A study of innovative techniques in jet noise attenuation

A previous study, by the current investigators, determined that a flexible filament attached to the centeiiine of an underexpanded, supersonic jet is quite effective at attenuating jet noise over a wide frequency range. As this study focused primarily on discrete inlet angles, an additional study has been completed in which the overall sound pressure level (OASPL) was mapped over the jet near field. The results of this study prove that the filament in effective over the entire mapped area in addition to the discrete inlet angles seen previously. Specifically, the filament eliminates the strong directionaf gradients in OASPL, which are associated with screech tones. This produces a much more uniform and stable sound field. The peak OASPL is reduced by 12dB from I52dB to I40dB and an attenuation of approximately 10dB is observed over all areas of the near field mapped. An interesting effect is observed as the location of the peak OASPL is displaced downstream of the nozzle lip by a distance of 6-7 nozzle diameters. This could possibly provide some physical explanation for the effectiveness with which the filament attenuates screech. A mapping of the near field of a subsonic jet indicated that the filament is successful in reducing turbulent mixing noise by as much as 2dB in the rear quadrant. It was determined that an additional study should be conducted using multiple filaments attached to the nozzle periphery where they can interact with structures in the shear layer of the jet. A detailed flow field analysis of the supersonic jet, using a stereoscopic PIV system, revealed that the filament is successful in extensively modifying the structures in the exhaust plume. The filament is observed to significantly weaken the shocks and reduce the spacing, as well as increasing the jet spread and reducing the downstream velocities. Each of these factors can be tied directly to the measured reduction in OASPL throughout the near field. Introduction Shortly after the introduetion-of thejet engine,, during the Second WorkLWarr iLbecame apparent that methods were needed to mitigate the negative effects of jet noise. These effects range from premature or unpredicted airframe or component failure due to acoustic excitation to general public annoyance. The first step to be taken to combat this problem was the establishment of dedicated research to understanding the problem of jet noise. This early research reached its pinnacle when Sir James Lighthill published the acoustic analogy theory inr 1952 . This concluded that the acoustic power produced by ar jet is proportional tcx the eighth power of the jet velocity. This discovery ted to the development of corrugated suppressor nozzles, which aimed ta decrease the exhaust velocity by both increasing the effective circumference of the nozzle and improving mixmg of the exhaust with the ambient air. By far the most important advance made in the field of jet noise reduction thus far is the introduction of the high by-pass turbofan engine in the WKf s*: This introduction drastically reduced engine noise levels from the !3fr-I46d& range to tBe K&-£ £5dB range commonly seen with: modem engines. Currently, there are many approaches being utilized ta altow both new design and existing engines to comply with the increasingly stringent federal and internatioflal noise regulations. These approaches include the use of acoustic liners to absorb acoustic energy in the nacelle as well as the rediscovery erf the corrugated or "lobed" suppressor type nozzles. The major drawback to these techniques is that they aft result in fairly significant performance penalties. This research program is focusing on t&eee innovative techniques that w2t impose little or na performance penalty on tfie engine. In addition, the techniques being considered are equally applicable to state erf the art engines still hr the design phase as well as to older engines in their final years of service. The first technique is a continuation of research into the use of flexible filaments attached to the jet centerline^. Results have proven the filament to be capable of interacting with and modifying the flow structure. In addition to modifying the largescale structures in the flow, the filament is (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. successful in modifying the shock cell structures and in near complete suppression of screech. Finally, the filament absorbs acoustical energy from the fine scale turbulence by converting it to mechanical vibrational energy. Previous results for the centerline filament focused primarily on acoustical measurements taken at discrete inlet angles. In order to better understand and quantify the filament effect on the entire sound field of the jety complete mapping_of the jet overall sound pressure level (OASPL) was performed for two. different operating_conditions. In addition to this, a stereoscopic particle imaging veloeimetry (PIV) system. wasL used to closely observe the effect of the filament on the flow structures. The second innovative technique still makes use erf the flexible filaments but is more applicable to subsonic jets, which lade significant large-scale flow structures hr the jet core. This technique consists of attaching small filaments to the circumference erf the irezzte. These filaments will then perform: much the same function as the centerline filament but by interacting with the jet shear layers rather than the jet flow itself Finally, the third technique consists of modifying the nozzle gsumcUy te include chevrons or non-circular geometries. Experimental Facility Experiments were conducted in the Louisiana State University Anechoic Jet Facility. The LSU Anechoic Jet Facility consists of a compressed air jet in an acoustically treated facility measuring 21' x 18' x 14' with a lower cutoff frequency of approximately 400Hz. The nozzle has a 1.5" exit diameter sad is capable of a maximum Mach number of 2.0. A complete description of the facility is given by CaUender and Gutmark. Near field sound surveys were conducted with the jet operating at nozzle pressure ratios of 2.7 and 1.7. These pressure ratios correspond to exit Mach numbers of 1.3 and 0.9 respectively. All tests were conducted with the jet operating at ambient temperature. The acoustical data was obtained using a Bruel & Kjaer model 4191, W condenser microphone, which has upper and lower frequency limits of 40 kHz and 3.15 Hz respectively. A two dimensional computer controlled traversing mechanism was used to accurately position the microphone, while data was acquired using a National Instrument PCI-MO-16E-1 data acquisition board. Two separate rectangular grid patterns were used in order to obtain a complete mapping of the near field. Figure 1 shows the coverage area and orientation of each of these grids. The downstream grid consisted of 195 points while the upstream grid contained only 75 points. In order to keep the microphone out of the jet flow, the downstream grid was oriented with a 7° divergence angle to the. jet centerline. The upstream grid was set at the nozzle angle of 12° in order to maintain a constant radial distance from the nozzle when traversing in the axial direction. In the downstream direction, the measurements extended ta a distance of 12 nozzle diameters, 12D, with a spacing of 1.5" (ID). In the upstream grid, the measurements extend to a distance of 4D upstream with ID spacing^ Measurements for both grids extend to a distance of 7D with 0.5D spacing in the radial direction. inlet Angle,? i Radial Distance, r Nozzle Diameter, 0 Figure 1: Position and Orientation of Grid Patterns As described in previous works, the filament is attached along the centerline of the jet . A centerbody device, which terminates two inches upstream of the nozzle exit plane, is used to hold the filament. The filament length, L, is defined as the distance from the nozzle lip to the end of the filament. A small knot is tied approximately A" from the end of the filament to help stabilize the filament in the flow. The method of attachment is illustrated in Figure 2. In the tests conducted in the LSU Anechoic Jet Facility, Kevlar strands were used as the filament material.