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Ahmed K.A.,Old Dominion University | Ahmed K.A.,State University of New York at Buffalo | Ali M.Y.,Florida A&M University | Ali M.Y.,Florida Center for Advanced Aerospace Propulsion | And 2 more authors.
AIAA Journal | Year: 2014

Mixing in self-ignited supersonic combustors continues to be an important process that influences the performance and limitations for propulsion applications. A common approach for current generation combustors involves the employment of fuel-jet-injection schemes that enhance shear-layer mixing. These fuel jets issuing in a supersonic crossflow induce a bow shock coupled with an oblique shock causing total pressure losses and consequently reducing scramjet-combustor efficiency. This paper explores a novel approach for enhancing mixing using transverse active microjet-based actuators injecting in a supersonic backward-facing-step flow, while reducing pressure losses through weak-shock formation. Shear-layer mixing and flow features were characterized using particle image velocimetry and shadowgraph imaging. Relative comparisons were made with and without microjet actuation under nonreacting conditions. The main flow mechanisms attributed to enhancing mixing are explained. A triple-microjet-injection configuration, in the form of three microjet arrays supplied with different pressures, was investigated. The multiple-microjet-injection configuration formed a virtual ramp at the trailing edge of the backward-facing step that further enhanced mixing and increased the shear-layer thickness. Copyright © 2014 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Kumar R.,Florida A&M University | Kumar R.,Florida Center for Advanced Aerospace Propulsion | Ali M.Y.,Florida A&M University | Ali M.Y.,Florida Center for Advanced Aerospace Propulsion | And 3 more authors.
AIAA Journal | Year: 2011

Jets in a supersonic crossflow are known to produce a three-dimensional bow-shock structure due to the blockage of the flow. In the present study, streamwise linear arrays of high-momentum microjets are used to generate either single or multiple oblique shocks in a supersonic crossflow. The shocks generated using microjets can be tailored in terms of their strength and be made either parallel or coalescing, depending on the application. Flow visualization using shadowgraph and density field measurements using background-oriented schlieren (BOS) technique were carried out for a range of microjet operating conditions. The results obtained using the two methods are consistent and complementary and show a linear variation of oblique shock angles with a microjet pressure ratio over the range of conditions tested. The density field obtained using BOS clearly shows the oblique shocks generated using these microjet arrays, the jump in density across the shock, the extent of the high-density field, the expansion fan, and the associated decrease in density. The results suggest that microjet arrays can be successfully used to develop techniques for sonic boom mitigation and high-performance supersonic inlets. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc.

Worden T.J.,Florida State University | Worden T.J.,Florida Center for Advanced Aerospace Propulsion | Upadhyay P.,Florida State University | Upadhyay P.,Florida Center for Advanced Aerospace Propulsion | And 4 more authors.
AIAA Journal | Year: 2014

This study examines the influence of jet temperature on the impinging jet flowfield and compares the effectiveness of microjet control for a Mach 1.5 ideally expanded jet. The tests were performed on a 36.2-mm-diam jet of air, heated to stagnation temperatures up to 1200 K. The jet impinged normally onto a traversable ground plane oriented between 1.5 and 12 throat diameters from the nozzle exit. Findings indicate that the impingement tones persist and generally become more prominent at higher temperatures, and the lift loss levels increase. Microjet injection produced substantial reductions of overall noise levels and considerable lift recovery, although the effectiveness is slightly lower for elevated jet temperatures. In the nozzle-to-ground range resulting in the highest pressure fluctuation levels on the lift plate, reductions reached 9 dB for the 1200 K jet and 10 dB for the cold jet. There was consistent behavior regarding the suppression of near-field and far-field overall sound pressure levels, with maximum reductions for the 811 K jet of 7 and 8 dB, respectively. The lift loss at small nozzle-to-ground distances for elevated jet temperatures increased by up to 30% compared to the cold jet, and microjets were able to reduce lift loss by as much as 34% for the 811 K jet. Copyright © 2013 by Theodore J. Worden, Puja Upadhyay, Jonas P.R. Gustavsson, and Famikh S. Alvi.

Roy A.,University of Florida | Gustavsson J.P.R.,Florida Center for Advanced Aerospace Propulsion | Segal C.,University of Florida
Experiments in Fluids | Year: 2011

This work identifies the fluorescence characteristics of a perfluorinated ketone, 2-trifluoromethyl-1,1,1,2,4,4,5,5,5-nonafluoro-3-pentanone, further referred to as fluoroketone. This compound is suitable for use with the third harmonic of an Nd:YAG laser for quantitative concentration measurements, as it exhibits strong emission even for relatively low excitation and has a near-linear response of fluorescence intensity with concentration. This makes it suitable for a broad range of fluorescence applications. The absorption cross-section of 3.81 × 10 -19 cm 2 was found to be constant for a temperature range of 293-441 K and a pressure range of 1-18 atm. A calibration line has been generated that relates the concentration of gaseous and liquid fluoroketone with its absorption coefficient. © 2011 Springer-Verlag.

Ali M.Y.,Florida Center for Advanced Aerospace Propulsion | Alvi F.S.,Florida Center for Advanced Aerospace Propulsion | Kumar R.,Florida Center for Advanced Aerospace Propulsion | Manisankar C.,National Aerospace Laboratories, Bangalore | And 2 more authors.
AIAA Journal | Year: 2013

An array of high-momentum microjets are used upstream of a compression corner to control the shock-wave/ boundary-layer interaction on a 24 deg unswept compression ramp in a Mach 2 flow. Measurements include schlieren flow visualization and unsteady pressure measurements using fast-response pressure sensors of the interaction region. Results show that the array of microjets issuing in the supersonic crossflow create oblique shocks, which effectively reduce the incoming Mach number at the compression corner. This leads to a modified separation shock of significantly reduced strength. The location of the modified shock is moved upstream by as much as 4δ0 from its mean undisturbed location. The mean pressure distribution on the surface is altered with microjet control leading to a more gradual compression of the incoming flow relative to the separation shock without control. The wall-pressure fluctuations in the interaction region are reduced by approximately 50%, and the flow near the compression corner appears to be energized with control, based on the unsteady surface-pressure measurements. The pressure spectra show that microjet control results in a redistribution of energy on the wall and the ramp surfaces.

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