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-   -   Time Step with mesh deformation (http://www.cfd-online.com/Forums/cfx/90144-time-step-mesh-deformation.html)

 rbarrett July 1, 2011 13:20

Time Step with mesh deformation

Hi,

I was just wondering if anyone has any advice on the size of a time step to use with a deforming model.

I am attempting to simulate the compression of a piston within a cylinder, which has an initial acceleration phase, a CV phase and a deceleration phase. The total compression time is 16.6 ms, the acceleration is 2079 m/s^2, deceleration is 23493 m/s^2 and the CV is 12 m/s. What I am seeing is that the simulation fails when I reach the short deceleration phase. If I reduce the value of this deceleration by an order of magnitude, the simulation runs to completion perfectly, with a timestep of 16.6ms/32.

Is there any guidelines available on what size of a time step I should be using with this type of simulation and is there a specific way to deal with this high of a deceleration term?

 ghorrocks July 2, 2011 06:55

The fluid time scales are usually far smaller than the motion timescales, so whatever time step you need to resolve the motion, you are bound to need something far smaller for the fluids to be modelled accurately.

But obviously with such massive decelerations you need an appropriate time step to capture it, and the fluid mechanics associated with it are likely to require an even smaller timestep.

 rbarrett July 3, 2011 07:43

Thanks a million for your reply. What I am seeing with a very small timestep is that the mesh folds on itself; does this mean that my mesh must be much more dense to accommodate this much reduced timestep?

Also, in terms of a numerical value for the timestep, is there any guideline as to what size of a timestep I should be using, or is it a matter of reducing it until it works for this particular simulation? Could I use a function that has a larger value of timestep for the acceleration, CV and post compression stages and refine it for the deceleration phase in a bid to reduce computational time?

Thanks in advance for any help.

 ghorrocks July 3, 2011 07:56

The folding should have nothing to do with the timestep size. The fact you say it runs at a coarser time step is weird. But I would fix the root cause of the problem rather than trying different time step sizes. There are a few things you can do, varying mesh stiffness, remeshing, different mesh types etc.

Can you post an image of what your geometry looks like near where you have the problem?

 rbarrett July 4, 2011 04:18

1 Attachment(s)
Please find attached below an image of the mesh just prior to the mesh folding. As the mesh is quite dense, I have only included a portion of the mesh. As you can see from the image, the mesh seems to be ok at this stage.
Attachment 8276

 stumpy July 4, 2011 09:17

Double precision?

 rbarrett July 6, 2011 10:08

Thanks a million for all your help. I had already double precision selected. I think that I have solved the problem as it was related to a poor mesh and is no longer occurring.

 jfmorissette July 7, 2011 08:30

Stumpy, do you think that with mesh deformation and FSI, double precision is a must?

 stumpy July 7, 2011 09:44

I would use DP for all mesh deformation and FSI cases.

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