Worldwide, jet engine noise emissions are subject to limits by legislation. Aerospace manufacturers, consequently, are looking to better understand static-to-flight effects on coaxial (outer fan and inner turbine core) jet noise. A new software package, recently announced by the Aircraft Noise & Structural Dynamics Group at IHS ESDU, estimates the change in coaxial jet noise spectrum levels when moving from static to flight conditions.

Computerized methods for predicting jet noise from static jet engines, both single-stream and coaxial, have been previously available from ESDU. A method for estimating single-stream jet noise in flight has also been available, but a reliable method for predicting coaxial jet noise in flight has, until now, proven elusive.

The new software package—10 years in development—reflects the known aerodynamic characteristics of both single-stream and coaxial jets, and should be applicable to aircraft turbofan engines with separate core and bypass exhaust nozzles with a hot core flow.

In this Engineering360 interview, Cyrus Chinoy, head of ESDU's Aircraft Noise & Structural Dynamics Group, and Willie Bryce, a member of the organization's Aircraft Noise Committee, describe ESDU 14014 and the challenges of bringing this new software to market.

DeMeis: How does the ESDU 14014 software package predict in-flight coaxial jet noise?

Chinoy and Bryce: In simple terms, the method breaks up the coaxial jet mixing noise into three discrete single-stream jets, which we term the secondary jet, the mixed jet and the interaction jet. Each of these discrete jets is characterized by different combinations of the coaxial jet parameters.

Considering velocity alone, the single-stream jet that represents the mixed jet has a velocity that is a function of the velocity ratio, the temperature ratio and the area ratio of the coaxial jet. The velocity of the interaction jet is taken to be the same as that of the primary (core) flow of the coaxial jet, and the velocity of the secondary jet taken to be that of the secondary (fan) flow of the coaxial jet. The other parameters of jet temperature, jet flow area and jet diameter are treated in similar but individual ways.

After applying the single-stream static-to-flight effect prediction procedure to each of these discrete jets, a further summation gives the predicted total coaxial jet noise in flight, and the complete static to flight changes.

The ESDU 14014 software package breaks up the coaxial jet mixing noise into three single-stream jet noise sources. The noise from each source is then predicted, modified appropriately and summed to provide a static coaxial jet noise spectrum.

DeMeis: How did QinetiQ Ltd and Rolls-Royce plc help you in the preparation and release of ESDU 14014?

Chinoy and Bryce: QinetiQ Ltd and Rolls-Royce plc have made both single-stream and coaxial jet noise measured data available to us. The static and flight coaxial jet noise data have been invaluable for validating the prediction method. The single-stream data are an integral part of the software package accompanying ESDU 14014 because they form the database of the noise from which the noise from each of the single-stream sources in the three-source model is estimated.

Data made available to ESDU by QinetiQ Ltd and Rolls-Royce plc helped validate ESDU's static-to-flight modeling method.

DeMeis: How versatile is this method?

Chinoy and Bryce: The method is a theoretically-based one, reflecting the known aerodynamic characteristic of both single-stream and coaxial jets. This enables applicability over a wide range of jet and flight conditions. However, the calculations required to perform the predictions are extremely complex and lengthy, and rely on the computing power of modern computers for their execution.Click image to enlarge. Image source: IHS ESDU

DeMeis: What challenges were encountered in the development and how were they overcome?

Chinoy and Bryce: For a single-stream jet of known size, the velocity and temperature of the jet are the principal parameters in the generation of jet noise. For a coaxial jet with separate bypass (secondary) and core (primary) flow nozzles, the number of variables is markedly increased, as are the theoretical complexities, exacerbating the problem.

The three-source coaxial jet model was postulated a number of years ago, but the technical complexities which arise in a flight environment, together with the limited amount of experimental data available under flight-simulation conditions, meant that it was more than 10 years before we were satisfied we could make reliable predictions of in-flight noise levels.

DeMeis: What parameters are incorporated in the computer model and typically varied to arrive at a solution?

Chinoy and Bryce: For a coaxial jet of a given geometry, the number of variables is large under static conditions. ESDU uses four independent variables to interpolate on a database: the primary jet velocity, the velocity ratio, the nozzle area ratio and the temperature ratio.

But in formulating the three-source model, each of the three sources is represented by different combinations of the primary and secondary jet parameters. To generate in-flight predictions, the flight velocity is applied in a complex manner to each of the sources.

(For a detailed description of the development of the prediction method, read the ESDU TM179 report, which covers in depth dependence of noise on various variables.)

DeMeis: How does the velocity difference between the coaxial bypass fan and core streams affect the results? Are there any limits, say, on velocity ratio between streams?

Chinoy and Bryce: The velocity difference between the coaxial streams is not a relevant acoustic parameter. The method has been validated for velocity ratios (secondary velocity/primary velocity) ranging from 0.63 through unity up to an inverted velocity profile where the velocity ratio is 1.25. These conditions cover all currently known aero-engine operating conditions. However, extrapolation to even higher velocity ratios is not recommended because of known aerodynamic processes, relevant to the acoustics, which are not yet quantifiable.

The method has been validated for a large range of secondary-to-primary area ratios, from 0.8 to 4.0. The theoretical basis of the method suggests that it should be applicable at substantially higher area ratios but such circumstances have not been validated.

DeMeis: How are noise spectra results applied?

Chinoy and Bryce: The computer program calculates the spectral changes in going from static to flight conditions. Users have the option to apply these calculated changes to their own measured or predicted static spectra for the greatest accuracy. If static spectra are unavailable, the program offers the option to predict the flight spectra directly.

DeMeis: Were there any surprises along the way in development?

Chinoy and Bryce: It was particularly surprising that all of the significant discrepancies between the predicted static-to-flight effects and those measured by flight simulation in an anechoic chamber could be convincingly explained by inadequacies or unavoidable limitations in the experimental measurements.

DeMeis: Who are the primary users of this software, and is the development indicative of any technology trends?

Chinoy and Bryce: The prediction of jet noise in flight is a subject of interest to university researchers, in both theoretical and experimental fields, government researchers and legislators, and most of all, aero-engine and aircraft manufacturing companies. As the aerospace industry is trending toward higher bypass ratios in jet engine design, the ESDU software will become an increasingly useful tool in estimating the effects on the jet noise generated.