Robert H. Schlinker

Unstructured Large Eddy Simulation Technology for Prediction and Control of Jet Noise

Development of concepts for reduction of jet noise has relied heavily on expensive experimental testing of various nozzle designs. For example, the design of nozzle serrations (chevron) and internal mixer/ejector nozzles have relied largely on laboratory and full-scale testing. Without a deeper understanding of the sources of high-speed jet noise it is very difficult to effectively design configurations that reduce the noise and maintain other performance metrics such as nozzle thrust. In addition, the high complexity of the flow limits the success of a parametric black-box optimization.Copyright

Sound Radiated by Large-Scale Wave-Packets in Subsonic and Supersonic Jets

A wave-packet Ansatz is used to model jet noise generation by large-scale turbulence. In this approach, an equivalent source is deflned based on the two-point space-time correlation of pressure on a conical surface surrounding the jet plume. The surface is su‐ciently near the jet to be dominated by pressure signatures of large-scale turbulence, yet su‐ciently far that linear behavior can be assumed in extending the near-fleld pressure to the acoustic fleld. In the present study, a rotating near-fleld microphone array is used to measure multipoint pressure statistics on the conical surface in order to identify model parameters, and a Green’s function based method is used to project the near-fleld pressure to the acoustic fleld. It is shown that the near-fleld pressure signatures are well-represented by a Gaussian wave-packet model for both subsonic and supersonic jets. The acoustic fleld re-constructed from the near-fleld data is shown to compare very well with far-fleld measurements. Results suggest that the source of low-frequency aft-angle sound in subsonic jets is dynamically similar to that in supersonic jets.

Measurement of Source Wave-Packets in High-Speed Jets and Connection to Far-Field Sound

A novel rotating near-fleld microphone array is used to measure multi-point pressure statistics on a conical surface surrounding the jet plume, just outside the turbulent shear layer. The microphone array extends axially to 10 jet diameters, with a maximum radial distance of 2 diameters from the jet centerline. A Green’s function based method is used to project the near-fleld pressure to the acoustic fleld. The diagnostic method is an extension of the approach previously described by Reba et al., 1 relying on measurement of equivalent noise sources described by second order statistics of a scalar quantity (pressure), rather than fourth-order statistics of a vector quantity over a volume as required by approaches based on the Lighthill Acoustic Analogy. Although the source description adopted here is less fundamental than that of Lighthill, it can be measured experimentally with relative ease. The diagnostic method is applied to a Mj = 1:5 ideally expanded jet over a range of temperature ratios, with acoustic Mach numbers from 1 to 2. It is shown that the near-fleld pressure statistics are well-represented by a Gaussian wave-packet model. Model parameters include convection speed, spatial source extent, and streamwise correlation scale. The acoustic fleld re-constructed from the near-fleld data is compared to direct far-fleld measurements and shown to give very good agreement.

Noise Prediction of Pressure-Mismatched Jets Using Unstructured Large Eddy Simulation

It is our premise that significant new advances in the understanding of noise generation mechanisms for jets and realistic methods for reducing this noise can be developed by exploiting high-fidelity computational fluid dynamics: namely large eddy simulation (LES). In LES, the important energy-containing structures in the flow are resolved explicitly, resulting in a time-dependent, three-dimensional realization of the turbulent flow. In the context of LES, the unsteady flow occurring in the jet plume (and its associated sound) can be accurately predicted without resort to adjustable empirical models. In such a framework, the nozzle geometry can be included to directly influence the turbulent flow including its coherent and fine-scale motions. The effects of propulsion system design choices and issues of integration with the airframe can also be logically addressed.Copyright

Unstructured large eddy simulation of a hot supersonic over-expanded jet with chevrons

Large eddy simulations (LES) are performed for a heated over-expanded supersonic turbulent jet issued from a converging-diverging round nozzle with chevrons. The unsteady flow processes and shock/turbulence interactions are investigated with the unstructured LES framework developed at Cascade Technologies. In this study, the complex geometry of the nozzle and chevrons (12 counts, 6 � penetration) are explicitly included in the computational domain using unstructured body-fitted mesh and adaptive grid refinement. Sound radiation from the jet is computed using an efficient frequency-domain implementation of the Ffowcs Williams–Hawkings equation. Noise predictions are compared to experimental measurements carried out at the United Technologies Research Center for the same nozzle and operating conditions. The initial blind comparisons show good agreement in terms of spectra shape and levels for both the near-field and far-field noise. Additional analysis of the large database generated by the LES is ongoing. Preliminary results show that the current simulation captures the main flow and noise features, including the shock cells, broadband shock-associated noise and turbulent mixing noise.

Decomposition of High Speed Jet Noise: Source Characteristics and Propagation Effects

Current research programs directed at supersonic engine exhaust noise reduction are demonstrating benefits of 3-4 dBA using passive methods to increase jet mixing and break up shock cells in over-expanded flows. While progress is being made, high speed jet noise continues to be a research challenge for small business jets and tactical military aircraft. The current work benchmarks high speed jet noise using laboratory scale jets for the purpose of a) identifying source and propagation mechanisms, and b) providing validation data for simulation/modeling methods. Laboratory scale experiments are presented over a Mach number range of M = 0.68 to 1.5 with static temperature ratio ranging from Tr = 0.68 to 2. A unique near field rotating phased microphone array technique was used to identify the large-scale turbulence structure noise source and Mach waves in supersonic shock-free jets. A companion paper documents the near field pressure statistics and projection of the convected wave packet to the far field. Validation against the directly measured far field levels quantitatively establishes the large scale structure noise contributions. The combined studies underpin a long term effort to develop modeling methods and new concepts for jet noise suppression based on controlling the evolution of the large-scale turbulence structures.

