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difference between compressible and imcompressible flow

Question asked by Loic Ancian on Jan 13, 2011
Latest reply on Nov 10, 2011 by Ruben Cornejo
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Hello all,

I read some document about flow simulation recently and it was said that Flow Sim can simulate both compressible and incompressible flows.

Can someone explain in simple words   what is exactly the difference between them.?? Is it related to "high mach" option?

 

Thanks

Bill McEachern
Bill McEachern Jan 13, 2011 10:56 AM
Correct Answer

generally speaking it refers to constant density or variable density. In practical terms density changes start to become significant around 0.3 Mach number though you could argue that number up and down a bit - lowest I have seen if 0.2 Mach. Flow sim handles it for you so no issue. The high Mach number setting is for flow that are dominated by flows over Mach 1.


here is what the help says:

 

In Flow Simulation, gases are always compressible and liquids can be incompressible or compressible. If your project deals with a high-velocity gas flow, where the Mach number maximum value exceeds about 3 for steady-state analyses or 1 for transient (time-dependent) analyses, you should consider the gas flow as a high Mach number flow. To consider high Mach number gas flow, select the High Mach number flow check box either on the Default Fluids step of the  Wizard or in the Fluids dialog box in General Settings.

The low Mach number gas flow is recommended for the tasks where the supersonic flow is localized in relatively small fluid volume and the major flow is subsonic. If the fluid volume in which the flow becomes supersonic is about a half of the computational domain size or greater, it is recommended that you consider the flow as a high Mach number gas flow.

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