Indentification, Validation, and Control of Jet Noise Sources

ABSTRACT: The noise generated by jet exhaust is a significant environmental concern, impacting both flight operations and ground crew. The jet noise is generated by small-scale turbulent structures in the jet that interact with shocks, and other jet structures. Current measurement techniques, such as hot wires provide good temporal resolution but limited spatial resolution while non-intrusive techniques such as PIV offer good spatial resolution but limited temporal resolution. Substantial progress has been made with high-speed lasers; however, these devices are expensive and offer a maximum of 10-kHz bandwidth. Particle Shadow Velocimetry is a new measurement technique that offers high spatial and bandwidth of over 20-kHz while performing non-intrusive 2D velocity measurements. PSV utilizes volumetric illumination provided by LED"s and the depth of field is set by the receiver optics. The displacement of seed particles in the flow are tracked, producing 2D velocity maps. The use of LED"s combined with high speed cameras have been used to configure systems with bandwidths over 15-kHz, and systems that operate at 50-kHz are possible. The spatial resolution of the system is a function of the receiver optics and resolutions similar to micro-PIV have been demonstrated, thus the system offers both high spatial and high bandwidth. BENEFIT: The focus of this proposal is the use of the high speed PSV system for velocity measurements with high spatial resolution in jets. Beyond the improved understanding of jet acoustics that would result from the high spatial and temporal resolution velocity measurements, the PSV technique could be used for several other commercial and research purposes. The PSV technique offers excellent near-wall velocity measurement capability, thus application such as heat transfer, film cooling, and skin friction could be investigated. The optical tool that is central to the proposal, the kHz PSV system, should have much broader appeal as a commercial system. Undergraduate institutions have expressed an interest in allowing students access to the newest tools and instruments, such as PIV. To date however, the safety concerns related to mixing students with high-power lasers have prevented this access. One key advantage of a PSV system is the use of low-power LED"s, thus eliminating the safety concerns. A second advantage of a PSV system is cost as a pulsed LED is much less expensive ($100 versus $50,000) than a PIV laser. This combination of safety and price should be attractive to this potential market. A second potential target is research institutions such as government labs and graduate schools at universities. In this case, the key advantages of PSV are bandwidth, cost, and capability. Traditional laser based kHz PIV systems are limited to about 1-kHz where a PSV system can easily attain 10"s of kHz. This increased bandwidth opens up the potential to investigate biological flows such as vocal cord research, heart valves and assist pumps, and artificial hearts commonly use high speed PIV systems.