Sound Radiation from an Open Pipe with a Mean Flow - Doctoral Forum

1. Doctoral Forum University of Music and Performing Arts Graz June 16, 2017 Drasko Masovic Sound Radiation from an Open Pipe with a Mean Flow Supervisors: Prof. Dr. R. Höldrich, Prof. Dr. G. Eckel, Dr. A. Fuchs

2. Contents • Case study • Main acoustic phenomena • Measurement results • Estimation of sound radiation • analytical approach • numerical approach

3. Case study • Geometry • Mean flow • Sound wave

4. Geometry • Axisymmetric geometry (2D) • Pipe • straight circular semi-infinite • thin rigid wall • Opening • straight cut • sharp edge

5. Mean flow • Jet • gas – dry air • low Mach number: M = V/c < 0.3 • temperature: 20ºC < T < 300ºC • Surrounding gas • cold and still air

6. Sound wave • Sound source • deep inside the pipe • Sound wave • low frequency (Helmholtz number: ) • example: • a = 2cm, c = 343m/s → f < 2.5kHz • small amplitude (SPL < 150dB)

7. Applications • Automotive applications • exhaust system, tail-pipe • pass-by noise • excitation of vehicle’s body • air-conditioning outlets • Musical applications • wind instruments • sound field in rooms • spatial sound synthesis

8. Main flow/acoustic phenomena • incident sound wave • diffraction at the edge • refraction in the mixing region directivity diffraction refraction

9. Analytical approaches • Levine and Schwinger (1948) • no mean flow • low frequencies

10. Analytical approaches • Munt (1977) • (hot) mean flow • full frequency range • + accurate for low frequencies and cold flows • – mathematically involved, few physical interpretations • – oversimplified mean flow (→ inaccurate refraction)

11. Laboratory measurements M = 0.3, T = 20°C, ka = 0.18, 0.36... Atvars et al. (1965) M = 0.2, ka = 0.53, T = 38°C, 149°C, 260°C T = 20°C, ka = 0.53, M = 0, 0.1, 0.3…

12. Experimental setup flow equipment acoustic equipment

13. Experimental setup Blower Elektroror SD 900 max. volume flow 14.5m3/min power 11kW

14. Experimental setup Heaters total power 110kW

15. Experimental setup Loudspeaker DAP audio AB-12 freq. range 55-2500Hz RMS power 300W

16. Experimental setup Microphones NTi Audio M2230 equivalent noise level 16 dB(A) accuracy ±1dB @ 20-4000Hz

17. Experimental setup Excitation signal – swept-sine Sound acquisition – 18 microphones Mean flow: M = 0, 0.05, ... 0.25 T = 40°C, 100°C, 200°C, 300°C Measured: the directivity patterns

18. No-flow (reference) case Comparison with Levine & Schwinger (1948)

19. Very low frequencies (negligible refraction) M = 0.1 T = 40...300°C • ka = 0.1 M = 0...0.25 T = 40°C • ka = 0.1 ·· measured – Munt (1977) ·· measured – Munt (1977)

20. Increasing frequency M = 0.25 T = 300°C ka = 0.1...0.9 ·· measured – Munt (1977)

21. Simple model • incident sound – plane wave inside the pipe • diffraction – vortex-sound interaction at the edge • (low M and low ka values) • refraction in the mixing region diffraction refraction

22. Estimation of the directivity diffraction refraction

23. Estimation of the directivity diffraction refraction

24. Numerical calculations • Fluid Dynamics • Acoustics M = 0.25

25. Numerical calculations • Computational Fluid Dynamics • Reynolds-averaged Navier-Stokes equations (RANS) • Computational Acoustics • Method 1: Linearized Euler Equations (LEE) • Method 2: Convected Wave Equation (CWE) M = 0.25

26. Computational acoustics M= 0.25, T = 300ºC, ka= 0.3 Method 1 (LEE) + accurate (includes vortices) – unstable – high computational costs Method 2 (CWE) + efficient and robust – inaccurate (no vortices)

27. Computational acoustics Method 2 + “vortex effect” + implicit vortex at the edge + efficient and robust – limited to low M and low ka values M= 0.15, T = 41ºC, ka= 0.5

28. Summary • Systematic measurements of the directivity • effects of the mean flow (velocity and temperature) and frequency • low Mach number, low frequency • Simple physical model • insight into the key physical phenomena • reasonable accuracy • simple geometries and flows

29. Summary • Numerical calculations • comparison of different acoustic equations and numerical techniques • Method 2 + “vortex effect” • improved accuracy compared to Method 2 • lower computational costs and more robustness compared to Method 1 • range of validity?

30. Publications • D. Masovic, F. Zotter, E. Nijman, J. Rejlek, and R. Höldrich, “Directivity measurements of low frequency sound field radiated from an open cylindrical pipe with a hot mean flow,” 9th ISNVH Congress, Graz, SAE Technical Paper 2016-01-1822, 2016 • D. Masovic, “Comments on convective amplification of sound sources in flows,”, ASRO Journal of Applied Mechanics, vol. 1, no. 1, pp. 20-23, 2016. • D. Masovic, E. Nijman, J. Rejlek, and R. Höldrich, “A simple model of the far-field directivity of an open circular pipe with a hot flow,” in Proceedings of DAGA 2017, Kiel, pp. 1222-1225, 2017. • D. Masovic, E. Nijman, J. Rejlek, and R. Höldrich, “Towards a boundary condition for convective wave equation and sound diffraction at a trailing edge inside a flow,” in Proceedings of DAGA 2017, Kiel, pp. 1301-1304, 2017. • Planned: • D. Masovic, E. Nijman, J. Rejlek, and R. Höldrich, “'Comparison of different approaches for calculation of sound radiation from an open pipe with a flow”, ICTCA 2017, Vienna, July/August 2017

31. Doctoral Forum University of Music and Performing Arts Graz June 16, 2017 THANK YOU FOR YOUR ATTENTION!Drasko Masovic