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Aeroacoustics of Muffler

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Old   July 8, 2021, 04:28
Post Aeroacoustics of Muffler
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Kai Han Goh
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I am now writing a bachelor thesis on the aeroacoustics of muffler. My muffler consists of one mass-flow inlet, one pressure outlet, with a few chamber in it and it is connected to a 3 cylinder 4 strokes engine running at a constant 1500rpm. I am interested in the noise level at the outlet and also the backpressure caused by the muffler.

So I have a few question regarding its simulation of aeroacoustics.

1. For my mass-flow inlet, do I use constant mass flow rate or fluctuating mass flow rate? How does this affect the sound pressure level-frequency graph? By fluctuating mass flow rate, I mean the almost instantaneous release of gas during exhaust phase which I will further simplify as half of a sine wave, followed by zero mass flow, for every exhaust of each cylinder.

2. As my muffler consists of a few expansion chamber, there is a big pressure difference(~2000Pa) in the narrow tube connecting them. So I am setting my air density as ideal gas, even though it doesn't exceed 0.3 Mach. In this case which method should i use to model the acoustics, direct method or using FW-H model? I will be getting the value right at the outlet boundary, as i dont feel the need to go into far-field.

3. For the transient simulation, how should I set the time step size and the frequency of data export to CFD-Post if i am interested in frequency range up to 250Hz? Here's what i think, time step size should be 0.0004s (1/10 of period), data export frequency should be every 10 step size (every period). Is that right? In CFD-post, i wil be plotting a graph of pressure against time and use FFT to transform it into sound pressure level against frequency graph.

4. Which properties should i monitor to know if my transient simulation has converged? Until now, i have only been using the residual chart as monitor, so not sure if thats the only way.

Any help or further suggestion or correction is very much welcomed. Thanks in advance.

Kai Han Goh
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aero-acoustics, massflowrate, transient convergence

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