Distributions of Noise Sources in Heated and Cold Jets: Are They Different?

Abstract

The axial extent of the high-speed flow region in a heated jet is much longer than its counterpart in an unheated jet at fixed Mach number. This trend has led some to believe that the source distributions would be different. Two specific questions are addressed in this study: (1) is the noise source distribution different for heated jets? and (2) is the distance to the true farfield longer for heated jets? The second question has practical significance as it impacts the measurement and processing of spectra. The assumption of a point source located at the nozzle exit plane is invoked in many tests – the condition for its validity is established for heated jets. Source distributions for heated and unheated jets have been measured with an elliptic mirror, for jets with a Mach number range of 0.5 to 1.9. The effect of jet temperature on the peak source location for the noise radiated to all the inlet angles in the range of 90° to 150° is negligible, both for the overall sound pressure level and individual frequencies. The effect of jet mach number is pronounced, with the peak source location moving downstream with increasing Mach number. Different approaches have been adopted for the establishment of the minimum distance to the true farfield for heated and unheated jets: microphone arrays at different distances together with nozzles of various diameters, microphones set at normal and grazing incidence, and the elliptic mirror. Both subsonic and supersonic jets have been considered. It is shown from detailed analyses that the distance to the true acoustic and geometric farfield is the same for heated and unheated subsonic jets, and is ∼37D (D is the jet diameter). The polar response characteristics of condenser microphones are examined to explain the observed good agreement of spectra between microphones located at ∼37D and larger distances. Finally, a minimum distance of ∼40D and a desirable distance of 50D or longer are recommended for true farfield measurements for subsonic jets. The minimum distance for supersonic jets is more nuanced: for unheated jets, a distance of 45D has been shown to represent the farfield. For highly heated supersonic jets, 45D again represents the farfield for the lower polar angles ≤∼100°. Rapid change in source directivity in the angular range of ∼100° to ∼125° dictates that a minimum distance of ∼70D for angles ≥∼100° might be prudent even though a shorter distance might be adequate for angles ≥∼130°. A good understanding of the source distributions and the radiation characteristics for different jet operating conditions, coupled with prudent test planning should help eliminate the nearfield effects in acoustic measurements.