Coaxial Jet Noise Research Articles

Empirical prediction of flight effect on subsonic coaxial-jet noise by introducing an adjusted flight velocity term

The effect of forward flight on jet noise is difficult to quantify through flyover tests since only the total noise is measured in a full-scale flyover test, and the contribution of the jet noise is difficult and sometimes nearly impossible to identify. Thus, most studies on the flight effect have been carried out through model-scale experiments with a single-stream jet simulator in a free jet facility. In this paper, the effect of forward flight was captured by using an adjusted flight velocity term (αV) to describe jet velocity in a new prediction of coaxial-jet noise. The new jet noise prediction method assumes that there are three components: primary, secondary, and mixed components with no filter functions. The coefficient α is determined by a thorough investigation of the model-scale data gained from an experiment in the anechoic wind tunnel of ONERA. The value of α is 1 for the primary component, 0.5 for the secondary component, and a linear function of the angle for the mixed component. The simple adjustment of the flight velocity successfully embodied the effect of forward flight at all angles, with no separate velocity exponent or an additional term.

Aeroacoustics Analysis of a Dual-Stream Subsonic Jet

This work was part of a major investigation for coaxial jet noise prediction and refers to a study of geometrical and velocity parameters and their influence on the noise generated by coaxial nozzles. The Reynolds Averaged Navier-Stokes (RANS) approach coupled with a fluctuation synthetization model and the integral formulation of Curle's Acoustic Analogy were employed to calculate the noise spectrum and compare it to experimental data from JEAN EU project.

Flow and Noise Predictions of Coaxial Jets

Flow and noise solutions of the two Large-Eddy Simulation (LES) approaches are evaluated for the jet flow conditions corresponding to a benchmark coaxial jet case from the European Union Computation of Coaxial Jet Noise experiment. The jet is heated and issues for a short-cowl axisymmetric nozzle with a central body at a transonic speed. The first LES method is based on the Compact Accurately Boundary-Adjusting High-Resolution Technique (CABARET) scheme, for which implementation features include asynchronous time stepping at an optimal Courant–Friedrichs–Lewy number, a wall model, and a synthetic turbulence inflow boundary condition. The CABARET LES is implemented on Graphics Processing Units. The second LES approach is based on the hybrid Reynolds-average Navier–Stokes (RANS)/implicit LES method that uses a mixture of a high-order Roe and Monotonicity-Preserving reconstruction of the 9th order (MP9 scheme) and a wall distance model of the improved delayed detached-eddy simulation type. The RANS/implicit LES method is run on a Message Passing Interface (MPI) cluster. Two grid generation approaches are considered: the unstructured grid using OpenFOAM utility snappyHexMesh and the conventional structured multiblock body-fitted curvilinear grid. The LES flow solutions are compared with the experiment and also with solutions obtained from the standard axisymmetric RANS method using the turbulence model. For noise predictions, the LES solutions are coupled with the penetrable surface formation of the Ffowcs Williams–Hawkings method. The results of noise predictions are compared with the experiment, and the effect of different LES grids and acoustic integration surfaces is discussed.

Simulations of co-axial jet flows on graphics processing units: the flow and noise analysis.

Large-eddy simulations (LES) are performed for a range of perfectly expanded co-axial jet cases corresponding to conditions of the computation of coaxial jet noise (CoJeN) experiment by QinetiQ. In all simulations, the high-resolution Compact Accurately Boundary-Adjusting high-REsolution Technique (CABARET) is used for solving the Navier-Stokes equations on unstructured meshes. The Ffowcs Williams-Hawkings method based on the penetrable integral surfaces is applied for far-field noise predictions. To correctly model the turbulent flow downstream of the complex nozzle that includes a central body, a wall modelled LES approach is implemented together with a turbulent inflow condition based on synthetic turbulence. All models are run on graphics processing units to enable a considerable reduction of the flow solution time in comparison with the conventional LES. The flow and noise solutions are validated against the experimental data available with 1-2 dB accuracy being reported for noise spectra predictions on the fine grid. To analyse the structure of effective noise sources of the jets, the covariance of turbulent fluctuating Reynolds stresses is computed and their characteristic scales are analysed in the context of the generalized acoustic analogy jet noise models. Motivated by self-similarity of single-stream axi-symmetric jet flows, a suitable non-dimensionalization of the effective jet noise sources of the CoJeN jets is tested, and its implications for low-order jet noise models are discussed. This article is part of the theme issue 'Frontiers of aeroacoustics research: theory, computation and experiment'.

