CFD Online Logo CFD Online URL
Home > Forums > CFX

Moving Mesh

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Display Modes
Old   February 15, 2007, 11:34
Default Moving Mesh
Posts: n/a
Hallo everybody!

I'm simulating a closed gas spring (Piston-Cylinder combination, no valves, no inlet/outlet) underlying sinusoidal compression/expansion. Movement of one boundary (Piston surface) is modeled by a simple expression, making the boundary Piston make cyclic displacements (BottomDeadCentre - TopDeadCentre) and, thus, moving and deforming the mesh. The simulation works (convergence of RMS=10^-4 is reached only with extremely small timesteps - ~10^-7s; with moderate timestep of 5e^-4 RMS is ~10^-2), but what I can't resolve is why the timestep has no relation to the frequency that my Piston is moving with. Namely, if I run it with 300 RPM and a timestep of 5e^-4, it runs and I get the results, with a lot of "numerical noise" though, as RMS is as mentioned 10^-2 - 10^-3, but it works. But if I run it at 2 RPM, the lowest timestep still has to be 0.001s or 5e^-4, otherwise the simulation stops after a couple of time steps. Cycle in the first example takes 0.2s, while in the later 30s. Thus the conclusion there is no relation between the Piston (and mesh) displacement in time and the actual time discretization!

Any ideas??
  Reply With Quote

Old   February 15, 2007, 17:24
Default Re: Moving Mesh
Glenn Horrocks
Posts: n/a

CFX is an implicit solver so the following comment is not strictly speaking a constraint, but the principle is certainly a guide.

For explicit solvers in incompressible flow it is well known the Courant number is a constraint on the size timestep which is stable, CN=U*DT/DX where U is the fluid velocity, DT is the timestep and DX is the mesh spacing.

For compressible flow the constraint is the Courant Freidrichs-Lewy criteon (CFL), where CFL=(U+a)*DT/DX where a is the local acoustic velocity.

In your flow the velocities are small compared to the acoustic velocity, so even though you run the simulation physically faster, the CFL number does not change much so the allowable timestep is roughly the same.

Regards, Glenn Horrocks
  Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
SnappyHexMesh for internal Flow vishwa OpenFOAM Native Meshers: snappyHexMesh and Others 24 June 27, 2016 08:54
How to let the mesh motion solver just solve a small region near a moving boundary? zhajingjing OpenFOAM Running, Solving & CFD 9 April 28, 2016 04:15
how to set periodic boundary conditions Ganesh FLUENT 13 January 22, 2014 05:11
external flow with snappyHexMesh chelvistero OpenFOAM 11 January 15, 2010 20:43
Waves halted by moving mesh Piotr Czajko Main CFD Forum 1 November 24, 2007 03:00

All times are GMT -4. The time now is 16:53.