Noise, turbulence, and thrust of subsonic freejets from lobed nozzles

A study of noise benefit, vis-à-vis thrust penalty, and its correlation to turbulence intensities was conducted for free jets issuing from lobed nozzles. Four convergent nozzles with constant exit area were used in the experiments. Three of these were of rectangular lobed configuration having six, ten and fourteen lobes; the fourth was a circular nozzle. Increasing the number of lobes resulted in a progressive reduction in the turbulence intensities as well as in the overall radiated noise. The noise reduction was pronounced at the low frequency end of the spectrum. However, there was an increase in the high frequency noise that rendered the overall benefit less attractive when compared on a scaled-up A-weighted basis. A reduction in noise was accompanied by a commensurate reduction in the turbulent kinetic energy in the flow field. As expected, increasing the number of lobes involved progressive reduction in the thrust coefficient. Among the cases studied, the six-lobed nozzle had the optimum reduction in turbulence and noise with the least thrust penalty. INTRODUCTION This experimental investigation was prompted by an earlier study in which far-field noise of high subsonic jets from rectangular, tabbed and lobed nozzles were reported. A six-lobed nozzle exhibited significantly lower noise compared to the other cases. Later, through limited experiments, the lobed nozzle was also found to involve remarkably lower turbulence intensities. A detailed study of the noise benefit, the Copyright © 2002 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty –free license to exercise all rights under the copyright claimed herein for Governmental Purposes. All other rights are reserved by the copyright owner. * Aerospace Engineer, Turbomachinery and Propulsion Systems Division, Associate Fellow, AIAA. Presently with John A. Volpe Center, U.S. Department of Transportation, Cambridge, MA, Senior Member, AIAA. corresponding thrust penalty, and its correlation to the structure of the turbulent flow field was considered a worthy effort. This led to the present investigation. ‘Lobed forced mixers’ or corrugated splitter plates are often used in the Industry in order to achieve efficient mixing between two streams. There have been several studies of a basic two-stream mixing layer originating from such a splitter plate. One obvious effect of the lobed geometry is the increase in the interfacial area between the two streams that enhances the mixing. Depending on the geometry, the lobes can also introduce pairs of counter-rotating streamwise vortices that efficiently transport momentum and species across the mixing layer. Cross-stream components of vorticity shed from the trailing edges of the lobes, and their subsequent dynamics, can also play a role in the mixing enhancement. Many researchers have investigated jet flows from nozzles with lobed exit lips or other modified shapes. Faster spreading of the jet was observed in all instances. Furthermore, some reduction in jet noise with the use of lobed nozzles has been observed in some of the cited work as well as in the Industry. Unfortunately, because of the complex geometry and large parameter-space the underlying flow mechanisms have remained far from being completely understood. The processes that impact the noise field of these nozzles are even less understood at this time. It is apparent that further model-scale experiments would be helpful for advancing the understanding and providing a database for developing engineering correlations for the prediction of mixing and noise. It is with this spirit the present investigation was initiated. The objective has been to measure the flow and noise fields in detail for a systematic parametric variation. Because of the many parameters, only limited variation was possible. The goal was set to examine the effect of the number of lobes for a fixed exit area of the nozzle. The nozzles were convergent and the experiments involved ‘cold’, free jets. Most of the data were