Flight Effects on Supersonic Jet Noise from Chevron Nozzles

Currently tactical aircraft supersonic jet noise studies are conducted at near static conditions motivated by the need to develop nozzle concepts for mitigating aircraft carrier launch crew noise levels. Effects of forward flight on supersonic jet noise source mechanisms need to be included as the next step to demonstrating concept benefits for community noise reduction during training scenarios where aircraft operate from land based airfields. Hence, the current study was directed at the flight effects on far field noise from round and chevron nozzles in model scale experiments conducted in the United Technologies Research Center (UTRC) open jet acoustic wind tunnel. The objective was to document the statistical characteristics of the noise at tunnel Mach numbers up to Mt =0.4 since there is limited generic data for military engine supersonic exhaust conditions under such forward flight scenarios. The study focuses on the noise characteristics in the aft quadrant which dominates the tactical aircraft noise signature. A generic chevron nozzle, developed for noise reduction, was included in the evaluation as part of tracking the noise reduction consistency with increasing forward flight. The chevron geometry consisted of the baseline shock-free round nozzle with 12 chevrons attached at the nozzle exit. Exit diameter corresponded to 2” for both nozzles. Static temperature ratios ranged from isothermal to Tr =2. The impact of chevrons on impulsive signature skewness reduction was tracked in the acoustic far field at near static conditions. Both the round and chevron nozzle were also operated at off-design conditions with over and underexpanded nozzle pressure ratios (NPR) for the skewness study.

Towards Prediction and Control of Large Scale Turbulent Structure Supersonic Jet Noise

In this paper, we report on progress towards developing physics-based models of sound generation by large-scale turbulent structures in supersonic jet shear layers generally accepted to be the source of aft-angle noise. Aside from obtaining better engineering prediction schemes, the development and optimization of long term jet noise reduction strategies based on controlling instability wave generated largescale turbulence structures in the shear layer can be more successful if based on predictive flow-noise models, rather than on build and test approaches alone. Such models, if successful, may also provide a path by which laboratory scale demonstrations can be more reliably translated to engine scale. Results show that the noise radiated by large-scale structures in turbulent jet shear layers may be modeled using a RANS based PSE method and projected to the far-field using a Kirchhoff surface approach. A key enabler in this procedure is the development of near-field microphone arrays capable of providing the pressure statistics needed to validate the instability wave models. Our framework provides, for the first time, a deterministic model that will allow understanding and predicting noise radiated by large-scale turbulence.

Chevron Nozzle Eects on Wavepacket Sources in a Supersonic Jet

A rotating microphone array is used to detect pressure signatures of large-scale turbulence in the hydrodynamic near eld of a supersonic isothermal jet both with and without chevrons. Structure eduction techniques based on proper orthogonal decomposition (POD) are combined with a Green’s function based acoustic projection method to identify the dominant noise producing event. It is found that the dominant event contributing to Mach wave emission has the form of a space-time wavepacket. Results show that the observed far eld noise reduction with addition of chevrons can be linked quantitatively to suppression of the wavepacket growth rate. Implications of these ndings for instability-based modeling of chevrons are discussed.

Simulated Flight Effects on Supersonic Jet Noise from Round Nozzles

Given the increased understanding of organized structure noise generation and the evolving efforts to control this mechanism in supersonic jet exhaust flows, it is appropriate to look ahead and verify that the physics of this mechanism continue to hold under forward flight conditions. For this reason, a forward flight acoustic study was initiated as part of the supersonic jet noise diagnostics being conducted by UTRC. The current study documents the jet noise statistical characteristics as a first step before introducing a near field hydrodynamic array to detect the wavepacket-far field link previously confirmed at static conditions. In support of these longer term goals, flight effects are presented for a laboratory scale 3” diameter converging-diverging nozzle designed for ideal expansion at a jet Mach number of Mj=1.5 and operated at a static temperature ratio of Tr=0.98. Overexpanded and underexpanded nozzle pressure ratios (NPR’s) are presented for forward flight Mach numbers Mt=0.1, 0.2, 0.3 and in some cases 0.4. The data base benchmarks the overall sound pressure level (OASPL), far field spectra, and directivity patterns as a function of NPR and forward flight. OASPL trends were found to show deep noise minima ranging from 4-8dB near the design NPR for directivity angles corresponding to sideline angles of 80-90 degrees. A weaker impact was observed at 110 degrees from the inlet with minimal impact at extreme aft angles. One third octave band and narrow band reductions were found to be approximately uniform across the spectrum at angles below 130 degrees and Strouhal (St) values below 2. Reductions are approximately 4-5 dB as Mt increases by 0.2 at these conditions. At extreme aft angles, reductions are more pronounced and on the order of 10dB or larger. However, reductions are no longer uniform across the spectrum, as at lower tunnel speeds, becoming weaker and tending towards 5dB or less as St values approach 10. Directivity pattern reductions are much more pronounced at 150 degrees compared to the reductions in the 90-120 degree range. However, the aft directivity pattern dominance is retained for low frequencies as forward flight speed increases suggesting that organized structure noise is still operative and dominates this portion of the spectrum.