Steven Martens

Broadband Shock Associated Noise Suppression by Chevrons

Tests were conducted to determine the ef fect of chevrons on the acoustic emissions from nozzles operating at under expanded conditions. Conical (as baseline) and chevron nozzles were tested on the core (primary) stream in a coaxial nozzle configuration that simulated a high bypass ratio engine e xhaust. The secondary (fan) stream was varied from quiescent condition to Mach=0.85. In a static condition, the chevron nozzle was shown to result in higher shock noise levels by 0.7 dB (OASPL) . Additionally, the shock noise frequency for the chevron had shifted to higher levels . For the M=0.85 condition, the chevron nozzle was shown to result in lower shock noise levels by 2.1 dB (OASPL ). As with the static case, the shock noise frequency for the chevron had shifted to higher values. For all secondary fa n stream velocities, it was shown that the chevron nozzle reduced the shock cell spacing, which lead to a higher frequency for shock noise. These tests can also be considered as an approximation to the effect of free stream on fan chevrons efficiency durin g cruise.

Jet Noise Reduction for High Speed Exhaust Systems

Jet noise has been an environmental issue since the advent of jet aircraft. The past five decades have seen much research into solving this very difficult challenge for a variety of applications. The Supersonic Transport (SST), High Speed Civil Transport (HSCT), and a variety of supersonic business jet (SSBJ) applications all face significant jet noise challenges. Jet noise from high performance military aircraft has also received growing attention. The continuous drive to higher specific thrust results in increasing jet noise levels. Compounding this is that many military bases, Naval in particular, are located in desirable locations on the coast, and surrounding communities are encroaching closer to these bases. In this paper we will conduct a survey of some jet noise reduction technologies for high-speed exhaust systems investigated in the past, as well as some of the implementation issues associated with them. Specific technologies aimed at changing the mixing characteristics of the jet plume after it leaves the nozzle will be discussed in detail, including chevrons and fluidic injection. Other noise reduction technologies, such as the inverted velocity profile, and fluid shield can also change the mixing characteristics of the jet plume. This includes the added benefit of noise reflection or shielding. Measured data will be presented to show the effect these technologies have on high-speed jets.© 2008 ASME

Shock Cell Modification due to Chevrons

*† ‡ Tests were conducted to determine the effect of chevrons on the turbulence and shock cell strength of nozzles operated at under expanded conditions. Baseline (or conical) and chevron nozzles were tested on the core (primary) stream in a coaxial nozzle configuration that simulated a high bypass ratio engine exhaust. The secondary (fan) stream was varied from quiescent condition to Mach = 0.85 to simulate external flow. For all cycle points, turbulence in the region of the shock cells was higher for the chevron configuration than for the baseline configuration. For the three lower coflow Mach numbers (0, 0.28, and 0.50) the strength of the shock cells with the chevron nozzle were comparable in strength to those with the baseline nozzle. For the highest coflow (Mach 0.85), the strength of the shock cells with the chevron nozzle was significantly less than the strength of the shock cells with the baseline nozzle. These results explain acoustic trends from previous work 1 . For the three lower values of coflow, higher turbulence and comparable strength shock cells from the chevron configuration lead to higher shock noise compared to baseline configuration. Alternatively, at the highest coflow (Mach 0.85) slightly higher turbulence and significantly reduced strength shock cells for the chevron configuration lead to lower shock noise compared to the baseline configuration.

Jet Noise Reduction by Fluidic Injection On a Separate Flow Exhaust System

This paper investigates the potential for using fluidic injection on both fan and core streams of a separate flow exhaust system to reduce subsonic turbofan jet noise. Fluidic injection consists of small jets of air injected directly into the shear layer between two jet streams. This injection induces stream-wise vorticity in the flow, which entrains air between the streams and yields greater mixing relative to conventional configurations without fluidic injection. Enhanced mixing has been shown to reduce the mean jet velocity and temperature, therefore, reducing jet noise. Fluidic injection experimental testing was performed in threestream flow at the GE Aviation Anechoic Free-Jet Test Cell 41 Facility. Test results demonstrate jet noise peak sound pressure level reductions of 1 to 2 dB, resulting in a maximum overall sound pressure level reduction of 1.6 dB. The greatest jet noise reduction occurs after optimizing the balance between low frequency noise reduction and high frequency noise generation.

