TURBULENCE MEASUREMENT OF A LOW-DENSITY SUPERSONIC JETS WITH A LASER-INDUCED FLUORESCENCE METHOD

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The fluctuations of the flow parameters in the supersonic freejets play dominant role in the generation of noise and/or vibration. Therefore, it is important to clarify the characteristics of these fluctuations. The fluctuations in a jet are generated in the shear layer of the jet boundary as the backpressure is increased. To investigate fluctuations, it is desirable to perform experiments with nonintrusive diagnostic methods. In the present research, the laser-induced fluorescence method was used to measure the fluctuations of low-density axisymmetric argon jets with high spatial and temporal resolution. With this technique, the following results were obtained.1. By changing the pressure ratio across a sonic orifice, the generation and distribution of turbulence in the jet were investigated. It was found that the turbulence was generated in the shear layer of the jet boundary, and its intensity was maximum in the periphery of the Mach disk. The turbulence was also found to be generate … More d with the Reynolds number based on the orifice exit condition above 4000.2. With the data acquisition system developed in the present research, the probability density function, auto-correlation function and power spectral density function were obtained, and the statistical characteristics of the fluctuation was clarified. Also, by chopping the laser beam by an acoustooptic modulator, the cross-correlation function was obtained.3. The analysis of the power spectral density function showed that the dominant spectrum existed near 3kHz, and the intensity of this spectrum depended on the orifice shape.4. With the detailed analyses of the power spectral density, auto-correlation and cross-correlation functions, it was found that the fluctuation generated in the shear layer was propagated in the downstream direction. However, the dominant frequency of the fluctuation was an order of magnitude lower than that was expected by the existing stability theory. This point seems to need further investigation. Less