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Hello,

I am having a technical problem with the heaviside step function in COMSOL.

My model is composed of a transient, Darcy flow (pressure) problem. I have attached the model.

There are two subdomains; namely rock and fluid. The permeabilities are 1e-22 and 1e-10, respectively. This is a pressure decay experiment, i.e. the fluid is given an initial pressure of 1 (normalized) and the rock is maintained at 0 (t=0).

I noticed that the pressure oscillates when being solved and realize that this is a stability problem (a numerical abberation) which generates a negative pulse wave and a sudden drop inside the fluid cavity at the first timestep.

I realize that this can be solved with brute force, i.e. mesh discretization and timestep control. But this will only work for unidirectional flow patterns and not bi-directional flow patterns. bi-directional flow would occur if the fluid cavity was smaller than the rock sample.

Could anyone help me define a heaviside step function that can control the negative pressure fluctuations in this problem?

my email address is pa.selvadurai@gmail.com


Thanks,

Paul Selvadurai