阻塞喷注的冲击噪声

冲击噪声是阻塞喷注噪声的重要部份。我们对它的声场和频谱作了详细的实验测量,由于采用外差分析技术,冲击噪声的离散谱和宽带谱可同时记录。实验表明,声场和频谱与驻压比的关系很复杂,但在与喷注垂直的方向上,距喷口1米处的总声压级,如驻压比在3到8.5之间,约为97+20logd分贝,基本不受驻压变化的影响,其中d是喷口直径,以毫米计。进一步又给出了全部驻压比范围内的总声压级的表达式。宽带谱在驻压比为2.5到7之间,离散谱在5到7之间最突出,驻压比小于2或大于8.5,阻塞喷注噪声则主要是湍流噪声。本文还给出了计算离散谱频率和宽带谱峰频的新公式;对实验中观察到的离散谱的非谐频成份,也作了解释。在喷注上游方向,观察到阻塞喷注噪声频谱具有一个或几个凹陷部份,表明在这些频率范围存在干涉现象,可能是湍流噪声与宽带冲击噪声之间的干涉现象,文中说明了这种干涉的可能性。

SHOCK ASSOCIATED NOISE FROM CHOKED JETS

Shock associated noise is an important part of choked jet noise. Sound field and spectra thereof were investigated experimentally by utilizing the heterodyne analysis technique, so that both the screech and the wide-band noise can be recorded and studied. The sound pressures and spectra vary widely with the stagnation pressure, but the overall sound pressure level at 90°to the jet shows good regularity, it is nearly a constant between pressure ratios 3 and 8.5, given by 97 + 201og d, dB, d being nozzle diameter in mm., and empirical formulae are given to facilitate the SPL prediction on the whole range of pressures. The wide-band shock associated noise was found to be predominant in the range of pressure ratio between 2.5 and 7, and screech only up to slightly more than 5. For pressure ratios less than 2 or more than 8.5, the jet noise is entirely dominated by turbulent noise. New formulae for screech frequency and peak frequency of wide-band shock associated noise are given, which agree well with experiments. The existance of non-harmonic components in screech is explained. Interference effect seems to exist in the upstream direction between shock associated noise and turbulent noise, but not in the 90°direction. This phenomenon is explained by considering the phase relations of the noise.