[fresty]

Hi,

I am trying to model a piston like assembly, well to make it very simple just a hollow cylinder with one end open immersed solid (modeled as rigid body solution) inside a larger cylindrical flow domain.. I plan to restrict all DOFs except the axial movement... the boundary conditions are set in a way that they would force the body to move forward and backward.. However, i would need the immersed solid to be permitted only to move in either forward or backward axial direction.. which means to further constraint the 1 DOF motion to one direction (this is to create virtual mechanical stops which are present in real time)..
The question is if I could get some help on setting up the CELs to restrict the immersed solid motion..(if that's possible)? I realize mesh motion is another option but would really want to head that way if there is no other option... I did try and search a lot related to CEL expressions to alter the DOFs of motion but could not find anything more than the usual rbstate function (which I presume is just a quantitative type function)..

Thanks for any help.. cheers.

[ghorrocks]

The normal way to model pistons is using moving mesh. Can you explain why this is not appropriate in your case?

[fresty]

Moving Mesh method is is without doubt appropriate for my case but probably slightly extravagant & overwhelming considering the objective of the simulation.. I could save a lot of time on meshing and then associated mesh deformation/ re-meshing, computation time etc. only if i could restrict the immersed solid motion somehow.. as mentioned, i did that already to 1 DOF but further restriction of direction is also required.. Could a CEL be applied on Immersed Solid (Rigid Body Solution)..?

[ghorrocks]

Moving Mesh method is is without doubt appropriate for my case

Is that right? Can you explain why it is more appropriate than moving mesh?

You seem to think the immersed solid approach is much simpler and easier to solve than moving mesh. In my experience moving mesh is quick to set up and works well and does not take up too much CPU time.

And don't forget the restrictions on immersed solids:
* No mesh deformation
* No heat transfer, additional variables, combustion, compressible or multiphase flow etc
* No wall functions
* and many others.

Most piston/cylinder arrangements are modelling compressible gases so that means the immersed solid approach is not appropriate.

Anyway, let's have a look at your question...

Assuming this piston motion is coupled with the fluid, then isn't the dynamics tab options (18.1.3 in the CFX-Pre doco) what you are looking for?

[fresty]

I guess I failed to convey that one correctly, I actually meant to say that Moving Mesh method is of course more appropriate than the immersed solid approach that comes with massive limitations, major ones you just mentioned..

I agree with you about moving mesh ease but in my case i am just trying to solve the momentum equations i believe with a preset pressure result already obtained in form of a sinusoidal pulse from an earlier compressible flow simulation.. all i need now is to observe the movement of the rigid body due to the pulsating pressure.. the problem only arises when i would like the rigid body to have a limited or no motion in +ve X direction while free movement in -ve X direction...(more of like 1/2 DOF)..
The reference (18.1.3) has explanation & details of 6 DOF options and usage however once again the most you could assign is a free axial movement not the +ve/ -ve directional constraint..

I am in parallel working on moving mesh but needed to request any help on if CELs work on further restraining the immersed solid rigid bodies motion..

[fresty]

Hi Glenn,

I am working on moving mesh rigid body model now and have performed basic checks of mesh motion.. could i please request some guidance on setting a constraint by using a CEL expression..? The case is actually pretty similar to the one in thread: http://www.cfd-online.com/Forums/cfx...body-wall.html

Again, the requirement is to have unidirectional motion rather than both +ve & -ve axial freedom.

[singer1812]

Are you using the built in 6DOF to move this or your own CEL equations?

Either way, constraint in the direction is easy.

[fresty]

I am using the built in 6-DOF (with only X direction free) and trying to figure out the CEL expression to restrict the motion of piston to either +ve or -ve direction while maintaining the rigid body motion by virtue of pressure pulsation.. any advise on the CEL expression?
Could I infuse something in CCL, like:

NewX = Total Mesh Displacement X
NewX <= 0 [m]

[singer1812]

Have you actually setup a 6DOF rigid body?

You can turn on only the equaitons you need within GUI for the body.

Based on you stating that you are using an expression leads me to beleive you are not using 6DOF but using mesh motion.

There is no need for that expression in the built in rigid body dynamics.

[fresty]

Sorry for the confusion, i may be using the terms wrongly or mixing it up here..
i have setup the rigid body as it shows in the image.. apart form that, the surrounding cylindrical domain of the rigid body is set to rigid body solution.. i would rather attach the CCL for your quick review.. I tried following the ball valve setup tutorial for the entire system... which means that the piston is modeled as rigid body while the sub-domain which contains the piston is set to solve as rigid body solution.. the mesh motion works fine as in the piston moves axially backward & forward without constraints as expected however, the constraint in forward direction is required (to emulate mechanical stops in real system)..



1.JPG

1.txt

[singer1812]

I dont have time to go through the ccl.

But, I take it you mean you have set up the rigid body, the motion is in the direction you want it to go, and you want the rigid body to stop moving when it reaches a hard stop?

This is a bit more complicated using rigid bodies as CFX doesnt have built in interferance detection (as this is approximately that).

I havent done it with rigid body before but have done it with pure mesh motion and this should be easy to do with prescribed mesh motion. But you will need to build in the equations of motion into CEL. You will have control on the hard stop this way.

[fresty]

Yes exactly.. the direction (X DOF) is what i have selected but need the body to stop at collision/ hard stop.. as you have advised and somewhat from all i have been able to search, i guess i could definitively conclude that specified displacement with equations of motion would be needed to model a hard stop.. any further advise or a push towards anything similar that you've come across (CEL expressions/ UDFs/ setup etc.) would be great..

Thanks a lot for your time & help. Cheers.

[singer1812]

Look to this post for putting in motion equations in CEL.

http://www.cfd-online.com/Forums/cfx...-language.html

Then you can add logic based on X location of your body to zero out any more displacements.

[fresty]

Thanks a lot Edmund!
Would get on with it and i guess it's pretty much clear now... thanks again. May comeback knocking for advise during implementaion though.. cheers..