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High Resolution Measurement of the Flow Velocity Field in a Supersonic Jet Plume


OBJECTIVE: Develop a non-invasive (non-seeded) approach to measure the unsteady, 3-D velocity field of a supersonic jet plume for a stationary aircraft. Looking also to make high resolution, time resolved measurements of the turbulent flow field for Short Take-Off/Vertical Landing (STOVL) aircraft with both subsonic and supersonic flow regions. DESCRIPTION: Modern supersonic jet aircraft engines produce a high amplitude noise field with complicated characteristics. The apparent dominant noise source as measured and mapped using acoustic holography methods occurs from 1 to 30 nozzle diameters behind the engine due to a variety of turbulent behaviors of the hot jet. This complicated aeroacoustic problem is not easily modeled using classical analytical approaches. Significant effort is being expended to model the flow field aft of the engine exhaust nozzle using Large Eddy Simulation (LES) methods. Researchers are looking to better understand the characteristics of the turbulent structures in the jet plume in the hopes of developing treatments to engines that might reduce the noise emissions. A significant obstacle to making these simulations practical and realistic for engine design purposes is the lack of methodology to measure the velocity field of the jet plume for purposes of correlating computational results. High quality measurements of the velocity field at and ahead of the exhaust nozzle exit plane would improve the upstream boundary condition definition for use by these analytical methods as well. A further need for this technology is for imaging the supersonic and subsonic turbulent flow field around a STOVL aircraft. This is needed to correlate CFD models that seek to understand the safety and other impacts of the flow field on support personnel and equipment. The various commonly used current velocity field measurement methods are inadequate. Hot wire anemometry methods are limited to flows of Mach 0.5 or less. Flow rakes using pitot tubes significantly disrupt the flow field and have very coarse spatial resolution. Supersonic jet flows in wind tunnels have been mapped using Particle Image Velocimetry (PIV) methods. This method makes use of either solid"seed"particles or some other added"seed"material such as olive oil. However, the"seed"particles used are judged to be impractical and damaging to use to image flow velocities in a turbo machine. Laser based methods, such as Light Detection and Ranging (LIDAR) methods have shown very good results for measuring low speed turbulent air flows in large regions of the atmosphere, and is a promising weather tool. This tool does not require a"seed"material (herein termed a"non-invasive"approach), but rather makes use of aerosols naturally occurring in the air, which either phosphoresce from or reflect the projected laser light in order to image the flow. Innovative non-invasive approaches to measure the velocity fields of supersonic jet plumes for a stationary aircraft are sought. Electromagnetic imaging methods may be a good solution to this problem. Light based methods such as LIDAR are expected to hold excellent promise, particularly as various light frequencies have been shown to have the ability to image various gases. It is expected that either one of the various naturally occurring or combustion byproduct gases in a supersonic engine jet plume may be amenable to such an imaging method. Proposed solutions must work without the addition of imaging particles or fluids to the jet engine intake or exhaust. Spatial resolution of measurement methods must capture the features of the unsteady velocity field with fine enough resolution to capture large and small scale eddies, turbulent boundary layer characteristics and shock structures. Method must also work with flows that are not combustion byproducts, and in the subsonic case. Measurement methods must capture results with sufficient time resolution to track the advection of both large and small scale turbulent structures in a supersonic jet plume. Any velocity measurement method that meets the above requirements will be considered. PHASE I: Demonstrate the feasibility of an imaging method capable of measuring the flow field of a subsonic or supersonic jet plume. Evaluate the three dimensional spatial resolution limits and explore methods to improve the resolution. Demonstrate the limits of temporal resolution of the measurement and the technological limits defined. Improvement to temporal resolution is desired. Contractor may make use of public domain jet flow facilities, such as wind tunnels operated by academic institutions for demonstration purposes. PHASE II: Extend the Phase I methodology to improve any deficiencies, such as spatial or temporal resolution issues. Develop and deliver a prototype measurement system capable of meeting the objectives outlined above. Improve upon the Phase I laboratory method to make it practical and affordable for future commercialization. Any differences between Phase I and Phase II methodologies are to be noted and explained. PHASE III: Further develop the measurement method into one that may be sold, or the service provided for private sector and other government uses. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The ability of current technology tools to image high speed flows is limited by several factors. If successful, it is expected that this technology will provide excellent benefits for aircraft designers in both the commercial and military sectors and will be generally useful as a research and engineering tool. Significant benefits will be gained in high speed flow imaging for a wide variety of aerospace applications from rockets to commercial jet engines. A large segment of the aerospace community would be potential customers of this method. REFERENCES: 1. Kelley, N.D., Jonkman, B.J., Scott, G.N., & Pichugina, Y.L. (2007). Presented at the American Wind Energy Association WindPower 2007 Conference and Exhibition:"Comparing Pulsed Doppler LIDAR with SODAR and Direct Measurements for Wind Assessment". Los Angeles, California. 2. Frelich, R., & Cornman, L. (2002). Estimating Spatial Velocity Statistics with Coherent Doppler Lidar."Journal of Atmospheric and Oceanic Technology", 19(3), 355-366. 3. Mikkelsen, T., Hansen, K., Angelou, N., & Sjoholm, M. (2010). Proceedings from European Wind Energy Conference (EWEC) 2010:"LIDAR Wind Speed Measurement From a Rotating Spinner". Warsaw, Poland. 4. Hewitt, J. (03 Jul 2008)."Doppler Lidar Gives Olympic Sailors the Edge". Retreived from
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