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Considerations of Aeroacoustics in Turbojet-Engine Testing

Recommendations for preventing aeroacoustic damage to test cells have been made.

A study of the aeroacoustics of a turbojet-engine test cell has been performed as one step in the development of a computational aeroacoustics capability (CAA) that could provide guidance for the design and operation of such cells. Ground testing of turbojet engines in test cells necessarily involves very high acoustic amplitudes, often severe enough to cause damage to test-cell equipment and to engines under test. Heretofore, the acoustic responses of test cells containing energetic jets have been poorly understood and generally unpredictable. The CAA capability is intended to enable prediction of deleterious acoustic events, making it possible to design test cells and choose operating conditions to prevent damage and thereby avoid the costly interruption of test schedules.

Figure 1. Super Resonance is caused by feedback of energy between a helical or flapping normal duct mode and a corresponding large, equal-frequency helical or flapping instability wave of the jet flow.
The study consolidated what is known about the aeroacoustics of jets and of flows in ducts like those of turbojet-engine test cells. In broad terms, the concepts addressed in the study are summarized in what follows: A turbojet- engine test cell is either a closed or a partially closed system. The source of acoustic energy in the cell is the jet flow. The solid duct surfaces that bound the cell reflect incident acoustic waves. Thus, the cell creates a special acoustic environment that, depending on the cell geometry and engine design, may interact directly with the jet flow.

Figure 2. Barriers Would Be Inserted in a duct to block helical and flapping duct modes.
One important part of the noise in the cell is broadband noise that propagates directly from the jet and is essentially the same as the noise propagating from a jet in open air. Another important part of the noise is that of acoustic resonances that arise in coupling of instability waves of the jet flow with acoustic normal modes of the cell (see Figure 1). Usually, this coupling results in tones at the resonance frequencies. In a case in which the frequency of a duct normal mode coincides or nearly coincides with the frequency of maximum growth of the associated instability wave, then the resonance is denoted a super resonance. Super resonance is violent: once it begins, it must be suppressed immediately, or else the engine test must be interrupted immediately, to prevent structural damage to the cell.

As a potential superior alternative to suppression or interruption, it has been proposed to design and construct cells to disrupt the duct modes that participate in super resonance. In particular, in the case of a hot circular-cross-section jet, the most amplified instability waves are those characterized by azimuthal wave number ±1. These are helical or flapping-mode jet waves that interact with acoustic waves in the corresponding helical and flapping duct normal modes. In principle, it should be possible to prevent super resonance by blocking the helical and flapping duct modes. This could be done by, for example, inserting barriers into the duct as shown in Figure 2.

Another product of the study is a set of recommendations concerning the development of a CAA capability for a turbojet-engine-testing laboratory. Summarizing the recommendations, the CAA capability should be developed and maintained by an in-house engineer, perhaps augmented with an engineering assistant who preferably has some expertise in CAA and/or computational fluid dynamics (CFD). The engineer and/or the assistant (perhaps augmented with consulting engineers having CAA and/or CFD expertise) should develop a battery of special-purpose CAA computer codes, each optimized for a specific class of CAA problems.

This work was done by Christopher Tam of Florida State University for the Air Force Research Laboratory. AFRL-0057

Topics:
Computational fluid dynamics Electronics Engines Fluid Handling Jet engines Physical Sciences Powertrains Simulation Software Simulation and modeling Test facilities Turbojet engines
This Brief includes a Technical Support Package (TSP).
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Considerations of Aeroacoustics in Turbojet-Engine Testing

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    Defense Tech Briefs Magazine

    This article first appeared in the June, 2009 issue of Defense Tech Briefs Magazine (Vol. 3 No. 3).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document titled "Engine Test Cell Aeroacoustics and Recommendations" is a final report prepared by Dr. Christopher Tam from Florida State University, under the auspices of the Arnold Engineering Development Center (AEDC) and the Air Force Materiel Command (AFMC). The research was conducted to address the challenges associated with aeroacoustics in engine test cells, particularly focusing on noise prediction and resonance suppression.

    The report details extensive comparisons between semi-empirical noise prediction formulas and experimental measurements, demonstrating that the predictions align well with actual data across various conditions, including both hot and cold jets and different expansion states. The nozzle design Mach number and fully expanded jet Mach number were tested within a range of 1.0 to slightly over 2.0, yielding satisfactory results for engineering applications.

    A significant portion of the report is dedicated to the phenomenon of super resonance, which can cause severe structural damage to test cells if not controlled. The document outlines methods for suppressing super resonance, including the use of large quantities of water to dampen vibrations and the installation of Helmholtz resonators tuned to the resonance frequency, which have proven effective in reducing resonance amplitude.

    The report emphasizes the importance of further testing to enhance the confidence in the semi-empirical theory used for noise predictions, as the current data set is limited. It suggests that additional research could provide more robust validation of the theoretical models.

    Overall, the document serves as a comprehensive resource for understanding the complexities of aeroacoustics in engine testing environments. It provides practical recommendations for mitigating noise and resonance issues, ensuring that engine development processes can proceed without interruption. The findings are particularly relevant for engineers and researchers involved in aerospace testing and development, offering insights that can lead to improved testing methodologies and enhanced engine performance.

    In conclusion, this report not only presents valuable experimental data and theoretical insights but also highlights the critical need for ongoing research in the field of aeroacoustics to support the advancement of aerospace technologies.

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