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yeo March 6, 2003 07:09

Compressible Flow Modelling?
Hi there!

I would like to seek your advice for matter regarding Incompressible and Compressible Flow for my 3D train model and matter about setting the operating pressure.

(1) To quality for incompressible flow field modelling, the Mach number must be low (much less than 1) and density does not vary significantly with time and space. My current Mach number is 0.06, that is train speed 22 m/s, local sound speed 340 m/s.

I had successfully completed this task with incompressible flow field modelling and, Now, if the model is to be modelled with Compressible Flow technique, will the results differ a lot? I had seen in Fluent tutorial about the Sliding Mesh method with similar parameters but model with Compressible Flow technique! Some advise that to create a pressure pulse at the tunnel entry of a train, the density and temperature must change drastically. Actually, my intention is to ensure that my way of modelling is correct.

(2) Is that any difference between setting the operating pressure as 0 Pa or 101325 Pa? Most tutorial suggest the user to to set it at 0 Pa.


Christian March 6, 2003 10:50

Re: Compressible Flow Modelling?
1) A rule of thumb I use is that if M<0.3, then the flow is incompressible. I have never done a control of this number however. But if it is correct, your case is very much incompressible!

2) If the pressure level is raised, then the density and viscosity will change also. So stick to the pressure that you would expect in the real world. Make sure if the operating pressure is absolute or gauge.


yeo March 7, 2003 00:48

Re: Compressible Flow Modelling?
Thanks Christian for your reply.

What can i get the guidelines for determining the criteria for compressible flow or incompressible flow. That is, the M => 0.3 rule of thumb for gauging compressible. I need the reference to show my supervisor.

thanks a lot.

Christian March 7, 2003 04:25

Re: Compressible Flow Modelling?
Hi Yeo.

The guideline should be in any fluid dynamics textbook.

yeo March 7, 2003 08:08

Re: Compressible Flow Modelling?
hi Christian, thanks a lot. I recall now and found it.

best regards

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