Exploration of temperature effects on the far-field acoustic radiation from a supersonic jet

Paper in proceeding, 2014

Jet engines designed for high-speed aircraft commonly include C-D nozzles to obtain supersonic speeds. The radiated noise from the jet exhaust reaches acoustic levels which may cause hearing damage to the pilot and the air-field personnel even though state of the art noise protection such as noise-canceling ear muffs are employed. It is therefore extremely important to keep the noise levels as low as possible. Understanding the noise generation mechanism is of great importance in order to reduce strength of the noise sources. Typical far-field noise spectral characteristics from the supersonic jet exhaust consist of turbulent mixing noise and shock-associated noise. Another noise component named'crackle' is radiated from the jet under certain circumstances. Although it does not appear in the noise spectra due to its characteristics, its rasping character is perceived as a dominant annoyance factor by the human ear. Since it does not appear in the spectrum other measures are needed to identify the existence of'crackle'. Statistical tools like Skewness and Kurtosis applied to the far- and near-field pressure signals and the time derivate of the pressure signal have been shown in literature to be useful for identification of'crackle' events. In this paper the near-field and far-field acoustic radiation from a supersonic jet is analyzed using LES with a code developed at Chalmers University of Technology. The code has previously shown to accurately capture far-field noise spectra of supersonic jets under a variety of moderately cool jet conditions. In the present study we move towards more realistic high-speed aircraft conditions with higher jet exhaust temperatures. The nozzle is operated at slightly underexpanded conditions (NPR = 4.0) and three different stagnation temperature ratios NTR = 1.0, NTR = 2.0 and NTR = 3.0. The LES results are compared with experiments conducted at the Gas Dynamics and Propulsion Laboratory at the University of Cincinnati.