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Zmur June 21, 2012 04:12

Heated Cylinder with moving wall
I failed at setting up the following problem:
The inner volume of a cylindrical tank is separated into two parts by another cylinder. It can move up and down freely (for now I'm not taking friction into account) it also is weightless.
In other words, I have two isolated volumes, filled with air that are separated by
a moving wall.
There is no inlet or outlet.
No remeshing is used, just mesh deformation.
I use an expert parameter to override isolated fluid regions check.
The mesh is swept and consists only of quadrilaterals. About 10 000 cells, maybe less.
Time step is 0.01s Total time is 1s. (link will be active for a month or so).
The inner cylinder should start to move upwards when the temperature of the air below it raises. But it doesn't.

At first I tried to create a "rigid body" and use "rigid body solution" in the wall BC. This resulted in small cylinder "jumping" outside of the large cylinder after some calculation time.

After that I wrote an equation in CEL for the movement but it also didn't work (according to Newton's second law). The small cylinder still won't move.

I tried to separate both volumes into two domains and set a higher starting temperature to the lower one. Also I tried putting an Opening BC on the top wall of the big cylinder.

How can I setup this problem correctly?

Zmur June 22, 2012 07:11

I simplified the problem. Now I'm trying to calculate only compression of air inside the upper part of cylinder. One wall is a "rigid body" and a force of 500N applied to it.
Here are some results:

And the output chart:

The pressure of air inside the cylinder should stop the wall, but it doesn't. What could be wrong?

ghorrocks June 23, 2012 06:44

Does it give you the correct answer (ie pressure, temperature) if you define the bottom wall displacement (ie simple mesh motion) rather than rigid body motion?

Zmur June 23, 2012 08:12

I haven't tried it yet. Thanks for the advice.

Also, how can the pressure be negative? Is it bad or it depends on location or something?

ghorrocks June 23, 2012 08:17

Are you using an incompressible fluid?

Zmur June 23, 2012 08:33

I use air ideal gas from standard library. Heat transfer also enabled.
To be honest I don't completely understand how compressibility is enabled.

ghorrocks June 23, 2012 08:40

You need to enable total energy to get the fluid to be compressible.

Is the fluid going to show compressibility effects?

Zmur June 23, 2012 08:56

It should, because there is no openings in the domain, the air has no way out, and the volume of the cylinder changes.

Zmur June 23, 2012 14:55

The simulation with mesh displacement looks correct after I turned on th Total Energy setting.

Made the same settings in the simulation with Rigid body. Also seems OK. The velocity of the moving wall started to decrease at some point of the simulation.

I'll try the same with two cylinders. Hope that it will work.
Many thanks for the help ghorrocks. I would have never solved this out on my own.

Zmur June 24, 2012 08:37

Here are some results with both cylinders:
and pressure:

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