Impact of Mechanical Chevrons on Supersonic Jet Flow and Noise

In this paper, we present observations on the impact of mechanical chevrons on modifying the flow field and noise emanated by supersonic jet flows. These observations are derived from both a monotonically integrated large-eddy simulation (MILES) approach to simulate the near fields of supersonic jet flows and laboratory experiments. The nozzle geometries used in this research are representative of practical engine nozzles. A finite-element flow solver using unstructured grids allows us to model the nozzle geometry accurately and the MILES approach directly computes the large-scale turbulent flow structures. The emphasis of the work is on “off-design” or non-ideally expanded flow conditions. LES for several total pressure ratios under non-ideally expanded flow conditions were simulated and compared to experimental data. The agreement between the predictions and the measurements on the flow field and near-field acoustics is good. After this initial step on validating the computational methodology, the impact of mechanical chevrons on modifying the flow field and hence the near-field acoustics is being investigated. This paper presents the results to date and further details will be presented at the meeting.Copyright

Simultaneous Noise and Performance Measurements for High-Speed Jet Noise Reduction Technologies

Model scale tests are conducted to assess the Noise/Performance trade for high speed jet noise reduction technologies. It is demonstrated that measuring the near field acoustic signature with a microphone array can be used to assess the far field noise using a procedure known as acoustic holography. The near field noise measurement is mathematically propagated producing an estimate of the noise level at the new location. Outward propagation produces an estimate of the far field noise. Propagation toward the jet axis produces the source distribution. Tests are conducted on convergent/divergent nozzles with three different area ratios, and several different chevron geometries. Noise is characterized by two independent processes: Shock cell noise radiating in the forward quadrant is produced when the nozzle is operated at non-ideally expanded conditions. Mach wave radiation propagates into the aft quadrant when the exhaust temperature is elevated. These results show good agreement with actual far field measurements from tests in the GE Cell 41 Acoustic Test Facility. Simultaneous performance measurement shows the change in thrust coefficient for different test conditions and configurations. Chevrons attached to the nozzle exit can reduce the noise by several dB at the expense of a minimal thrust loss.Copyright

Computational aeroacoustics of a s eparate flow exhaust system with eccentric inner nozzle

A major issue related to our modern way of life is the noise caused by jet engine powered airliners. Methods to reduce the jet noise were proposed in the last decades starting with the introduction of high bypass turbofan engines. A numerical investigation of flow and acoustics of a separate flow exhaust system is performed in the present paper. The goal is to investigate how the noise generated by a separate flow jet engine exhaust is influenced if the symmetry of the jet exhaust geometry is broken by having the inner nozzle and the center body off the center line axis of the fan nozzle. A decomposition of flow variables is used that allows separation of flow and acoustic computations. Large Eddy Simulation approach is employed to compute the flow field and the acoustic sources. The inhomogeneous wave equation is used to compute the acoustic near- and far-fields. Using this method, previous numerical studies of the symmetric (baseline) configuration showed good agreement between the computed data and the experimental results. Comparisons between the near- and far-field baseline concentric case and the computations obtained using the eccentric configuration are performed showing an acoustic benefit for the offset case.

Jet Noise Reduction by Fluidicly Enhanced Chevrons On Separate Flow Exhaust Systems

This paper investigates the potential for using fluidic injection to enhance the acoustic performance of conventional chevron exhaust nozzles for separate flow exhaust systems. Fluidic injection consists of small jets of air strategically injected around the nozzle exit. This injection can induce stream-wise vorticity in the flow, which entrains air between the streams and yields greater mixing relative to conventional configurations without fluidic injection. When applied to chevron nozzles, the fluidic injection can also strengthen the vortices generated by the chevrons, delaying vortex breakdown and increasing the mixing across the shear layer. Enhanced mixing has been shown to reduce the mean jet velocity and temperature, thereby reducing jet noise. The experimental testing reported here was performed in two stages. The first stage was conducted at the University of Cincinnati Aeroacoustic Test Facility in single-stream flow. The success of these tests led to the second stage, which was conducted on a dual flow nozzle with simulated external flow, at the GE Aviation Anechoic Free-Jet Test Cell 41 Facility. Test results demonstrated overall sound pressure level jet noise reductions of 1 to 2 dB at each directivity angle for a single-stream exhaust, and approximately 1 dB of reduction for a dual-stream exhaust with a simulated external flow.