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Numerical Study of Aeroacoustics of a Round Supersonic Impinging Jet

Principal Investigator: Jörn Sesterhenn, CFD - Technische Universität Berlin (Germany)
HPC Platform: Hermit/Hornet/Hazel Hen of HLRS

Impinging jets occur within various technical applications, for example as part of the cooling system of gas turbines or within the flow of a vertical take-off and landing aircraft. Especially supersonic impinging jet flow is characterised by extremely loud noise. The occurring strong pressure waves can damage the human hearing system as well as technical components. For this reason impinging jet noise became an important research topic within the last decades. Researchers were able to describe the occurrences of these tones, but the question for the sources of this phenomenon remained open.

An important influence on the generation of impinging tones is the ratio between total pressure at the nozzle and ambient pressure. This study concentrates on relatively low nozzle pressure ratios that however generate a supersonic flow. These conditions are referred to as the pre-silence region.

To study the sound source mechanisms of impinging tones within the pre-silence region, direct numerical simulations (DNS) have been carried out at the chair of computational fluid dynamics at the Technical University of Berlin. DNS is the most precise method within computational fluid dynamics and allows the visualisation and the statistical description of vortices and shocks that appear in front of the impinging plate. These so-called standoff shocks can be seen in figure 1. The numeric precision can be afforded only by the world’s most powerful supercomputers. This project started on the Cray XE6 Hermit and was continued on the Cray XC40 Hornet / Hazel Hen of the HLRS. Grid sizes of up to 1 billion points (1024 x 1024 x 1024) have been used on up to 16384 cores in parallel so that a Reynolds number of 8000 could be realised. Around a quarter million time steps were needed to provide statistical information.

Two different sound source mechanisms of impinging tones were found. Both involve interactions between vortices and standoff shocks. In the first one, a vortex crashes asymmetrical into a standoff shock. As a result, the shock moves out of the jet and transforms into a strong pressure wave, which is observed as impinging tone. The second mechanism is caused by a subsonic embedding within a supersonic flow. The collapse of this embedding creates strong spherical pressure waves.

The knowledge of the origin of impinging tones is the basis for a successful suppression of tones resulting in a more silent application of impinging jets with a longer life time of the technical components.

Numerical Simulation of Impinging JetsFigure 1: Flow field of a supersonic impinging jet showing standoff shocks as well as traveling sound waves which are referred to as impinging tones.
Copyright: CFD, TU Berlin (Germany)

Publications:

R. Wilke and J. Sesterhenn. Numerical simulation of subsonic and supersonic impinging jets. In High Performance Computing in Science and Engineering ’15, pages 349–369. Springer, 2016.

R. Wilke and J. Sesterhenn. On the origin of impinging tones at low supersonic flow. arXiv preprint arXiv:1604.05624, 2016.

R. Wilke and J. Sesterhenn. Statistics of fully turbulent impinging jets. arXiv preprint arXiv:1606.09167, 2016.

R. Wilke. The impinging jet. Unpublished doctoral thesis, Technische Universität Berlin, 2017..

Research Team & Contact Information:

Robert Wilke, Jörn Sesterhenn

Prof. Dr. Sc. techn. habil. Jörn Sesterhenn
Technische Universität Berlin
Department of Computational Fluid Dynamics
Müller-Breslau-Str. 15, D-10623 Berlin (Germany)
e-mail: Joern.Sesterhenn [at] TU-Berlin.DE

February 2017

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