# Question about Pressure Field in ANSYS Fluent

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 March 3, 2016, 14:01 Question about Pressure Field in ANSYS Fluent #1 New Member   Jian Feng Join Date: Mar 2016 Posts: 6 Rep Power: 9 Hi, I am a beginner of ANSYS Fluent, and looking for help. I have got a question about setting a oscillating pressure field in ANSYS Fluent. Basically, I am trying to simulate a bubble oscillation behavior in a sinusoidal acoustic pressure field. It looks like the 'floating operating pressure' can only set a steady pressure field. What I need is a pressure field applied to all of the domain, but with a time depended function. Does any one have the idea how to make it? Thanks.

 March 6, 2016, 11:23 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,631 Rep Power: 65 You could just specify a time-dependent pressure at your domain boundaries. Pressure inlet or outlet. Run an incompressible or constant density simulation (very non-physical) and that will give you the infinite sound speed needed to achieve a uniform pressure field. fjmaxfjmax likes this.

 March 6, 2016, 21:35 #3 New Member   Jian Feng Join Date: Mar 2016 Posts: 6 Rep Power: 9 Thanks for your information! I am just a beginner...so I have one more question. What should I do after I get the sound speed? How to put this parameter into fluent?

 March 6, 2016, 23:56 #4 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,631 Rep Power: 65 Why do you care what the sound speed is? If you want a uniform pressure field, the only way to achieve that is to have an infinite sound speed which can only be done via an incompressible or constant density substance. The sound speed is specified by your equation of state.

 March 7, 2016, 11:48 #5 New Member   Jian Feng Join Date: Mar 2016 Posts: 6 Rep Power: 9 Thanks for your reply. I have a stupid question. So basically, the acoustic field is just a result of pressure inlet/outlet boundary condition? I should only give the oscillating pressure to boundary instead of the whole domain (bubble and surrounding fluid)?

March 7, 2016, 13:38
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Lucky
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 Originally Posted by fjmaxfjmax Thanks for your reply. I have a stupid question. So basically, the acoustic field is just a result of pressure inlet/outlet boundary condition? I should only give the oscillating pressure to boundary instead of the whole domain (bubble and surrounding fluid)?
In Fluent that is the only thing you can do. In reality, that is how you would achieve acoustic forcing.

If you want to impose an oscillating pressure field to the whole domain, you no longer have a boundary value problem and the Navier-Stokes equations are no longer applicable. This is a low-order model and has a completely different set of governing equations.

Differential equations can be boundary value problems or initial value problems, where you specify constraints at the initial time or at spatial ends of the problem. You cannot specify a time-dependent pressure field (you cannot put constraints on the interior).

An incompressible substance will react instantaneously to changes in pressure at domain boundaries. If you want finite speed of sound (then you should use a better equation of state).

 March 7, 2016, 15:42 #7 New Member   Jian Feng Join Date: Mar 2016 Posts: 6 Rep Power: 9 I am pretty clear what to do now. Thank you so much!!!

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