Experimental study of physical mechanisms in the control of supersonic impinging jets using microjets

Supersonic impinging jet(s) inherently produce a highly unsteady flow field. The occurrence of such flows leads to many adverse effects for short take-off and vertical landing (STOVL) aircraft such as: a significant increase in the noise level, very high unsteady loads on nearby structures and an appreciable loss in lift during hover. In prior studies, we have demonstrated that arrays of microjets, appropriately placed near the nozzle exit, effectively disrupt the feedback loop inherent in impinging jet flows. In these studies, the effectiveness of the control was found to be strongly dependent on a number of geometric and flow parameters, such as the impingement plane distance, microjet orientation and jet operating conditions. In this paper, the effects of some of these parameters that appear to determine control efficiency are examined and some of the fundamental mechanisms behind this control approach are explored. Through comprehensive two- and three-component velocity (and vorticity) field measurements it has been clearly demonstrated that the activation of microjets leads to a local thickening of the jet shear layer, near the nozzle exit, making it more stable and less receptive to disturbances. Furthermore, microjets generate strong streamwise vorticity in the form of well-organized, counter-rotating vortex pairs. This increase in streamwise vorticity is concomitant with a reduction in the azimuthal vorticity of the primary jet. Based on these results and a simplified analysis of vorticity transport, it is suggested that the generation of these streamwise vortices is mainly a result of the redirection of the azimuthal vorticity by vorticity tilting and stretching mechanisms. The emergence of these longitudinal structures weakens the large-scale axisymmetric structures in the jet shear layer while introducing substantial three-dimensionality into the flow. Together, these factors lead to the attenuation of the feedback loop and a significant reduction of flow unsteadiness.

[1]  Farrukh S. Alvi,et al.  Flow field and noise characteristics of a supersonic impinging jet , 1998, Journal of Fluid Mechanics.

[2]  M. Gharib,et al.  The role of streamwise vorticity in the near-field entrainment of round jets , 1992, Journal of Fluid Mechanics.

[3]  Eric F. Spina,et al.  Mode-switching and nonlinear effects in compressible flow over a cavity , 2004 .

[4]  Brenda Henderson,et al.  The connection between sound production and jet structure of the supersonic impinging jet. , 2002, The Journal of the Acoustical Society of America.

[5]  Christopher K. W. Tam,et al.  The shock-cell structures and screech tone frequencies of rectangular and non-axisymmetric supersonic jets , 1988 .

[6]  Farrukh S. Alvi,et al.  Active and Passive Control of Supersonic Impinging Jets , 2006 .

[7]  Donald B. Bliss,et al.  The instability of short waves on a vortex ring , 1974, Journal of Fluid Mechanics.

[8]  P. Bradshaw.  Turbulent secondary flows , 1987 .

[9]  B. L. Hunt,et al.  The near wall jet of a normally impinging, uniform, axisymmetric, supersonic jet , 1974 .

[10]  M. Samimy,et al.  Mixing enhancement via nozzle trailing edge modifications in a high speed rectangular jet , 1999 .

[11]  Mark F. Reeder,et al.  Control of an axisymmetric jet using vortex generators , 1994 .

[12]  Coleman duP. Donaldson,et al.  A study of free jet impingement. Part 1. Mean properties of free and impinging jets , 1971, Journal of Fluid Mechanics.

[13]  Alan Powell,et al.  The sound‐producing oscillations of round underexpanded jets impinging on normal plates , 1988 .

[14]  Christopher K. W. Tam,et al.  Theoretical model of discrete tone generation by impinging jets , 1990, Journal of Fluid Mechanics.

[15]  Eckart Meiburg,et al.  Three‐dimensional vorticity modes in the wake of a flat plate , 1990 .

[16]  Eric F. Spina,et al.  Control of high-speed impinging-jet resonance , 1994 .

[17]  Eckart Meiburg,et al.  Experimental and numerical investigation of the three-dimensional transition in plane wakes , 1988, Journal of Fluid Mechanics.

[18]  B. L. Hunt,et al.  The Occurrence of Stagnation Bubbles in Supersonic Jet Impingement Flows , 1976 .

[19]  Frank E. Marble,et al.  Shock enhancement and control of hypersonic mixing and combustion , 1990 .

[20]  L. Venkatakrishnan,et al.  A PIV study of a supersonic impinging jet , 1999 .

[21]  J. Lasheras,et al.  Three-dimensional instability and vorticity patterns in the wake of a flat plate , 2003, Journal of Fluid Mechanics.

[22]  K. Karamcheti,et al.  Some Features of AN Edge-Tone Flow Field , 1969 .

[23]  Farrukh S. Alvi,et al.  Effects of Counterflow on the Aeroacoustic Properties of a Supersonic Jet , 1999 .

[24]  Wolfgang Rodi,et al.  Calculation of turbulence-driven secondary motion in non-circular ducts , 1982, Journal of Fluid Mechanics.

[25]  Anjaneyulu Krothapalli,et al.  The effect of streamwise vortices on the aeroacoustics of a Mach 0.9 jet , 2007, Journal of Fluid Mechanics.

[26]  Dimitri Papamoschou,et al.  Visual observations of supersonic transverse jets , 1993 .

[27]  Brenton J. Greska.  Supersonic Jet Noise and its Reduction Using Microjet Injection , 2005 .

[28]  Sheila E. Widnall,et al.  The two- and three-dimensional instabilities of a spatially periodic shear layer , 1982, Journal of Fluid Mechanics.

[29]  Anjaneyulu Krothapalli,et al.  On the use of microjets to suppress turbulence in a Mach 0.9 axisymmetric jet , 2003, Journal of Fluid Mechanics.

[30]  Khairul Q. Zaman,et al.  Spreading characteristics of compressible jets from nozzles of various geometries , 1999, Journal of Fluid Mechanics.

[31]  Farrukh S. Alvi,et al.  Control of supersonic impinging jet flows using supersonic microjets , 2003 .

[32]  Coleman duP. Donaldson,et al.  A study of free jet impingement. Part 2. Free jet turbulent structure and impingement heat transfer , 1971, Journal of Fluid Mechanics.

[33]  Frank E. Marble,et al.  Vorticity Generation by Contoured Wall Injectors , 1992 .

[34]  Mark F. Reeder,et al.  Effect of tabs on the flow and noise field of an axisymmetric jet , 1993 .

[35]  James Bridges,et al.  An experimental study of the oscillatory flow structure of tone-producing supersonic impinging jets , 2005, Journal of Fluid Mechanics.