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

利用数字图像处理技术研究了涡环的结构和行为对含有离散频率的喷流噪声的影响。特别地，利用包括纹影系统在内的高速摄像机，研究了具有扩张出口角的喷管释放出的一系列涡环的行为。涡环的结构可分为前导涡环被释放的涡环追赶的现象、释放的涡环穿过前导涡环后合并为前导涡环的合并现象和主流收缩后迅速卷起的卷曲现象。结果表明，旋涡的结构和行为对离散频率有较大影响，并给出了上述现象的特征频率分布。此外，尽管压力低得多，但仍会产生剧烈的离散频率和超音速喷流所产生的尖叫声。

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.