Measurements of Scalar Convection Velocity in Heated and Unheated High-Speed Jets
Shea, Sean Patrick
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Jet noise has been a growing concern in recent years due to the costs associated with hearing loss of United States service members. Jet noise is also becoming more of a concern due to the rise of civilian complaints regarding the noise of jets near civilian and military air stations. One source of noise generation is from packets of air called eddies, which move with a convection velocity Uc. The current work seeks to expand upon the understanding of jet noise by collecting data using Time-resolved Doppler global velocimetry (TR-DGV) from regions of the jet known to produce high levels of acoustic radiation. Past experiments in studying convection velocity are reviewed based on the technique for obtaining the velocities. To add to these experiments, the current work analyzes data obtained using TR-DGV applied to a perfectly expanded Mach 1.65 flow with total temperature ratio (TTR) equal to 1. Additional measurements were obtained on a Mach 1.5 nozzle operated at a slightly over expanded condition and at TTR = 2. The cold jet flow is compared to the past experiments on unheated jets and demonstrates good agreement with respect to normalized convection velocities based on the jet exit speed. The data is then compared to past experiments conducted on the same nozzle at heated conditions. Shadowgraph imaging is used as a qualitative tool to locate shock cells within the jet plume. TR-DGV data from near the lipline (r = 0.5D) is axially aligned with the shadowgraph images to demonstrate that the shock structure within the potential core causes detectable variations in the scalar convective velocity. Additionally, it is shown that in the heated and unheated Mach 1.65 jet and the over expanded heated Mach 1.48 jet that the convection velocity does increase beyond the potential core. The Mach 1.48 jet is also compared to mean velocities obtained using Particle Image Velocimetry and found that the convective and mean velocities were only similar in some regions of the jet. A discussion is provided on suggestions of future work on where to obtain data within the jet plume and how to collect the data using current capabilities. Suggestions are also provided for improving data quality in future experiments, as well as ideas for future investigations into convection velocity along the length of the jet plume using TR-DGV.
General Audience Abstract
Jet noise has been a growing concern in recent year due to the costs associated with hearing loss of United States service members. Additionally, many civilians complain about the noise of aircraft flying both out of military facilities and commercial airports. One source of noise generation is from packets of air called eddies which move with a convection velocity. Researchers have identified that by affecting the convection velocities of these eddies, there is a larger benefit than other traditional methods such as engine chevrons. The current work summarizes techniques used to investigate convective velocity as well as to provide evidence for other unconfirmed theories. This study focuses on using a laser-based technique to obtain data within the flow of an unheated supersonic jet. An unheated jet is studied to allow for easy comparison to other experiments that have used different diagnostic techniques. Additionally, this case is studied to complete a set of experiments that were previously conducted on the same nozzle so that there is a true base-line or “control” case for future work. Later in this paper, analysis will be done to show how shocks within the jet affect the convective velocity. A combination of both quantitative and qualitative efforts are performed to accomplish this. Additionally, it will be shown that after the potential core of the jet breaks down, there is an increase in the local convective velocity in this region immediately after the potential core. Finally, a brief summary will be given and suggestions for future work will be presented.
- Masters Theses