Kristjan Gudmundsson

Supersonic Jet Noise from Round and Chevron Nozzles: Experimental Studies

High speed exhaust noise reduction continues to be a research challenge for supersonic cruise business jets as well as for current and future tactical military aircraft. Significant noise reduction may be possible from advanced concepts for controlling instability generated large-scale turbulence structures in the jet shear layer, generally accepted to be the source of aft-angle noise. In response to this opportunity, our team is focused on experimental diagnostic studies and unique instability modeling suited for identifying control strategies to reduce large scale structure noise. The current paper benchmarks the jet noise from supersonic nozzles designed to provide the supporting experimental data and validation of the modeling. Laboratory scale jet noise experiments are presented for a Mach number of Mj = 1.5 with stagnation temperature ratios ranging from Tr=0.75 to 2. The baseline configuration is represented by a round converging-diverging (CD) ideal expansion nozzle. A round CD nozzle with chevrons is included as the first of several planned non-circular geometries directed at demonstrating the impact on large scale structure noise and validating noise prediction methods for geometries of future technological interest. Overexpanded and underexpanded conditions were tested on both nozzle configurations. The resulting data base provides an opportunity to benchmark the statistical characteristics of round and chevron nozzle data. The current paper examines far field spectra, directivity patterns, and overall sound pressure level dependence comparing observed characteristics with the fine scale turbulence noise and large-scale turbulence structure noise characteristics identified by Tam. In addition, the paper probes the effect of chevrons on the developing flow field and suppression of screech tones. Measurements are also reported from a far-field narrow aperture phased array system used to map the acoustic source distribution on the jet axis. The dominant source region, situated between the end of the potential core and the sonic point, was found to agree with the peak amplitude location of the jet near field wavepackets measured using a unique near field array. This observation supports the cause-effect link between large-scale turbulence structures in the shear layer and their dominant contribution to aft radiated far field noise.

Parabolized Stability Equation Models for Turbulent Jets and Their Radiated Sound

In this paper we present several refinements to a wave-packet model of sound generation from large-scale turbulence. We examine heated and unheated jets at Mach numbers of 0.5 and 0.9. Pressure fluctuations associated with large-scale structures are modeled with the Parabolized Stability Equations (PSE) for linear disturbances to the turbulent mean-flow. We show that PSE provides better agreement with near-field microphone-array data at low frequencies than previous models based on linear stability theory. We examine the extent to which microphone data is contaminated by fluctuations uncorrelated with largescale structures. By filtering out the uncorrelated fluctuations, via the proper orthogonal decomposition (POD), better agreement between data and theory is obtained.

Spatial Stability Analysis of Chevron Jet Profiles

We investigate the linear stability characteristics of mean flows produced by round, and chevron nozzles. We derive a Rayleigh equation for the chevron profile, which allows the fast solution of the chevron stability problem. Using PIV and RANS data, we compute the stability characteristics of various chevron/round nozzles. We find there are two main dierences between the chevron and round jet: chevron jet growth rates are highly suppressed and peak growth rates shifted to lower frequencies, and phase speeds are somewhat increased. Some preliminary implications on sound generation are discussed. We compare our instability wave results to microphone measurements taken with a phased hydrodynamic array. Our results indicate that the hydrodynamic pressure field of both round, and chevron jets is consistent with that of the instability modes of the turbulent, spreading mean flow.

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.

An Integrated RANS-PSE-Wave Packet Tool for the Prediction of Subsonic and Supersonic Jet Noise

Few engineering tools are suitable for predicting supersonic jet noise, and the development of engine exhaust noise reduction technology for tactical aircraft continues to rely heavily on laboratory scale parametric testing. Aside from intuition and experience with the generally subtle issues involved in low-noise design, there is currently no way to rapidly and cheaply assess whether proposed designs will be effective, and no way to determine whether such designs are optimal. Arguably, this gap in jet noise modeling capability is an impediment toward achieving significant noise reduction for tactical aircraft. The objective of the on-going research program presented in this paper is to develop and demonstrate innovative, highly-efficient computational methodologies for simultaneous nozzle acoustic and aerodynamic design applicable to subsonic and supersonic jet exhaust noise reduction in tactical aircraft. The approach comprises of three major elements: (1) Reynolds-averaged NavierStokes (RANS)-CFD for computing the jet turbulent mean flow, (2) pressure wave packet-based methods for predicting near-field sound generation from the largest scales, based on the Linear Parabolized Stability Equations (LPSE) and the aforementioned RANS solutions, and (3) a method based on the solution of the linear wave equation for determining the acoustic radiation field from the LPSE solution. While these three procedures have received significant attention in the literature, their integration into a single tool for far-field noise prediction has not. To assess the accuracy and robustness of the simulation tool, experimental data has been acquired with near field array directed at detecting changes in the organized turbulence scale structure to link cause (nozzle geometry) and effect (near field and far field noise changes). Both ideallyand non-ideally expanded conditions are being investigated. Forward flight effects have also been measured using an open jet acoustic wind tunnel (at United Technologies Research Center) to evaluate the noise dependence on different operating conditions. The development and preliminary validation of the integrated tool is presented in this paper, with a focus on individual components and their translation into a common format for integration.

Linear Stability Analysis of Chevron Jet Profiles

We investigate the linear stability characteristics of the mean velocity profiles produced by chevron nozzles. We show that chevron instability waves can be decomposed into azimuthal modes analogously to those of round jets. This facilitates a direct comparison of growth rates and mode structure between different nozzles. We find that the three nozzles used in this study share a set of modes, referred to as primary modes. In addition, we find that there exist modes unique to the chevrons nozzles, termed secondary modes. While chevron jets possess a much larger number of unstable modes, the modes with lowest azimuthal structure show strong suppression of growth rates in two different chevron jets. Some preliminary implications on sound generation are discussed.© 2006 ASME