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Hello all,

I am new to using comsol and for an university assignment I have to simulate the behaviour of an IV infusion tube. So what I did so far is:

I created an tube with inner diameter of 1mm and outer diameter of 2mm and a length of 100mm. Moreover I used a boundary load at the inner surface of the tube of 100mmHg, to represent the pressure applied by a fluid, I specifically didnot have to simulate the fluid. What I did then was creating a small ring around the tube with a thickness of 0.4mm and a width of 2mm. This ring represents a force sensor, and using a boundary probe on the inside surface of the ring, I tried to find the force acting on this surface and thus what force the sensor would measure.

I am currently facing 2 problems. For the ends of the tube, I wanted to use the periodic boundary condition, to represent a really long tube, without having to simulate it. However, when doing the stationary study I immediately get the following error:

Failed to find a solution. The relative residual (0.0043) is greater than the relative tolerance. There was a warning message from the linear solver. Iterative refinement triggered. Returned solution is not converged. Not all parameter steps returned.

Therefore I used a fixed boundary condition on both ends and simulated it then. However the problem is now that for different lengths I obtain different pressures at the inside of the ring. namely: 100mm = 6749.5Pa, 200mm= 7219.2Pa, 300mm= 6369.6Pa, 400mm= 7124.1Pa, 500mm= 9434.1Pa, 600mm= 7020Pa, 700mm= 5077Pa, 800mm= 5252.6Pa, 900mm= 7466.2Pa, 1000mm=11026Pa

I first assumed that the pressure would remain the same, since the pressure doesnot change inside the tube. Moreover, there is no trend in the values and they change randomely.

Does one of you have any suggestion on how to fix this, or understands what I am doing wrong? I am not able to share my file since it is too large, but if you need additional information please let me know

With kind regards