Comsol-Pressure drop fluid flow in rectangular channel problem

[baranii2000]

Dear all,

I am trying to solve a simple model with Comsol to find the pressure drop in a rectangular channel W=5mm, h=0.6mm, L=10mm. When i try to set the inlet velocity to 0.5m/s the solver shows the error message "Maximum number of Newton iterations reached". The solver works fine if i set the input velocity to lower velocity(eg., 0.005m/s). when i increase the velocity to 0.05 m/s the solver ends up with the error message. I even tried to increase the height of the channel, still get the same error sequence.

I am using a Stationary, Direct (Pardiso)solver.

Any comments or help is much appreciated.

Thanks in advance

Arun

 

[minger]

I'm not familiar with COMSOL, but here are a couple things you can try. First, try ramping the velocity up. Solvers hate suddenly seeing a flow.

Also, you can try changing the boundary condition to total pressure at the inlet, and static at the outlet for a first cut. These tend to be more well posed than others and can have better stability.

 

[baranii2000]

Thanks Minger,

Actually your comment on the solver crash on seeing sudden high velocity at inlet was very useful. I remodelled as a time dependent file and now it works fine...

Many thanks

Arun

 

[minger]

Right, just to clarify a little more on what happens. Unless you normalize your mean flow at t=0 to some value, at that time 0, you suddenly impose a velocity at the inlet. One point inwards your trying to either take a derivative or calculate an integral with a discontinous flow.

You will calculate an artificially high flux at that point, which will then give you an artificially high time derivative. This will just continue until your solution blows up.