1. Drasko Masovic 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 Opening
straight circular semi-infinite
thin rigid wall
Opening
straight cut
sharp edge

5. Mean flow Jet Surrounding gas 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 Sound wave deep inside the pipe
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,
Atvars et al. (1965)
T = 20°C, ka = 0.53,
M = 0, 0.1, 0.3…
M = 0.2, ka = 0.53,
T = 38°C, 149°C, 260°C

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 Hz

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

18. No-flow (reference) case
Comparison with Levine & Schwinger (1948)
f [Hz] ka σ
Lmin [dB]
Lmax [dB]
281
0.1
0.20
-0.60
0.24
1407
0.5
0.30
-0.54
0.63
2533
0.9
0.47
-0.79
1.18

19. Very low frequencies (negligible refraction)
M =
T = 40°C
ka = 0.1
M = 0.1
T = °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 Planned:
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
D. Masovic, “Comments on convective amplification of sound sources in flows,”, ASRO Journal of Applied Mechanics, vol. 1, no. 1, pp , 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.  , 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.  , 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. THANK YOU FOR YOUR ATTENTION! Drasko Masovic
Doctoral Forum
University of Music and Performing Arts Graz
June 16, 2017
THANK YOU FOR YOUR ATTENTION! Drasko Masovic