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Capability of Fluent to model compressible flows

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Old   December 4, 2015, 15:05
Post Capability of Fluent to model compressible flows
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hamid
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Hi every body,

please share any information about the following question:

Does fluent capable of modelling shock-bubble interaction (shock in water with a gas bubble)?

( In the case of interaction of a shock in water with a gas bubble, since both phases should be compressible and in fluent there should be only one compressible phase, I doubted and the above question arose)

thank you all,
Hamid
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Old   December 9, 2015, 14:47
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You mean like a cavitating bubble? Otherwise how do you get shocks in water (in a liquid)?
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Old   December 9, 2015, 15:35
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Quote:
Originally Posted by LuckyTran View Post
You mean like a cavitating bubble? Otherwise how do you get shocks in water (in a liquid)?
Dear Lucky Tran,

by shock in a water, I mean a pressure wave. consider my desired problem as follows:

a domain consist of water with an initial higher pressure and a gas bubble with much lower initial pressure. Does fluent capable of solve such problem? in this situation both the liquid (water) and the gas should be compressible but there is a warning in Fluent using VOF which says only one of the phases can be compressible. I tried all of the schemes in the Fluent but the solution diverged every time. I thought it is because of mesh at first but at last I concluded that Fluent (Ansys 14.5) is not capable of modelling such problem. I ask this question here because I wanted to be sure that I have made a right conclusion otherwise find the my errors.

Thank you very much
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Old   December 9, 2015, 15:49
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Fluent has the capability, except for the equation of state. If you want compressible liquid, you need a udf for the equation of state.

VOF method in Fluent is limited to one compressible ideal gas phase.

Water at a uniform initial high pressure surrounding a gas bubble with a slightly lower pressure is a simple bubble shrinking problem and not a pressure wave problem. This does not require the water to be compressible. So although it may not simulate the physical situation that you want, you should have been able to get a result with Fluent. If you still have divergence problems even with small pressure differences, then you likely have an error.

Also I recommend the full Eulerian approach if you want best results.
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Old   December 10, 2015, 01:18
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Quote:
Originally Posted by LuckyTran View Post
Fluent has the capability, except for the equation of state. If you want compressible liquid, you need a udf for the equation of state.

VOF method in Fluent is limited to one compressible ideal gas phase.

Water at a uniform initial high pressure surrounding a gas bubble with a slightly lower pressure is a simple bubble shrinking problem and not a pressure wave problem. This does not require the water to be compressible. So although it may not simulate the physical situation that you want, you should have been able to get a result with Fluent. If you still have divergence problems even with small pressure differences, then you likely have an error.

Also I recommend the full Eulerian approach if you want best results.
Thank you again and I appreciate your time,

I have attached the picture of my desired problem which is a common shock bubble interaction. I wrote a CFD code for this problem and in that, the water was also considered as compressible (stiffened EOS was used for both liquid and gas phases). Now I want to simulate the same problem with a commercial software package. Regarding your discussion I have two questions:

1. Although from Ansys 14 on, Fluent has compressible liquid in its material density, should I use a UDF for the equation of state?


2. After clarifying my problem and my explanations, do you think the water does not need to be compressible? Or it should be compressible and a UDF for EOS should be given?


The answers of above the questions are so important to me and can save my time a lot.

Thank you very much sir,
Hamid

Last edited by Hamid-Moezzi; December 10, 2015 at 03:03.
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Old   December 10, 2015, 02:06
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I wasn't aware that Fluent already had some compressible fluid EOS's available. If that's the case than there's no reason why Fluent can't model your problem with or w/o shockwave. You only need a UDF if the EOS in Fluent is insufficient.

The bubble shrinking problem by itself does not need the water to be compressible, as the gas inside can expand and compress as a result of the externally applied pressure. But you do need a compressible fluid for there to be pressure waves and shockwaves in the water.

But Fluent/FVM does have trouble with shockwaves in general because of the discretization schemes used.
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Old   October 29, 2018, 07:36
Default Help with Shock Bubble Interaction
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Hello, please I am currently trying to simulate a shock bubble interaction case involving a supersonic flow through air and the subsequent compression of an helium bubble. I am doing this on the ANSYS Fluent package using the VOF model and Level set function. Fundamentally, I do understand that the density based solver is more suited to compressible and high velocity flows but this solver is not compatible with the VOF model on the ANSYS Fluent package, hence I have had to use the pressure based solver which is more suited to low speed incompressible flows. I have modelled a few cases but have not gotten any good results. A few cases has seen me choose air and helium as both incompressible gases (which clearly shouldn't be the case); air as an incompressible gas and modelled helium as compressible liquid with a very low bulk modulus value; and both as incompressible liquids. I have even tried the boussinesq approximation for helium which is most suited to bouyancy driven flows and treated air as an ideal gas but still didnt get the right results. Please, I need help urgently. Also, will I need to write a CFD code or a udf. Thank you and I look forward to any response.
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