Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc. THE ROLE OF WATER INJECTION ON THE MIXING NOISE SUPERSONIC JET D. Washington* and A. Krothapalli 1 " Department of Mechanical Engineering Florida A&M University and Florida State University 2525 Pottsdamer St., Tallahassee, FL 32310 donnell @ eng.fsu.edu Recent studies indicate that the turbulent mixing noise is the main contributor to the far-field overall sound pressure level (OASPL) in supersonic jets. Evidence from the current investigation suggest that the dominant sources of the turbulent mixing noise are located at the end of the potential core region of the jet. A novel approach using water injection into the jet shear-layers is employed as the noise suppression technique. In contrast to previous investigations, this study utilizes only a small amount (less than 10% of jet exit mass flow) of water to affect the dominant noise producing region. A converging-diverging rectangular nozzle of aspect ratio 4 and a design Mach number of 1.44 is used in this investigation This preliminary study indicates that a minimal amount of water injected at the nozzle exit has produced a modest decrease in the near-field OASPL. The narrow band frequency spectra show that both the shock associated and the mixing noise was reduced by the injection of water. I. INTRODUCTION It is known that supersonic jet noise can be broadly classified into three distinct noise components, the broadband shock associated noise, screech tones, and the turbulent mixing noise. The former two are associated with imperfectly expanded jets. The sources of the shock associated noise are generated by the interaction of shear-layer instabilities with the shock cell structure. 1 ' 2 The screech tones, which are generated by a feedback loop are easily suppressed. 3 However, the suppression of broadband noise is quite difficult and received little attention. One of the objectives of this investigation is to suppress this noise component by the use of water injection. The dominant component of the supersonic jet noise is the turbulent mixing noise. It consists of two parts: the large scale turbulent mixing noise and the fine scale mixing noise. The large scale turbulent mixing noise is created by the shear-layer instabilities of the jet traveling at supersonic speeds relative to the ambient speed of sound. This is analogous to the Mach wave radiation produced by a wavy wall traveling at supersonic speed. The fine scale turbulent mixing noise makes up the background noise of the jet. A more comprehensive explanation of this subject can be found in Tarn. 1 Based upon the success of Tarn's universal spectra model one can conclude that the turbulent mixing noise of the jet is perhaps the most dominant contributor to its OASPL. 4 After studying more than 1500 spectra Tarn et al., concluded that the large scale turbulent mixing noise dominates the downstream direction while the fine scale mixing noise dominates the upstream direction of the jet. 4 A region in between both directions has approximately equal contributions from both turbulent mixing noise components. A novel approach to the noise reduction problem is to use water injection to minimize ' Ph.D. Student Student Member AIAA, f Don Fuqua Professor, Associate Fellow, AIAA 1 American Institute of Aeronautics and Astronautics