Role of Jet Temperature in Correlating Jet Noise

Abstract

The correlation of jet noise spectra, obtained at various jet velocities, has practical relevance; the description and the characterization of the spectral characteristics as functions of the jet operating conditions continue to be of interest. A variety of formulations have been proposed in the last three decades. Recently, new scaling laws, which explicitly identify the jet temperature ratio as an independent controlling parameter, have been developed. Perfect collapse of the spectra, from jets at constant stagnation temperature ratios, over the entire frequency range was demonstrated. With a newly acquired database at fixed static temperature ratio, a thorough investigation of the role of static and stagnation temperature ratios in correlating jet noise is carried out. Similar excellent collapse of the spectra is obtained with the static temperature ratio as the independent parameter. Most of the past theoretical studies have relied on Tanna’s database to provide the justification for their formulations. A careful assessment of the quality of Tanna’s data, both at unheated and heated conditions, has been made through direct comparison with the current data and through the use of the current scaling laws. It is unambiguously established that the high frequency portion of the spectra from unheated jets at all subsonic Mach numbers have elevated levels due to noise contamination. The affected frequency range and the magnitude of the contamination are functions of Mach number. Even spectra from heated jets at lower temperature ratios but at high subsonic Mach number are corrupted; Tanna’s data show incorrect spectral trends and are therefore unreliable. It is clearly shown that there is no change in spectral shape due to heating a jet at fixed low jet velocity, contrary to past belief. Thus, there is no experimental basis for many of the theoretical models which identify multiple sources of jet noise at 90o. The shortcomings with past scaling laws based on variants of the acoustic analogy are highlighted. Finally, the scaling law proposed here with either temperature ratio, is simple, elegant and does not contain any adjustable constants whatsoever.