A method for predicting static-to-flight effects on coaxial jet noise

Previously-published work has provided a theoretical modelling of the jet noise from coaxial nozzle configurations in the form of component sources which can each be quantified in terms of modified single-stream jets. This modelling has been refined and extended to cover a wide range of the operating conditions of aircraft turbofan engines with separate exhaust flows, encompassing area ratios from 0.8 to 4. The objective has been to establish a basis for predicting the static-to-flight changes in the coaxial jet noise by applying single-stream flight effects to each of the sources comprising the modelling of the coaxial jet noise under static conditions. Relatively few experimental test points are available for validation although these do cover the full extent of the jet conditions and area ratios considered. The experimental results are limited in their frequency range by practical considerations but the static-to-flight changes in the third-octave SPLs are predicted to within a standard deviation of 0.4dB although the complex effects of jet refraction and convection cause the errors to increase at low flight emission angles to the jet axis. The modelling also provides useful insights into the mechanisms involved in the generation of coaxial jet noise and has facilitated the identification of inadequacies in the experimental simulation of flight effects.

Noise Source Distribution of Coaxial Subsonic Jet-Short-Cowl Nozzle

The noise source distribution of a short-cowl coaxial jet operating at different velocity ratios is described in this work. This was motivated by an ongoing research about noise prediction of coaxial jets through Acoustic Analogy with purposes of industrial engine application. This research has been carried out between Universidade Federal de Uberlândia (UFU), Brazil and the Institute of Sound and Vibration Research (ISVR) at Southampton University, UK. The numerical approach employed is originally based on Lighthill Acoustic Analogy. This technique, although likely known, is associated with an improved energy transfer time-scale, used in the turbulence two-point correlation function, in order to enhance the source model. The source model is coupled with the aerodynamic calculation of flow through turbulence quantities evaluated by using a standard k-e turbulence modeling. The Computational Fluid Dynamics data have also been used to provide complementary information about the coaxial jet noise production mechanisms. Experimental data were used in order to corroborate the results from the current model. Good agreement has been found, showing that high and low frequency contributors to the radiated noise for low velocity ratio are aggregated in a region about seven to ten secondary diameters downstream, while at higher velocity ratios sources are continuously spread from about one up to ten secondary diameters from the jet exit.

Aeroacoustics of Turbulent Jets: Flow Structure, Noise Sources, and Control

The paper reviews research performed to advance the understanding of state-of-the-art technologies capable of reducing coaxial jet noise simulating the exhaust flow of turbofan engines. The review focuses on an emerging jet noise passive control technology known as chevron nozzles. The fundamental physical mechanisms responsible for the acoustic benefits provided by these nozzles are discussed. Additionally, the relationship between these physical mechanisms and some of the primary chevron geometric parameters are highlighted. Far-field acoustic measurements over a wide range of nozzle operating conditions illustrated the ability of the chevron nozzles to provide acoustic benefits. Detailed mappings of the acoustic near-field provided more insight into the chevron noise suppression mechanisms by successfully identifying two primary chevron effects consistent with the results of the far-field measurements: chevrons penetration and shear velocity across them. Mean and turbulence data identified the physical flow mechanisms responsible for the effects documented in the far- and near-field studies.

