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Shaoming August 24, 2000 04:31

some queries
 
I've a couple of queries:

1. What is the difference between static temperature and total temperature?

2. Is there any guide to the extent I can adjust the under-relaxation factors to achieve convergence?

3. It says somewhere in the Fluent manuals that convergence problems might be encountered for long thin geometries. How can I avoid this problem for a long pipe?

Thanks!

John C. Chien August 25, 2000 00:59

Re: some queries
 
(1). Static temperature is the temperature. (2). Total temperature is a derived property. Check out most gasdynamics books for the definition. (3). The relaxation parameters are there for you to use In other words, these are used to adjust the newly computed "guessed value". If the newly computed value (guess) is 60.0, you can adjust it to zero by setting the relaxation factor to 0. In this way, the solution will not be updated at all. It will not change from the initial guess. (4).There are two ways to avoid the problem. One is not to compute it. The other is to test it out, and fool around with it. They don't want you to sue them for getting into the trouble. But if you decided to take the second approach, there is a possibility to discover a new solution.

Maurizio Barbato August 25, 2000 08:16

Re: some queries
 
Dear Shaoming,

just few answers.

1.)Static temperature is what you measure with a thermometer. Total temperature is the temperature of a gas brought at quiescence adiabatically. This means that for a gas at rest the two temperatures are equal. In summary, the total temperature (or stagnation temperature) is a measure of the gas energy (internal + kinetic).

2.) There are not universal rules for that. There are good suggestions that you can find in the Fluent manual. As a rule, the lower they are the safer you are, BUT the slower is your convergence. Some times especially starting some calculation which is very stiff can be necessary to use values < 0.1. Anyway their value is problem dependent and experience (yours with time or of some CFD expert) will teach you.

Convergnece problems in a long pipe can be due: - to a bad grid (too high aspect ratio of the cells) - to wrong settings of boundary condition at exit. - to a bad initial solution Suggestions: - check the grid cells aspect ratio. If excessive modify the grid. - check boundary conditions - use a initial solution which is not too far from the guessed solution.

Cheers

Maurizio


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