STUDY ON STRUCTURE OF SHOCK WAVES BY DIGITAL IMAGE PROCESSING AND BASIC RESEARCH ON MEASUREMENT TECHNOLOGY OF DIGITAL IMAGE

本文用数字图像处理方法研究了涡环的结构和特性对含离散频率的喷流噪声的影响。特别是，一系列的涡环从喷嘴与发散出口角释放的行为澄清了高速摄像机包括纹影系统。涡环的结构可分为：前涡环被释放涡环追逐的追逐现象、释放涡环通过后与前涡环汇合的汇合现象和主流收缩后迅速卷起的卷起现象。结果表明，涡的结构和行为对离散频率有很大的影响，并给出了上述现象的特征频率分布。此外，尽管压力相当低，但产生了剧烈的离散频率和尖叫噪声，例如由超音速射流产生的噪声。

The effect of the structure and behavior of vortex rings on jet noise containing discrete frequency has been studied by digital image processing. In particular, the behavior of a series of vortex rings released from a nozzle with divergent exit angle was clarified by a high-speed video camera including Schlieren systems. The structures of vortex rings were classified into the following phenomena : chasing phenomenon in which a leading vortex ring was chased by a released vortex ring, merging phenomenon in which a released vortex ring merged after passing-through to a leading vortex ring, and rolling up phenomenon in which the main flow was rapidly rolled up after contraction. It was found that the discrete frequency was affected considerably by the vortex structure and behavior, and the characteristic frequency distributions for the above phenomena were presented. Furthermore, in spite of considerably lower pressure, violent discrete frequency and screech noise such as that generated by a supersonic jet were generated.