Attempted cylinder simulation

[Anonymous]

Hi,

I am attempting to model a cylinder motion with one flow driven valve on the inlet and a type of check valve on the outlet.

MY issue is that the boundaries I have to work with are pressure based and as such, dont have direction in the software whereas they do in the scenario I am attempting to simulate. 

This is how its supposed to work:

inlet has a small positive pressure and as the cylinder moves, the fluid should open the valve and flow into the chamber as the piston creates negative pressure. Once the cylinder reaches BDC and starts pressuring the fluid (incompressible), the pressure becomes much higher than the inlet pressure, thus causing the valve to shut. the pressure buildup should then force a check valve of sorts to open, which operates at a much higher pressure than inlet.

The setup of the valve reacting to the flow is easy and so is the piston motion, however I cannot make my boundaries work properly as I cannot give the pressure a direction so fluid comes into the cylinder from the outlet.

I come from ansys CFX so im not used to the autodesk CFD yet. I dod some reading but couldnt figure it out. it is very simple to do those boundaries in CFX but im stuck here. help please!

[Anonymous]

Hi,

I am also new to Autodesk CFD. But, as far as I can understand you want to do transient Simulation not a steady one.

For pressure as BC, you could not give direction. But, the values define the direction. Like flow will be from higher point/potential to lower point/potential, unless and until something forcibly moves it in reverese direction like external force. In your case, as you are saying, after BDC other check valve should open which is at Higher pressure, so you are having High Pressure point at outlet and that is why flow comes from outlet.

I think you should define transient BC (like pressure varying with time). You have to figure it out, at what time pressure at outlet should reach maximum value which you want. Every BC you have to make time transient. And  outlet pressure should reach that particular higher value only when piston has pressurized the fluid traped and check valve should open at that instant. Then due to pressure of fluid being more that outlet one will make it go from inside to outside. But, you have to figure out timing. I might not be 100% right. But, logically I can think you have to go that way or may be some experts could comment.

But, what is the purpose of it? You just want to see how fluid flows or you want to study some parameters?

Regards,

Rajdeep Rajput

[Jon.Wilde]

Be really mindful of the mesh here too - you are going to need a fine mesh around the moving parts. Have these parts outside of the model initially and use the 'initial position' to move them in to place.

This way you can re-enable the mesh enhancement (off by default in motion analyses).

Also, be sure you have a small enough time step. 

You results needs to be time step and mesh independent 🙂