Discussion Forum

Hi all, I’m new to physics simulation and I’m trying out comsol to see if I can learn a little about this.

I wanted to start with something I thought would be a really simple problem, but now have a problem figuring out how to get the force acting on my objects.

1. I created a 2 dimensional model with laminar flow and stationary study.
2. Under Component 1 I created a first circle c1, radius 1000 mm with center at x y coordinate 0,0
3. Then I created two small circles with radius 10 mm; c2 at coordinate -50,0 and c3 at coordinate 50,0
4. I added a Difference operation to subtract c2 and c3 from the area of c1, so now I had a large circular area with two small holes in it on each side of it’s center.
5. I added hydrogen to materials.
6. Under laminar flow I added an inlet and selected the 4 boundaries of c1 and a velocity of 1 m/s.
7. I added an outlet and selected the 8 boundaries of c2 and c3 and a pressure of 1 Pa.
8. Under study 1 I had to set Stationary Solver 1 to Linear to get rid of convergence problems.

When the above is computed I get a velocity field that looks like what I expected, and a pressure field that looks like it could be at least close to correct. Wow, so far so good!

Now I wanted to study the net force (the resultant) acting on the outlet c2. So I added an Integral to Results and set the dataset to sol1 and method to Summation over nodes. I set Integral 1 Selection to the 4 boundaries of c2.

The resultant should be 1 vector (strength and direction), but I haven’t figured out yet how to display this in a table so I created a 1D plot group with Dataset set to Integral 1. Under this plot group I created a Point graph and set y-Axis Data to expression reacf(comp1.u).

All I get out of this is one point (which is expected) at 0 on the y-axis, which to me looks like the force is 0, which it can’t be.

I suspect the expression reacf(comp1.u) might not be correct, but can’t figure out what it should be.

How can I compute and display the resultant force acting on c2?