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Thread: Triphasic model problems

  1. #1
    Join Date
    Apr 2016
    Posts
    2

    Default Triphasic model problems

    I am trying to run the attached Triphasic model but unfortunately it keeps crashing at around 11 seconds in, towards the end of a ramp compressive strain. Postview suggests that the fluid pressure rises just before the model crashes but I am struggling to understand why this is the case. Could anyone help?

    triphasic.feb

  2. #2
    Join Date
    Dec 2007
    Posts
    639

    Default

    Hi Fahd,

    For the given material properties and ramp speed you have chosen, the elements facing the rigid porous interfaces are being compacted such that the triphasic material is losing all of the fluid in its pore space. You can check this result by plotting the relative volume J: You will notice that your analysis struggles and stops when J approaches 0.2, which also happens to be the solid volume fraction you selected for this material (i.e., the porosity was initially 80% and now has reduced to 0%). As the fluid volume reduces to zero, the fixed charge density (which is the number of fixed charges per volume of fluid) tends to infinity, which is a problem from a numerical perspective. This is the primary reason why the analysis is unable to continue past this point. The multiphasic material does not function when the pores are completely compacted.

    To fix this problem you can choose to load the triphasic material more slowly (more practical), or take tiny time steps (less practical). In theory, if you take tiny time steps, the fixed charge density may increase more progressively without shooting up to near infinity, which will cause the material to stiffen and perhaps prevent a complete collapse of the pore volume, but that also depends on the ramp speed. Reducing the time steps is just a numerical workaround to what is essentially a physics problem: If the pores truly collapse in an actual experiment, then fluid can no longer flow through the porous platens (since the permeability goes to zero). If the tissue is truly perfectly confined, that would imply that it now behaves as a nearly incompressible material loaded in confined compression, which means that the load response will shoot up (theoretically to infinity) and damage the load cell.

    Other suggestions: Since you are using more than one solute, pleas switch the Module from "solute" to "multiphasic". Also, since you have opted to use an osmotic coefficient Phi less than unity, please remember to adjust your effective fluid pressure initial and boundary conditions to be consistent with this choice. In particular, if the Na and Cl ions have concentrations of 150, the effective fluid pressure is pe=-R*T*Phi*([Na]+[Cl]), where R=gas constant, T=absolute temperature, Phi=osmotic coefficient, [Na]= Na concentration, etc.

    Best,

    Gerard

  3. #3
    Join Date
    Apr 2016
    Posts
    2

    Default

    Thank you for the explanation Professor. I will try changing the parameters as you suggested and try running the model again.

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