler anemometry. As an application of the instantaneous and unseeded LIG measurement technique, temperatures in heated air jets were determined simultaneously with the flow velocity by quick fits from the power spectrum.