[vinerm]

The mesh in the tet region, particularly the small duct, which I suppose is the inlet, is very coarse. This needs to be refined. Do note that only mesh is received by the solver, i.e., Fluent. Therefore, the mesh should represent geometry as closely as possible. As you can observer, the small duct does not have a cylindrical surface at all. For FVM, this does not pose a conservation challenge but certainly the numerics would have oscillations, possibly leading to what you observed.

[RJE]

no i get oscillations in the first simulation with part1
but with part2 i get error after running it.

[vinerm]

Which one are you called Part1? Or does Part1 imply keeping everything as single body?

[RJE]

this one is part1

[RJE]

Originally Posted by srsel6

Have you made sure the upper section and the lower section are separate bodies? Meaning you should have 2 bodies. 1 body where it is completely static and the other where a dynamic mesh is applied. An interface will then be necessary to connect the two together without facing negative cell volume issues.

he told me also that connection must exist ? should i keep or delete it

[vinerm]

If you keep any connections, then you will have to create interfaces; leading to interpolation error and extra time for interpolation. Connections should only and only be used when you have a moving mesh. Not in dynamic mesh.

There must be some problem with the setup for system with two bodies that is causing trouble. But you should use Part2 and not Part1.

[RJE]

ok, for meshing there's no problem if i have photo 1 from one side and photo 2 from the other side ?
2-when running i get from the beginning reversed flow, could u explain why ?

[vinerm]

I ran a case using conditions based on the images you shared. The graph shows the motion of the piston for first few time-steps. I used two separate bodies. And I used air as working fluid. Since I do not know the material properties for your material nor do I know the pressure or mass flow rate, I applied some arbitrary pressure just enough to push the piston down. Spring constant is 78750 N/m and preload is 877 N; as per your first image. I do not see any wiggles and the motion is quite smooth with 1 ms as time-step.

Since the spring is preloaded, it will push the piston upward with a certain pressure and that depends upon the area of the piston. For a piston with 20 mm diameter, the pressure would be around 2794450 Pa. If the pressure in the cylinder is smaller than that, piston will push the fluid out. If it is higher, piston will be pushed out. So, now you know why there is flow reversal in your case.

[RJE]

u mean by motion the displacement of the piston ?

[vinerm]

Yes, you are correct. What you see in the graph is position of the piston starting from an initial location.

[RJE]

i'll try it again then.
u seperate body using split function ?

[vinerm]

The method does not matter. You can split it using any method. I did not split; I created it that way.

[RJE]

hey again,
i'm sorry for disturbing u this much but it still don't working
can i send u my project via mail so that u can discover what's wrong ?

[vinerm]

Could you just share the snapshots of your OneDOF setup and dynamic zone setup, including Meshing Option details within Dynamic zone setup? Also, could you tell me the height of each cell layer in the deforming zone?

[RJE]

ok the cylinder diameter is 120mm and the lenght is 30mm
for the orifice ; d=4mm and 30mm length
the cell taille du mesh =2mm

[RJE]

the piston surface (photo6)

[vinerm]

Why do you have a deforming zone called layering? Delete that. And then run.

[RJE]

yes i have call deforming zone layering.
i delete it but still having same error after 2 step

[vinerm]

Is the error related to negative volume?

[RJE]

Originally Posted by vinerm

Is the error related to negative volume?

yes , i try for time step 1e-3 and 1e-4