Reduction in Supersonic Jet Noise from a Laval Nozzle available in Various Industry and Development of a High Efficiency Nozzle

本文介绍了各种工业用拉瓦尔喷嘴降低超音速射流噪声的实验研究和高效喷嘴的研制。该喷管设计马赫数为2.0，喉道直径为20mm，喷管出口直径为26mm。实验在2.0 ~ 18.0的喷嘴压力比范围内进行，包括过膨胀和欠膨胀射流。测量是在提供自由场环境的消声室中进行的。采用纹影技术定性地研究了产生尖锐音调的主要激波模式。实验研究了喷嘴唇厚对噪声的影响。结果表明，当喷嘴唇厚小于一个出口直径时，过膨胀状态下的尖音振幅随喷嘴唇厚的增加而增大，射流振荡模式由轴向不对称转变为螺旋或扑动，而当喷嘴唇厚大于两个出口直径时，尖音振幅随喷嘴唇厚的增加而减小。提出了两种超声速射流降噪的新技术，一种是由直径很小的长圆柱体组成并在超声速射流中沿射流轴方向变化位置的交叉丝法，另一种是由直径很小的长不锈钢丝组成的筛网法。交叉丝是影响射流结构的重要因素。纹影照片显示，交叉导线破坏了冲击细胞结构，降低了强度。因此，电线降低了尖锐音调的振幅。在喷嘴出口平面垂直放置筛网，并在筛网的中心部位穿孔，以扰动初始射流剪切层。筛网导致一个实质性的抑制喷射尖叫音调和所有声压水平。孔尺寸是降低超音速射流噪声的一个重要因素。少

The present paper describes the experimental investigations on reduction in supersonic jet noise from a Laval nozzle available in various industry and development of a high efficiency nozzle. A design Mach number of the nozzle is 2.0 with the throat diameter of 20mm and the nozzle exit diameter of 26mm. The experiments have been conducted over a range of nozzle pressure ratios from 2.0 to 18.0 including the overexpanded and the underexpanded jets. The measurements were conducted in an anechoic room providing a free-field environment. Schlieren technique was used as the tool to study qualitatively the shock wave patterns which are primarily responsible for screech tones. The effect of the nozzle-lip thickness on screech tones has been investigated experimentally. As a result, for the nozzle-lip thickness below one nozzle exit diameter, the screech tone amplitude at the overexpanded condition increases with increasing the nozzle-lip thickness, and the jet oscillation mode changes from ax … More isymmetric to spiral or flapping, while for the nozzle-lip thickness beyond the two nozzle exit diameter, the screech tone amplitude decreases with increasing the nozzle-lip thickness. Two new techniques on supersonic jet noise reduction are proposed, one is the method by a cross-wire which is composed of long cylinders with a very small diameter and its location is varied in the jet axis direction in the supersonic jet stream, the other a mesh screen which is long stainless wires with a very small diameter. The cross-wire is significantly affects the jet structure. Schlieren photographs show that the cross-wire destroys the shock cell structure and weakens the strength. Therefore, the wire reduces the amplitude of screech tones. The mesh screen is perpendicularly placed at the nozzle exit plane and the hole is perforated in the central part of the screen to perturb the initial jet shear layer. The mesh screen leads to a substantial suppression of jet screech tones and the over all sound pressure level. The hole size is an important factor in reducing the supersonic jet noise. Less