Pressure inside a cavity

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  • sam2000
    Junior Member
    • Jun 2020
    • 2

    Pressure inside a cavity

    Hello,
    I want to model a 2D cavity that filled with an incompressible fluid in two steps. In the first step the pressure inside the cavity will increase inflate the cavity. In the second step, I want to use the last pressure magnitude inside the cavity from the first step as the initial value and then an external force will be applied to exterior side of the cavity to deform the cavity. I am indenting yo use FSI. My problem how to make the internal pressure changes while the volume kept constant in the second step? If I applied the pressure as load on the internal surface, it will be constant and wont change.
    Thanks..
    Sam
  • maas
    Lead Code Developer
    • Nov 2007
    • 3400

    #2
    Hi Sam,

    You could look into adding a "volume constraint". You can find it in the "Physics\Add Constraint" menu. It will calculate the pressure necessary to maintain the volume, bound by a surface. The way I would recommend using it, is in the first step you apply your pressure, then in the second step you apply the volume constraint.
    Since you mention FSI, I don't think this approach will work when the cavity is filled with a fluid. In that case, I'm not entirely sure, but I don't think you need any special boundary conditions, since the fluid (given the right material properties) will be incompressible and thus exert the necessary pressure to maintain its volume.
    Hope this helps.

    Cheers,

    Steve
    Department of Bioengineering, University of Utah
    Scientific Computing and Imaging institute, University of Utah

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    • sam2000
      Junior Member
      • Jun 2020
      • 2

      #3
      Many thanks Maas..
      I read some old post on similar questions, but still not sure how to use volume constraint. I am attaching a model, could please help me to setup the volume constraint.
      Regards,
      Sam
      Attached Files

      Comment

      • jshim777
        Junior Member
        • Mar 2018
        • 11

        #4
        Hi Sam

        After you inflate the cavity, if you want to maintain volume without fixing the pressure even with an external load, one thing you might be able to try is fixing the fluid at the inlet (preventing it from leaving). That way the same amount of fluid is within the cavity. It may help to make the fluid is as close to incompressible as possible, like K=1x10^12, but you should double check whether or not increasing K has a significant effect. Once you start this second part of your analysis you would have to make sure there is also no more pressure_offset (make load curve go to 0 at the time when second step starts). Also I noticed that you used a isotropic elastic material with a poisson ratio close to 0.5. For finite deformation it is better to use a neo-Hookean instead of the isotropic elastic solid. If you want to simulate an incompressible solid it might be better convergence wise to use a Mooney Rivlin solid with a reasonably high bulk modulus.

        However if you enclose the fluid the problem might become very stiff and not converge well. If that happens you may have to take some steps like refine the mesh, bias the mesh more, take smaller time steps, use quadratic elements, ramp up loads slower, etc.

        Hopefully this helps!

        Comment

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