The Turbulence Structure of Heated Supersonic Jets with Offset Total Temperature Non-Uniformities
Mayo Jr, David Earl
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Noise induced hearing loss is a large concern for the Department of Defense. Personnel on aircraft carriers are exposed to dangerous noise levels of noise from tactical aircraft, causing hearing damage which results in significant costs for medical care and treatment. Additionally, NASA and the FAA have begun to investigate the viability of reintroducing supersonic commercial transport in the United States and one of the largest problems to address is reducing the noise impact of these aircraft on communities. The overarching goal of jet noise research is to optimize noise reduction techniques for supersonic jets. In order to achieve this, a more complete theoretical framework which links the jet boundary conditions to the turbulence production in the jet plume and the far-field radiated noise must be established. The research presented herein was conducted on the hypothesis that introducing thermal non-uniformities into a heated supersonic jet flow can favorably alter the turbulence structure in the jet shear layer, leading to reductions in radiated noise. To investigate the impact of temperature on the turbulence development in the jet, spatially resolved three-component velocity vectors were acquired using particle image velocimetry (PIV) performed on two small-scale perfectly expanded Mach 1.5 jet flows, one with a uniform temperature profile and another containing a geometrically offset temperature non-uniformity. Using the PIV data, the mean velocities, Reynolds stresses, and correlation coefficients were obtained from both jet flows and compared to analyze changes in the mean turbulence field. Small but significant reductions in the shear layer turbulence were observed in the near nozzle region of the thermally offset jet when compared to the uniform jet case. The changes result in a thickening of the shear layer nearest the location of the cold plume which alters the integral length scales of the coherent turbulent structures in the offset jet in a manner consistent with other techniques presented in the literature that reduce jet noise. Applying quadrant analysis, a conditional averaging technique, to the jet turbulence plume revealed changes in the statistical flow field of Reynolds shear stress structures. The changes provide strong evidence of the presence of intermittent stream-wise vortical structures which serve to reduce the spatial correlation levels of turbulence in the thermally offset jet flow when compared to the uniform baseline jet.
General Audience Abstract
Increasingly large and powerful engines are required as the mission requirements for tactical aircraft become more advanced. These demands come at the cost of an increased production of noise which is particularly hazardous to crewpersons operating on Navy aircraft carriers during take-off and landing. Noise-induced hearing loss from extended exposure to high noise levels has become a major medical expenditure for the Navy. To address this issue in tactical aircraft engines, the sources of jet plume noise must be reduced, but doing so requires improved understanding of the connections between nozzle boundary conditions, the jet turbulence plume, and the radiated noise while keeping in consideration system constraints and performance requirements. The current study introduces a novel method for controlling supersonic jet noise induced by turbulence through the introduction of an offset non-uniform temperature perturbation at the nozzle mouth. Non-invasive flow measurements were conducted using stereoscopic particle image velocimetry to obtain high-resolution velocity and turbulence data. Analysis of the flow data indicate that an offset reduced temperature plume introduced at the nozzle exit has a first-order effect on the turbulence evolution which result in small, but significant reductions in jet noise levels. The reductions observed are attributed to a disruption in the coherence of the primary noise generating turbulence structures in the jet plume which are associated with the formation of stream-wise vortical structures induced by the cold plume.
- Doctoral Dissertations