[Ledeeb]

Hello; I am simulating a simple 2D piston. I have a mobile interface that is moving according to a field function.

The model I am working with:

Coupled Energy
Coupled Flow
Gas - Air
Gradients
Ideal Gas
Implicit Unsteady
Laminar
Two Dimensional

The inlet, wall and the oulet are concidered adiabatic.
P initial is 1 bar.
P reference is also 1 bar;
T initial is 300 K.

I am using morpher option to use the mobile mesh.

The inlet interface is translating upward with a constante velocity of 0.03 m/s in order to compress the fluide (air as ideal gaz). The physical time is 30 s with a time step of 0.01s.

The problem is that when I observe the PRESSURE in the scene plot; the pressure remains almost constant equal to 1 bar.

Why the pressure does not increase ?


Thanks

[Owain_Parry]

Hello.

Is the boundary you refer to as "Inlet" an inlet type of boundary or a wall?

[Ledeeb]

Hello,

Inlet, wall and the outlet are just names that I gave. All these three are
considered as type wall.

Symmetry wall is considered as type symmetry plane.

In the '' inlet '' conditions, and in the morpher properties I took it as rigid body. The field function associated to the linear displacement in the inlet morpher, is as follows: y=V*time.

As for " wall " and " outlet ", and in the morpher properties I took it as Fixed.

For the thermal specification for the all the boundaries, I considered as Adiabatic.

I visualize in the scene plot, the density. The density remains constant. So in other word the mass is decreasing. I don't know how can I have here a leakage.
So maybe I have to check that there is no leakage ? I don't know how.

I tried also to change to reference pressure to zero, and I also have the same problem.

I understand that my velocity is very small so my mach number is very small but this doesn't mean that the pressure will not change.

Thanks for helping me.

[marmot]

if you have a closed system, all 4 sides are walls but you give them different names, and you morph the bottom plane to compress the pressure will go up, but if you really do have an outlet condition than instead of the pressure increasing mass will leave the system and the pressure will remain constant

[Ledeeb]

Dear marmot,

My 4 sides are wall type. And I applied morpher to the bottom plane but I always have the problem. The pressure doesn't vary.

So the "Oulet" side is not an outlet condition. Outlet is just a name that I gave. It is taken as a wall type to be able to compress the air inside these walls.

So I don't know where is the problem.

Thank you.

[marmot]

OK than instead of looking at scenes make some reports. volume average report of the region and choose pressure, also do a surface average report and choose the outlet plane, right click to create monitor and plot from both reports. Also create a volume integral report and choose density ensure you are not loosing mass in the system, there could be a bug.

Now when running you can observe numbers instead of scenes,

Also very simple, check the dimensions of your problem, could be you think 100mm and instead it is 100m.

[Ledeeb]

Dear marmot,

Below I uploaded the mass and the pressure plot. The mass is decreasing; so there is leakage in my model and that's why the pressure is not increasing. How Can I assure that there is no leakage?

Thanks a lot for your help.

[marmot]

-Ensure at each time step the solution is converged but most likely you are doing that

-Just take one time step and examine the mesh, are their bad cells, did it converge, did it move in the right direction

-Create a flow report for all boundaries and make sure it is zero

-Make sure you have no source/sink terms for mass in your region

-There could be a bug with a 2D domain and morpher, try making a 3d model and use symmetry planes for front and back

contact support or see if there is a similar tutorial and see what is different between the cases

[Ledeeb]

Hello,

I think I found where is the problem:

Density= Absolute Pressure / R*T (As defined in Star-CCM+)

and Abs Pressure = Static P. + Reference P.

where I took Ref. P = 1 Bar ; and I plot the density. It remains cte and equal to 1.16. That means that the static P. equal to zero.

So Navier stokes equations aren't solved ? Can someone have an idea about it ?

Thanks in advanced.

[Send_help]

Dear Ghady,

I'm having the except same problem. Could you solve it in the end? I would appreciate your help a lot.