Hey everyone,
I'm trying to use LS-DYNA (R11) to perform ICFD analysis on blood flow through a porous medium, specifically a medical-grade foam. For a quick check of the program's capabilities, I've gathered some Darcy flow data through a soil sample and have aimed to recreate this in silico.
The setup simply consists of a digital cylinder of porous medium (i.e. one part volume and one fluid) with a pressure gradient applied across the length (i.e. a pressure at the inlet and the zero-pressure condition at the outlet). The problem is steady-state. The cylinder wall is set to freeslip, as I've experienced issues with the no-slip condition for whatever reason. The porosity and permeability match the calculated porosity and permeability from the real-life test data. Here, I'll clarify that I'm using the intrinsic permeability [m^2]. I'm solving using the isotropic Ergun correlation, and the simulated fluid is water.
When I run the simulations, the velocity is many orders of magnitude off. For reference, the velocity should be around 5e-8 m/s; I get around 1 m/s. I've checked units, the mesh, etc. multiple times. (I wouldn't expect it to be a mesh convergence issue anyway, since my mesh is quite fine and the flow is so slow.) I've also tried using different correlations (Ergun vs Darcy Forchheimer) despite the fact that the inertial term should be negligible for creeping flow. On another note, I've noticed that using the two different solvers (fractional step vs monolithic) gives wildly different results for the same input.
I ran an identical simulation in OpenFOAM and was able to get precisely the right results on the first simulation, so I know my general approach is correct. However, I have other reasons for not wishing to switch over to OpenFOAM.
My question, then, is: Am I misunderstanding how LS-DYNA handles the permeability term? (The porosity term should have little effect, and indeed sims with different porosities are basically the same.) Am I using a completely incorrect value? Dimensional analysis of the equations in the manual suggest it's treating the permeability as a value with units [m^2] as expected, but obviously that hasn't worked for me so far. Any ideas? Does anyone have experience with porous sims in LS-DYNA?
Thanks.
P.S. I'm still not opposed to thinking I screwed up the units if other people have had these sims go right.