Analysis of the Two Similarity Components of Turbulent Mixing Noise

It is now widely believed that the turbulence in free shear layers is not completely random, but more coherent and orderly, and that turbulent flows contain both fine-scale and large-scale structures. Both fine-scale turbulence and large-scale turbulence generate noise. Experimental measurements have shown conclusively that the mean flow as well as the turbulence statistics exhibit self-similarity. Based on these observations, Tam et al. (Tam, C. K. W., Golebiowski, M., and Seiner, J. M., Two Components of Turbulent Mixing Noise from Supersonic Jets, AIAA Paper 96-1716, 1996) proposed that because noise is generated by the turbulence of the jet, the noise spectra generated by fine-scale and large-scale turbulence should also exhibit self-similarity. By the examination of a large set of supersonic jet noise data acquired at NASA Langley Research Center, Tam et al. offered evidence that the turbulent mixing noise of high-speed jets does consist of two, independent self-similar components. We first provide additional independent confirmation of the universal shapes of the two components of mixing noise for a single jet. The significance of an important effect, due to atmospheric absorption, is illustrated with detailed analysis of The Boeing Company jet noise data. The Tam et al. analysis is based on the examination of data from single-stream nozzles. We provide evidence from the analysis of noise from dual-stream nozzles that the measured spectra in the forward quadrant and near-normal angles conform to the shape of the fine-scale spectrum, regardless of nozzle geometry and operating conditions. We clearly demonstrate that the spectral shape associated with the large-scale structures of single jets does not characterize the noise of coaxial jets at large aft angles. Finally, we show that the noise of hot subsonic jets at low angles also conform to the shape of the fine-scale spectrum.

A MODELLING OF THE NOISE FROM SIMPLE COAXIAL JETS, PART II: WITH HEATED PRIMARY FLOW

This paper reports the second part of a continuing study of the noise of coaxial jets, and describes modifications to the model developed previously to allow for the effects of a heated flow from the primary nozzle. The essential feature of the model described previously for the prediction of the noise from isothermal coaxial jets was the identification of three flow regions, within the coaxial jet flow, the noise production of which could be estimated from single-jet prediction methods. In particular, it was shown that noise from the principal interaction zone could be calculated by using single-jet prediction methods as long as account was taken of the fact that measured turbulence levels in this region were lower than those observed in a single isolated jet at the same centerline velocity. For isothermal flows, for which only quadrupole sources exist, allowance for this reduced turbulence level was entirely straightforward. However, for heated flow both dipole and quadrupole sources exist, and these have different dependencies on the turbulence level. Hence to predict the noise one needs to know the relative contributions of the dipole and quadrupole sources. In the present work, use has been made of previously published results for these relative contributions, as a function of jet velocity and temperature, for single jets. This then permits prediction of the noise from the interaction zone, which is subsequently combined with that from the secondary jet shear layer and fully mixed flow region, as before. Comparison between data and prediction over a range of jet velocity, temperature and angle of observation again show very acceptable agreement.

A MODELLING OF THE NOISE FROM SIMPLE COAXIAL JETS, PART I: WITH UNHEATED PRIMARY FLOW

The work described in this paper forms a portion of an on-going fundamental study of coaxial jet noise both statically and in flight. Three principal noise-producing regions are identified and their mean flow and turbulence characteristics classified from published data. The noise production from each region is then calculated by using single jet prediction methods for flows of similar mean velocity and turbulence profiles. The initial test of this prediction scheme has been conducted by comparison with data from an unheated, co-planar, coaxial jet configuration. The agreement, in terms of one-third octave spectral predictions, is significantly better than 1 dB over a wide range of both angle of observation and velocity ratio.

Conventional profile coaxial jet noise prediction

A semiempirical model for predicting the noise generated by conventional velocity-profile jets exhausting from coaxial nozzles is presented and compared with small-scale static data. The present method is an updated version of that part of the original NASA Aircraft Noise Prediction Program (1974) relating to coaxial jet noise. The new procedure is more theoretically based and has also been improved by some empirical adjustments.

On the connection between the aerodynamic and acoustic characteristics of coaxial jets

The results are given of experimental investigations into the distributions of the mean and pulsation velocities in the mixing region of isothermal coaxial jets with ordinary velocity profile and “inverted” velocity profile (velocity of the outer flow greater than that of the inner flow). These results are used in a comparative estimate of the noise of coaxial jets with different initial velocity profiles, and a comparison is made with the data of experimental investigations of the noise.

