Dear all,

I am a Biomedical Engineering Master's student, and for my thesis I am working on a Comsol time-dependent model that includes the following physics modules: Free flow in porous media (fp), Transport of diluted species in porous media (tds), and Darcy's law (dl). The model aims to reproduce the conditions inside a perfusion bioreactor chamber; therefore, it simulates the fluid flow in and around a porous polymeric tubular scaffold, with cells seeded on the luminal surface.

I am interested in observing the concentration profiles of 2 solutes (oxygen and glucose) in the lumen of the tube, as well as inside the wall of the construct, while also taking into account the cell consumption of both solutes.

The geometry is 2D axi-symmetric. The fluid inlet boundary condition is a velocity sine wave with respect to time, and the flow conditions are of co-current flow.

There is a periodic boundary condition defined in the fp module, to recreate the closed-circuit conditions of the experimental setup; accordingly, a periodic boundary condition is also defined for the tds module. Furthermore, while the concentration of the oxygen entering the system (inflow boundary condition) is kept constant due to its continuous supply through an air filter on the reservoir chamber, for the glucose concentration I put a boundary probe performing an averaging operation on the outlet, to then use the probe variable as the inflow glucose concentration condition. This is to emulate the depletion of glucose due to cell consumption.

As previously said, the study is time-dependent and carried out over the course of 60 seconds, with 10 s time steps.

What I get once I compute is a convergence plot (reciprocal of step size vs. time step) fluctuating between 20 and 320. I am struggling to understand how to interpret this in terms of error and solution tolerance, and I would appreciate if anyone could help me make it clearer.

Thanks,

Andrea.