Piston Cylinder with constant pressure applied
 

Hi,

I have done some CFD related coding before, but I'm new to FLUENT, so please bear with me.

My problem is pretty straightforward. There's a piston inside a cylinder filled with a compressible liquid. A constant pressure is acting on the outside of the piston. There's no inlet or outlet. Only the liquid getting compressed.

https://i.imgur.com/STv6ibG.png

The liquid is inviscid, there's no heat transfer, turbulence or any other complexity involved. I need to get the pressure distribution inside the liquid with time, and also the position and velocity of the piston with time.

I've tried to model this using 2 approaches, by making an axisymmetric 2D model and making a 3D one. For the 2D model, I simply drew a rectangle, and considered one of it's edges as the piston. I chose the axisymmetric transient analysis option in the solution, and specified the boundary condition on the piston as pressure-inlet. All this while, I did not use the dynamic mesh option, as the piston motion will be of the order of millimeters (known by other calculations)

https://i.imgur.com/VGGJa7m.png

The problem I faced was that the solver considered the edge I had specified as the piston as fixed, i.e. it's position did not change with time. Here's a sample of the results I obtained after running the problem for 500 timesteps (timestep = 1 microsecond) showing pressure distribution

https://i.imgur.com/S0Owk0V.png

Here's another sample of the results showing the velocity distribution at the 500th timestep. If you can see it in the picture, the line representing the piston is blue, i.e. it's at rest.

https://i.imgur.com/HveYRvF.png

What should I do to overcome this? Any help will be appreciated.

Shreyansh