Effect of excitation on coaxial jet noise

Coaxial model jets, including those of high bypass ratio engine exhaust hot gas conditions, were excited internally by tone and broadband noise. Acoustic excitation in the secondary (outer) duct was found to be most effective in jet noise amplification due to the sensitivity of the outer shear layer. Jet noise amplification at the subharmonic of the excitation frequency occurred in a number of cases. An acoustic elliptic mirror was used to observe the noise sources along the jet. It revealed local noise source characteristics in different shear layer regions and noise source location changes from unexcited to excited jets.

Some thoughts on the effects of flight on jet noise as observed in actual flight and in wind tunnels

Evidence on differences between flight effects on jet noise as observed in flight tests and in tests with model jets in anechoic wind tunnels is briefly reviewed and discussed. The effects on noise of single stream jets is distinguished from those on noise of coaxial jets. A way of explaining the differences, both between model and engine jets and single stream and coaxial jets is proposed, which involves ideas drawn from Ribner's self-noise/shear-noise model, recent results on noise amplification of excited jets, and new lines of research.

Acoustic and Flow Characteristics of Cold High-speed Coaxial Jets

Far-field noise of model coaxial supersonic cold jets exhausting from a coplanar convergent two-nozzle coaxial configuraton was investigated. The pressure ratio (velocity) of the outer annular jet was maintained higher than the inner round jet. For each outer-jet pressure ratio, the inner-jet operating pressure ratio for the minimum radiated noise was established. Experimental results on the noise reductions and the modifications of the shock structure and velocity distribution of coaxial jet flows at the minimum noise conditions are presented and are compared with those of a single convergent underexpanded round and annual jet at a wide range of operating pressure ratios.

Aerodynamics and Noise of Coaxial Jets

The objective of this investigation was to develop a unified prediction method for estimating the aerodynamic and noise characteristics of jets issuing from nozzles of arbitrary geometric shapes. The method has been developed and demonstrated for dual-flow coaxial jets. An extension of Reichardt's theory is utilized to predict the mean velocity, temperature, and axial turbulence intensity distributions throughout the jet plume. The generic noise intensity spectrum is synthesized by a slice-of-jet approach, wherein each axial location in the plume contributes to the sound generation in one dominant frequency band. The propagation of the source spectrum to the far field is modeled by means of convected quadrupoles embedded in a parallel slug flow. Extensive predictions of the aeroacoustic characteristics of coaxial jets were made and compared with experiment. The agreement between theory and experiment is quite good, except at high frequencies and shallow angles to the jet axis, where refraction is overestimated. A major conclusion drawn from these results is that the noise reduction attained by a coaxial jet conies primarily from a reduction in turbulence intensity.

A survey of low velocity and coaxial jet noise with application to prediction

A review of recent published data on low velocity jet noise is given together with previously unpublished results taken from the Rolls-Royce Noise Research Programme on model rigs and full-scale engines. Noise correlations are given which show that at low jet velocities, the low frequency exhaust noise which is commonly referred to as jet noise, emitted from the fan stream of a turbofan engine is considerably lower in level than that from the (hot) centre stream. From this result, a new prediction procedure for coaxial jet noise of turbofan engines is then developed. Comparisons are given which show that this method gives good correlation with measured results from a number of full-scale turbofan engines. The importance of accurate estimation of the “ground reflection effect” is clearly demonstrated. A critical review of published jet noise data from model coaxial jets is given and the need for further extensive testing emphasized.

Noise and Flow Characteristics of Coaxial Jets

The noise from cold subsonic coaxial jets issuing from concentric nozzles with external mixing has been examined and an attempt made to relate the results to the flow patterns of the jets. As the velocity of the annular jet was increased in relation to a given central jet velocity, an initial attenuation of the jet noise was observed. On further increasing the annular jet velocity until the velocities of the two streams were nearly equal, the noise was in excess of that of the central jet alone. An attempt has been made to predict the curve relating the attenuation attained to the coaxial jet velocity.In comparing the attenuation produced by a coaxial jet combination with that of a single jet, it could be argued that the total thrust in each case should be the same and experimental results are presented on this basis. Results are also available for a nozzle configuration in which the jets are mixed internally in a tube before discharge, and the paper is completed with the results obtained from an annular nozzle acting alone.