G. W. Butler

The effect of streamwise vortices on the aeroacoustics of a Mach 0.9 jet

The role of the streamwise vortices on the aeroacoustics of a Mach 0.9 axisymmetric jet is investigated using two different devices to generate streamwise vortices: microjets and chevrons. The resultant acoustic field is mapped by sideline microphones and a microphone phased array. The flow-field characteristics within the first few diameters of the nozzle exit are obtained using stereoscopic particle image velocimetry (PIV). The flow-field measurements reveal that the counter-rotating streamwise vortex pairs generated by microjets are located primarily at the high-speed side of the initial shear layer. In contrast, the chevrons generate vortices of greater strength that reside mostly on the low-speed side. Although the magnitude of the chevrons axial vorticity is initially higher, it decays more rapidly with downstream distance. As a result, their influence is confined to a smaller region of the jet. The axial vorticity generated by both devices produces an increase in local entrainment and mixing, increasing the near-field turbulence levels. It is argued that the increase in high-frequency sound pressure levels (SPL) commonly observed in the far-field noise spectrum is due to the increase in the turbulence levels close to the jet exit on the high-speed side of the shear layer. The greater persistence and lower strength of the streamwise vortices generated by microjets appear to shift the cross-over frequencies to higher values and minimize the high-frequency lift in the far-field spectrum. The measured overall sound pressure level (OASPL) shows that microjet injection provides relatively uniform noise suppression for a wider range of sound radiation angles when compared to that of a chevron nozzle.

Boeing's variable geometry chevron: morphing aerospace structures for jet noise reduction

Boeing is applying cutting edge smart material actuators to the next generation morphing technologies for aircraft. This effort has led to the Variable Geometry Chevrons (VGC), which utilize compact, light weight, and robust shape memory alloy (SMA) actuators. These actuators morph the shape of chevrons on the trailing edge of a jet engine in order to optimize acoustic and performance objectives at multiple flight conditions. We have demonstrated a technical readiness level of 7 by successfully flight testing the VGCs on a Boeing 777-300ER with GE-115B engines. In this paper we describe the VGC design, development and performance during flight test. Autonomous operation of the VGCs, which did not require a control system or aircraft power, was demonstrated. A parametric study was conducted showing the influence of VGC configurations on shockcell generated cabin noise reduction during cruise. The VGC system provided a robust test vehicle to explore chevron configurations for community and shockcell noise reduction. Most importantly, the VGC concept demonstrated an exciting capability to optimize jet nozzle performance at multiple flight conditions.

Variable Geometry Chevrons for Jet Noise Reduction

Boeing is applying cutting edge smart material actuators to the next generation morphing technologies for aircraft. This effort has led to the Variable Geometry Chevrons (VGC), which utilize compact, light weight, and robust shape memory alloy (SMA) actuators. These actuators morph the shape of chevrons on a jet engine fan nozzle trailing edge in order to optimize acoustic and performance objectives at multiple flight conditions. We have completed a flight test of the VGC system on a Boeing 777-300ER with GE-115B engines. In this paper we describe the VGC design, development and performance during flight test. We demonstrated autonomous operation of the VGCs, which did not require a control system or aircraft power. The VGC concept demonstrated an exciting capability to optimize jet nozzle performance at multiple flight conditions. The VGC system provided a robust test vehicle to explore chevron configurations for community and shock-cell noise reduction. This capability was demonstrated with two examples of a parametric study which showed the influence of VGC configurations on community noise reduction and shock-cell generated cabin noise reduction during cruise.

Fiber-Optic Shape Sensing and Distributed Strain Measurements on a Morphing Chevron

Boeing has recently flight tested a Variable Geometry Chevron (VGC) system which used shape memory alloy (NiTinol) actuators to drive changes in shape. At take off, the VGCs immersed into the fan stream to reduce jet noise; at cruise they were actively morphed to investigate shock cell noise and performance losses. A set of three strain gages mounted on each chevron provided estimates of its tip position (shape) and feedback to the on-board control system. During the development of the VGC flight system, Luna Innovations instrumented two VGC test articles with shape probes, a new technology in which multi-core fiber provides distributed and axially co-located differential strain measurements to generate complex shape data. This technology shows promise of providing a more direct correlation of NiTinol actuation to chevron shape and tip immersion. Additionally, Luna Innovations instrumented each VGC with high density distributed strain fiber to provide hundreds of discrete strain measurements over the surface of the chevron.

