Incompressible and compressible flow.

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September 24, 2013, 08:53
#21
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Filippo Maria Denaro
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Quote:
 Originally Posted by mercy.gh Hello, In addition to the interesting conversation; Measuring the Mach number also demands attention to the temperature of your environment, since the speed of sound would change correspondingly. I am trying to simulate a combustion chamber with a high velocity fuel -oxygen injection.Really close to the injectors(before combustion happens) flow is definitely compressible (velocity of injection is 500 and sound almost 380) but in a small distance, when combustion happens Temperature will rise highly and the velocity of sound will increase to 800 which makes the Mach number much less! Should I consider this compressible or in-compressible flow?
what about the velocity? if V/800 < 0.3 you can consider the flow incompressible in terms of acoustic wavew but you still have dependance of density due to high temperature gradients

September 24, 2013, 09:58
#22
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mersedeh
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Quote:
 Originally Posted by FMDenaro what about the velocity? if V/800 < 0.3 you can consider the flow incompressible in terms of acoustic wavew but you still have dependance of density due to high temperature gradients
True, I must consider this compressible. I just have never worked with that!
Thank you

 January 30, 2014, 06:32 Deflagration not Detonation #23 New Member   Richard Chinery Join Date: Jan 2014 Posts: 1 Rep Power: 0 with respect to the earlier posts on combustion effects on what would normally be incompressible flow, simple temperature correction accounts for change in density. you could use combustion intensity figures to plot the heat release and continually account for this? i don't see the need for use of compressible flow considerations unless you are considering detonation.

September 1, 2016, 10:52
#24
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larmes
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Quote:
 Originally Posted by Ananda Himansu ;31568 If the bulk compressibility of the fluid (the ratio of the change in specific volume to the change in pressure that causes it; this is a material property of the fluid) is small by comparison with the pressure variations encountered in the flow under consideration, then these pressure variations will cause only small changes in the density.
How high of a pressure difference do you need to have in order for the density to change due to the pressure difference?

 March 18, 2017, 04:01 #25 New Member   CHUNG Ching Chun Join Date: Aug 2016 Location: Tin Shui Wai, New Territories, Hong Kong Posts: 11 Rep Power: 9 Dear all, I have been doing 3D validation case of flow over a bump-in channel from NASA Turbulence Modelling Center. https://turbmodels.larc.nasa.gov/bump3d.html I get floating point errors when I am using an incompressible solver but it runs fine with an incompressible solver. Its velocity is 0.3 Mach number. Is it usual to assume that it is compressible flow so that my results will not be wrong? Also is it ok to use a compressible flow for a case that I am not sure, whether there are compressibility or not?

March 18, 2017, 04:31
#26
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Filippo Maria Denaro
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Quote:
 Originally Posted by richard.chung.jones Dear all, I have been doing 3D validation case of flow over a bump-in channel from NASA Turbulence Modelling Center. https://turbmodels.larc.nasa.gov/bump3d.html I get floating point errors when I am using an incompressible solver but it runs fine with an incompressible solver. Its velocity is 0.3 Mach number. Is it usual to assume that it is compressible flow so that my results will not be wrong? Also is it ok to use a compressible flow for a case that I am not sure, whether there are compressibility or not?

March 18, 2017, 04:46
#27
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CHUNG Ching Chun
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Quote:
 Originally Posted by FMDenaro
When I use a OpenFOAM incompressible CFD solver, like pimpleFoam, its continuity error goes up to around e+90 and its velocity field values are also absurdly high. According to the error messages, it says floating point errors and I think it means it is taking up too many bytes.

However, when I use a compressible solver, rhoPimpleFoam, everything goes fine.

To be frank, why does that happen? Do anyone has experience about that?

 March 18, 2017, 12:25 #28 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,793 Rep Power: 71 floating points error is not due to the lack of memory space but it means that the number is so big to be not representable on the registry machine... The first thing I could immagine is a wrong setup of the case or a numerical instability.