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Introduce a change in temperature through a subdomain

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Old   August 31, 2018, 11:47
Default Introduce a change in temperature through a subdomain
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In my simulation I would like to modify/change the temperature field after a certain time. For multiple reasons I cannot do this at the inlet and therefore I am using a subdomain. With the help of a energy source term I was able to inject energy into the system. The energy then creates the desired change in temperature.
The problem is that the injected energy also creates a pressure wave which travels both up- and downstream in the domain. The attached picture (Picture1) shows the created pressure wave (top) when introducing the temperature fluctuation (bottom).
When using NRBC at the outlet the pressure wave will eventually exit the domain. However in a more complex situation this pressure wave will reflect of the geometry which will mess up my results.
I need a situation shown in Picture 2 where there is no pressure fluctuation present but a temperature fluctuation.

Is there a way I can change the temperature field without causing this pressure wave?

Your help will be much appreciated!
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File Type: jpg Picture_1.jpg (52.0 KB, 10 views)
File Type: png Picture_2.png (150.2 KB, 6 views)
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Old   September 2, 2018, 06:56
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Glenn Horrocks
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I assume this is a compressible simulation - in which case heating up the gas will lower its density so it pushes the nearby gas out of the way to make room for itself. Pushing this gas out of the way is done by the pressure waves.

This means the pressure waves are inherent in using an energy source to heat up gas in a compressible simulation. It is not physically possible to heat it up with no pressure waves.

If this does match what you see in your system then you should look for a more physically realistic model for the source of the heat.

So what causes the heating in the actual system?

(All the previous comments assume a compressible fluid. If you are using an incompressible fluid you should not get pressure waves from heating it. If you are getting these waves but your fluid is incompressible then there is something wrong with your model.)
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
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