Noise reduction by interaction of flexible filaments with subsonic jets

Abstract

Experiments were conducted at the Boeing Interior Noise Test Facility (INTF) to evaluate the reduction of jet noise by a small diameter cord in a cold subsonic jet plume Different cord diameters, lengths and styles were used to determine the optimum configuration for noise reduction The optimized configuration of the cord was multiple stranded Kevlar fibers, 6 3 nozzle diameters long and having a thickness of 2% of the primary nozzle diameter A small knot at the downstream tip of the cord stabilized it along the jet centerline The tests were performed using a single jet and coaxial cold jet The jet velocities ranged from 7 to 9 Mach for the primary jet Acoustic and flow visualization data showed the effect of the cord on the jet noise and on the flow structure The filament changed both magnitude and directivity of the sound emission The sound field became nearly uniform, with predominant perpendicular orientation With a 6 3 nozzle diameter long multiple stranded cord in place noise reductions occurred in the lower frequencies up to 4OkHz with a maximum reduction of 2dB in the far field INTRODUCTION The advent of jet engine as a power plant for military and civil aircraft after the Second World War, and its counterpart, jet engine noise, initiated substantial research on the sources and causes of jet noise, as well as methods and devices for its reduction The noise level of jet engines, particularly during takeoff and climb, is in the range of 100 to 120 decibels (dB), and is often a concern for people living near airports Such high noise levels can limit future airport air traffic expansion, and force new airports to occupy remote sites New requirements for lower jet noise are a continued area of interest both by governmental agencies around the world and by neighborhoods located in close proximity to airports, flight paths and to engine and flight vehicle manufacturers Due to these concerns a need to further jet noise reduction technology is in demand Various approaches have been used or proposed to overcome the noise issue ‘. ’ Earlier solutions have proposed so-called “daisy nozzles” or tubular nozzles to break up the jet in an effort to reduce noise More recently, an “ejector suppresser” has been proposed, comprising a “shroud-like” device, which is acoustically lined to reduce noise during takeoff However, this approach may add as much as 5,000 pounds per engine to the wings of the airplane It is dead weight, and results in loss of thrust and increase in drag of the airframe It also reduces the aircraft overall efficiency and increase its specific fuel consumption Such noise suppresser should not add significant weight to the craft nor adversely affect thrust during cruise Development of such devise requires basic understanding of the noise generation mechanisms This paper presents a new technique for jet noise reduction that is easy to implement and which does not carry heavy penalty in extra dead weight, loss of thrust and reduced propulsion efficiency The technique was previously shown to be most effective for the underexpanded jet flow conditions described in a previous paper on this topic 3 The present paper shows that noise reductions can occur also in subsonic single jet flows The sources of jet noise were initially assumed to be related to the small eddies which form fine-scale turbulence However, Crow and Champagne4 and Brown and Roshko’ found that turbulence in jets and free shear flows comprised of not only fine-scale turbulence, but also large turbulent structures Largescale turbulence dominates the mixing process of jet and shear layers The two major sources of noise in subsonic jets and in fully expanded supersonic jets are the large scale coherent structures and the small-scale turbulence 6. 7 However, in subsonic jets the total acoustic power is much lower than in supersonic jets (-Uj*S) Also, even though coherent structures participate in the mixing process of subsonic jets, the exact mechanism of their contribution to noise generation is not clear Many researchers believe that noise produced by fine-scale turbulence constitute the dominant part of subsonic jets noise a. ’ Large scale 1 American Institute of Aeronautics and Astronautics