# Fluid particles penetrate boundary

Hi, everyone!

I am new to dualsphysics and I am testing the dambreak case with Freecad following https://design.sphysics.org/wiki/doku.php?id=dam-break-example.

I just found a strange behavior that in the initial condition, the fluid particles penetrate the boundary, please see the attached figure, blue is boundary and red are the fluid particles.

To give more details, I changed the case to 2D and the boundary wall have thickness 10 mm while the dp is set 5 mm.

does anyone have any ideas to avoid it?

Regards,

Duan

• It probably has to do something with the order of initialization. If you create the wall first and then the fluid, the fluid particles can replace the wall particles if the two initialization domains slightly overlap at the edges.
To give more details, I changed the case to 2D and the boundary wall have thickness 10 mm while the dp is set 5 mm.
Also I do not understand this concern. You set the wall to have a thickness of 10 mm, and set the particle spacing to 5mm, which means 2 particles will be "bound" and act as the wall.

What I did not understand is exactly why the fluid particles can replace the wall particles, because when I did the initialization, there is supposed to be no overlap, while in fact there is a overlap.

I found that I could make a small distance (equals to dp) between the fluid and water to avoid the overlap. But I am not sure it is the best solution or not.
You set the wall to have a thickness of 10 mm, and set the particle spacing to 5mm, which means 2 particles will be "bound" and act as the wall.
This is another doubt. By setting the wall thickness = 10 mm, and set the particle spacing to 5mm, "2" particles are supposed to be "bound" and act as the wall. However as shown in the picture, there is "3" particles bound as the wall. Why?

• There are three particles since, it starts from 0, then goes to 5 and then goes to 10 mm. Sometimes because of numerical error you would only get two because it might think that it starts at 0,001 or something. Getting three particles means that it has been done correctly in this case.

The fluid particles can penetrate the boundary if the particle spacing is too big, which I believe is what happened here. If the particle spacing was 1 mm I don't think you would encounter a problem.

Kind regards
• @duan Ah, I made the same mistake as you. With thickness =10mm and dp=5mm, there are 2 gaps present. Kind of hard to explain. Lets say particle A is placed at point 1=at 0mm, then there is a gap of 5mm, and point 2 particle=at 5mm, another gap and last particle/point=at 10mm. This means 3 particles were placed, with 2 gaps in total.
• @Asalih3d
The fluid particles can penetrate the boundary if the particle spacing is too big, which I believe is what happened here. If the particle spacing was 1 mm I don't think you would encounter a problem.
I tried with 1 mm, the problem remains at z=0 mm, see the attached figure.
Since the wall is set to (-10 mm to 0 mm) and the water (0 mm to 300 mm) on Z direction, I think it is probably both fluid particles and wall particles starts from z=0 mm, so there is one layer particle overlap at z=0 mm, no matter how small the dp is. And if the water is set to (dp mm to 300 mm), the overlap is gone.