Fluid Flux in Confined Compression

[jon98]

Hi. I am new to FeBio, and I am running the test suits to learn.
In the test suit bp04, which is confined compression of a biphasic material, I have two questions.
First: I have prescribed pressure only in the top face, which means that in all the others' faces I have zero fluid flux by default. But when I go to plot the fluid flux in those faces they are not zero.
Second: What is an easy way for me to obtain the total fluid flux by time? I want the cumulative fluid flux, the total amount of fluid that left or entered the body by time, and not the instantaneous value.
Thanks in advance.
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[ateshian]

Hi,

... when I go to plot the fluid flux in those faces they are not zero.

The fluid flux is a vector whereas the natural boundary condition (zero fluid flux) actually applies only to the component of this vector normal to that boundary. In bp04, if you display the fluid flux as a vector plot, you can see that the vectors for all the elements point only in the Z-direction, which means that the fluid flux component normal to the other boundaries is indeed zero. This can be verified by also displaying contour plots of the fluid flux components along X and Y (they are on the order of 2e-17 or smaller, i.e., negligible compared to the Z-component, which is 1e-3).

Also, please keep in mind that the fluid flux is evaluated at integration points inside the element, not at the boundaries. So, when displaying the Z-component of the fluid flux, you will not get an exact zero value at the back Z-face (since FEBio only evaluates the fluid flux inside the element), even though that's the boundary condition being enforced there. This becomes less evident when using a finer mesh.

What is an easy way for me to obtain the total fluid flux by time?

The fluid flux component normal to a boundary is equal to the volumetric flow rate per unit area across that boundary. There is a plot variable called "fluid flow rate" which integrates the normal fluid flux over a selected boundary surface to calculate the volumetric flow rate (in PreView/FEBioStudio, remember to first Edit->Name selection... to select and name a boundary, then edit plot variables, add "fluid flow rate" and Add Domain... to select the named surface).

If you want the volume of fluid (instead of the volumetric flow rate) coming across that boundary, you need to perform your own numerical time integration of the volumetric flow rate. A simple trapezoidal rule would work.

Best,

Gerard