Experimental study of the turbulent flow of an underexpanded jet

Measurements of full pressure pulsations in the mixing layer of a supersonic, strongly underexposed jet were performed. The resulting pressure was converted into a longitudinal velocity, which allowed determining the main characteristics of the layer: its growth rate and thickness, longitudinal and transverse dimensions of disturbances, intensity, and spectral composition of turbulence. The layer under consideration was in a state of saturation, with a wide range of disturbances being realized within. The measured growth rate of the layer turned out to be four times higher than that calculated by a statistically generalized method. The intensity of disturbances of the longitudinal velocity component was determined in the considered layer by two groups of disturbances — low-frequency and high-frequency — with approximately equal inputs. From the analysis of the measurement results, it follows that the mass transfer across the layer is inversely proportional to the lengths of the vortices, and in the upper part of the frequency range of intense disturbances it is many times higher. The high mass transfer across the layer causes its intensive mixing and rapid growth rate.