Type of Document Thesis Author Lazic, Sladana Author's Email Address firstname.lastname@example.org URN etd-07132008-225721 Title Active Control of Hot, Supersonic Impinging Jets Using Microjets Degree Master of Science Department Mechanical Engineering, Department of Advisory Committee
Advisor Name Title F.S. Alvi Committee Chair Juan Ordonez Committee Member Justin Schwartz Committee Member Keywords
- Microjet Control
- Supersonic Jets
Date of Defense 2008-06-03 Availability unrestricted AbstractSupersonic impinging jets, similar to the jets issued from a short takeoff and vertical landing (STOVL) aircraft, generate a highly unsteady flow with high unsteady pressure and thermal loads on the aircraft structure as well as the landing surface. These high-pressure, high-temperature and acoustic loads are also accompanied by dramatic lift loss, severe ground erosion and hot gas ingestion in the engine inlets. Previous studies have concentrated on characterizing the impingement flow and its control for cold jets, i.e. operating at ambient temperatures. They have shown that one of the major characteristics of a supersonic impinging jet is the dominance of the feedback loop mechanism. Previous work has also shown that active microjet control is successful at attenuating the feedback loop and therefore the negative effects associated with it.
The current studies attempt to examine and investigate the flow properties of a hot supersonic impinging jet issuing from a convergent-divergent, Mach 1.5 nozzle and operating at more realistic, higher temperatures,. This ideally expanded jet was heated up to a stagnation temperature of ~500K. The jet is impinging on a flat plate, called the ground plate that is appropriately larger than the nozzle exit diameter. The ground plate can be moved vertically in order to simulate different hover heights.
In order to compare the properties of a cold and a heated impinging jet, mean pressure and unsteady pressure measurements, temperature measurements as well as acoustic measurements were obtained. The mean and unsteady pressure measurements as well as temperature measurements were performed on the lift plate (representing the undersurface of a STOVL aircraft) as well as on the ground plate. Acoustic near-field measurements were obtained using a microphone placed at 15-diameters away from the nozzle exit.
Active microjet control was implemented as a way to attenuate the adverse effects of a jet impinging on a flat surface. It has already been shown that microjets are very effective when introduced to an impinging flow of a cold supersonic jet. Another aim of this study is to explore how effective microjet control is when the stagnation temperature of the primary jet is heated.
The results clearly indicate that when the primary jet is heated, the pressure fluctuations and the associated unsteady loads, are substantially higher then when the jet is cold. These high unsteady loads also persist over larger nozzle to plate distances. The hover lift loss at high temperatures increases dramatically as well, from ~50% of the primary jet thrust at cold temperatures to an astounding ~75% of the primary jet thrust when the jet is heated. The temperature recovery factor is strongly dependent on the nozzle to plate distance and the temperature of the jet. There is an indication of an increase in entrainment of ambient air when the jet is heated. Additionally the unsteady thermal loads seem to increase in frequency as the stagnation temperature of the jet increases.
These results show that the adverse side effects of an impinging supersonic jet are even more dramatic when the jet is at higher temperatures – a trend that is expected to continue as the temperatures are increased further to real jet exhaust conditions. However, this study also demonstrates that the activation of microjets can provide an effective way of reducing these negative effects even when the jet is heated. The pressure fluctuations have been drastically reduced, where the discrete impinging tones have been attenuated or even eliminated at both the cold and hot conditions. The overall pressure levels on the ground plane have been reduced up to 20dB and on the lift plate up to 15dB at small nozzle to ground plane distances while the jet was heated. Additionally up to 21% of the lift loss has been recovered at cold temperature jets and an astounding 35% of the lift loss was recovered when the jet is heated. The temperature recovery factor indicates a similar trend as the lift loss, which is that the entrainment of the ambient air is decreased when the microjets are applied. The thermal unsteady loads have been attenuated as well.
In summary, this study demonstrates that the adverse effects of impinging supersonic jets are even more pronounced when the jet is heated, however microjet control is very effective at both cold and hot conditions. These dramatic reductions due to microjet control are achieved using microjets with a mass flow rate less than 0.5% of the primary jet flux.
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