Subsonic Jet Noise Reduction Variable Geometry Chevron

Turbulent mixing of jet engine exhaust streams contributes significantly to airport community noise. Serrated aerodynamic devices along the trailing edge of exhaust nozzles, known as chevrons, have been shown to greatly reduce jet noise by promoting advantageous mixing of the streams. The design of these devices represents a delicate compromise between noise reduction at take off conditions, engine operability, and thrust loss at cruise. We report here the successful design and test of a full-scale variable geometry chevron for the fan-nozzle exhaust whose shape change is driven by a shape memory alloy actuator. The collected data supports the aerodynamic, thermal, and mechanical modeling of the chevron. The test results demonstrate that the device can operate autonomously and can reproducibly transition between take-off and cruise shapes. Having validated the concept, this data will facilitate the development of the next generation, variable geometry chevron.

High frequency jet nozzle actuators for jet noise reduction

Rules governing airport noise levels are becoming more restrictive and will soon affect the operation of commercial air traffic. Sound produced by jet engine exhaust, particularly during takeoff, is a major contributor to the community noise problem. The noise spectrum is broadband in character and is produced by turbulent mixing of primary, secondary, and ambient streams of the jet engine exhaust. As a potential approach to controlling the noise levels, piezoelectric bimorph actuators have been tailored to enhance the mixing of a single jet with its quiescent environment. The actuators are located at the edge of the nozzle and protrude into the exhaust stream. Several actuator configurations were considered to target two excitation frequencies, 250 Hz and 900 Hz, closely coupled to the naturally unstable frequencies of the mixing process. The piezoelectric actuators were constructed of 10 mil thick d31 poled wafer PZT-5A material bonded to either 10 or 20 mil thick spring steel substrates. Linear analytical beam models and NASTRAN finite element models were used to predict and assess the dynamic performance of the actuators. Experimental mechanical and electrical performance measurements were used to validate the models. A 3 inch diameter nozzle was fitted with actuators and tested in the Boeing Quiet Air Facility with the jet velocity varied from 50 to 1000 ft/s. Performance was evaluated using near-field and far-field acoustic data, flow visualization, and actuator health data. The overall sound pressure level produced from the 3 inch diameter jet illustrates the effect of both static and active actuators.

Significant Improvements on Jet Noise Reduction by Chevron-Microjet Combination

The two successful noise reduction methods, chevrons and microjets, are used simultaneously such that the combination will be more beneficial than each method alone. As such, the jet noise is lowered significantly below the previous levels observed from the best of either method. The unique feature of this method is that it applies microjets (small diameter fluidic injections) to the shear layer at the tip of the chevrons. The strategic location of the microjet-chevron injection is a critical design parameter and is based on our understanding of the chevron and microjet flow physics. Previous detailed flow measurements and improved understanding of the flow physics of both individual devices 1 indicated that a potential synergy might have existed. Subsequent proof-of-concept tests in Boeing’s Quiet Air Facility (QAF) have obtained a reduction in noise levels not previously achieved. Initially performed acoustic tests show that the OASPL benefits are nearly additive in all measured radiation directions. For instance, at a 150 degree nozzle inlet angle and at 100 D distance the isolated chevron and/or microjet reduction is only about 1 dB, whereas the microjet-chevron combination is 2 dB. The narrow band spectra at aft angles also show the better performance obtained by the combination at a large range of frequencies. Furthermore, no additional high frequency lift has been observed in any measured aft radiation angle with the application of microjets in combination compared to isolated chevron case.

Modeling of Piezoelectric Actuators for Active Control of Jet noise

Many investigators have successfully used acoustic or mechanical actuators to promote increased mixing in the initial shear layers of jets. Actuator designs for active noise control must be further refined to control the production of large scale turbulence and its contribution to the low frequency jet noise spectrum. Although many concepts reduce energy associated with large turbulence scales, a commensurate energy increase at smaller scales is often observed. Actuators better tuned to the flow and acoustic environment are needed if active flow control is to successfully negotiate this difficulty. This work focuses on the development of a numerical performance model to better understand the coupling between the actuators and the turbulent flow field. Los Alamos National Laboratory’s CFDLib provides the framework for our modeling effort. We have incorporated a “realizable” turbulence model, spatial filtering to better resolve the turbulent structures, and numerical sampling to generate time-averaged turbulence parameters. Performance predictions are presented for a cold, M=0.9 jet issuing from a nozzle fitted with 13 piezoelectric tab-type actuators. Comparisons of predictions with and without actuators demonstrate the potential benefit of this approach.