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

超音速自由射流中流动参数的波动是产生噪声和/或振动的主要原因。因此，澄清这些波动的特征是很重要的。随着背压的增加，射流中的波动在射流边界的剪切层中产生。为了研究波动，最好用非侵入性诊断方法进行实验。在本研究中，我们利用激光诱导荧光法，以高时空分辨率测量低密度轴对称Ar喷流的涨落。利用这一技术，获得了以下结果1。通过改变声速孔板上的压比，研究了射流中湍流的产生和分布。结果表明，湍流产生于射流边界的剪切层，其强度在马赫盘边缘最大。湍流也被发现是产生…更多的d与雷诺数的基础上的小孔出口条件超过4000.2。利用本研究开发的数据采集系统，得到了波动的概率密度函数、自相关函数和功率谱密度函数，阐明了波动的统计特征。同时，通过声光调制器对激光进行斩波，得到了相关函数。功率谱密度函数分析表明，在3 kHz附近存在主谱，且该主谱的强度与孔口形状有关。通过对功率谱密度、自相关函数和互相关函数的详细分析，发现剪切层中产生的波动沿下游方向传播。然而，波动的主频比现有稳定性理论预期的要低一个数量级。这一点似乎需要进一步调查。较少

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