Determination of lip noise by correlation measurements

Abstract

An experimental investigation of lip noise has been conducted at the UCLA Aerosonics Laboratory using a 112-in.-diameter model jet. Over the range of Mach numbers M for which the experiments were performed (0.3 ⩽ M ⩽ 0.7) the lip noise intensity IL was found to vary as the sixth power of the jet velocity (IL ∼ U6) consistent with a dipole source. Cross-correlation measurements of the nearfield acoustic pressure waves emitted perpendicular to the jet axis show that lip noise originates from two lip positions (off axis), the two contributions being out of phase for frequencies significantly above 2 kHz. Cross-correlation measurements of the farfield acoustic pressure waves emitted from symmetric positions on opposite sides of the jet axis suggest that lip noise reaches maximum intensity about 70°–80° from the jet axis. Third-octave spectral analysis shows that lip noise can be characterized as a wide-band (impulsive) type of noise peaked at a frequency f ∼ U/D, where D is the jet diameter. Measurements made with a small, 2-mm-o. d. probe tube mounted flush with the inside surface of the lip show, as expected, that the hydrodynamic pressure fluctuations arising from turbulence there are proportional to the square of the jet velocity (p ∼ U2. Calculations made from the probe tube measurements yield values of 0.03 for the turbulence intensity just inside the lip of the jet. Normalized cross correlations of 0.1 were found for the lip pressure with the sound field. [Partially supported by NASA-Ames Research Center.]