Analysis of the Two Similarity Components of Turbulent Mixing Noise

Abstract

It is now widely believed that the turbulence in free shear layers is not completely random, but more coherent and orderly, and that turbulent flows contain both fine-scale and large-scale structures. Both fine-scale turbulence and large-scale turbulence generate noise. Experimental measurements have shown conclusively that the mean flow as well as the turbulence statistics exhibit self-similarity. Based on these observations, Tam et al. (Tam, C. K. W., Golebiowski, M., and Seiner, J. M., Two Components of Turbulent Mixing Noise from Supersonic Jets, AIAA Paper 96-1716, 1996) proposed that because noise is generated by the turbulence of the jet, the noise spectra generated by fine-scale and large-scale turbulence should also exhibit self-similarity. By the examination of a large set of supersonic jet noise data acquired at NASA Langley Research Center, Tam et al. offered evidence that the turbulent mixing noise of high-speed jets does consist of two, independent self-similar components. We first provide additional independent confirmation of the universal shapes of the two components of mixing noise for a single jet. The significance of an important effect, due to atmospheric absorption, is illustrated with detailed analysis of The Boeing Company jet noise data. The Tam et al. analysis is based on the examination of data from single-stream nozzles. We provide evidence from the analysis of noise from dual-stream nozzles that the measured spectra in the forward quadrant and near-normal angles conform to the shape of the fine-scale spectrum, regardless of nozzle geometry and operating conditions. We clearly demonstrate that the spectral shape associated with the large-scale structures of single jets does not characterize the noise of coaxial jets at large aft angles. Finally, we show that the noise of hot subsonic jets at low angles also conform to the shape of the fine-scale spectrum.