Scaling Laws and a Method for Identifying Components of Jet Noise

Abstract

It is well established that there are three principal jet noise components for imperfectly expanded supersonic jets. However, to this date there has been no reliable and practical method for identifying the individual components. First, new scaling laws for the turbulent mixing noise component are developed from a comprehensive experimental database generated by the author. The scaling laws are based on the explicit recognition that a) the variation of the overall sound power level with jet velocity has a weak dependence on jet stagnation temperature ratio; b) the variation of the overall sound pressure level with velocity at every radiation angle is a function of jet stagnation temperature ratio. Therefore, the behavior of the turbulent mixing noise at each radiation angle can be characterized by the two independent parameters: the velocity ratio and the stagnation temperature ratio. These two findings set this study apart from past approaches and form the basis for the methodology developed here. It is demonstrated clearly that there is excellent collapse of the mixing noise spectra over the entire frequency range. Once the normalized or master spectra for the mixing noise are established, it is a trivial matter to subtract these from the total measured spectra to obtain the shock-associated noise. For moderately imperfectly expanded heated supersonic jets, the mixing noise component has the same spectral level as the shock-associated noise, over a wide range of higher frequencies. At the lower radiation angles in the forward quadrant, there is a substantial decrease in the values of the velocity exponents as the stagnation temperature ratio is increased. Proceeding aft, the values start to rise and in the peak sector of noise radiation, the velocity exponent becomes less sensitive to jet stagnation temperature ratio unlike at lower angles, and stays close to the values for the unheated jet.