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Old   May 14, 2009, 16:00
Default Question about grid scaling...
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Iaroslav
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Hi everyone,

I have a question about grid scaling in Fluent 6.3.16. At the moment I am working on modeling a dimensionless domain (axi-symmetric) for free jet flow (supersonic, compressible ideal gas). My intention is to have all my dimensions in terms of diameter lengths (ie, my domain in 41D long, 27D high with the inlet being 0.5D high which is located on the bottom left boundary, with the rest of the boundary being a wall).

Initially I believed that is I though of D being a meter in fluent, I would get a correct solution. However, when I found a new solution doing the each same thing, except scaling my grid by a factor of 0.01 (cm), I got a different result (jet core length is reduced by a factor of 3). Is there something I am missing?
(Note: I have used pressure inlet for inlet, pressure far field for the top boundary, pressure outlet for the right boundary, axis for the bottom boundary, and wall for the left boundary that is above the inlet).

Any help would be appreciated.

Thanks,
Iaroslav
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Old   May 14, 2009, 16:24
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Daniel Tanner
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You can use whatever dimensions you want, but they must be consistent!!
The length scale units must be consistent with the pressure scale units.

For instance, in dimensional space if length is in meters then velocity must be meters per unit time! Similarly if length is x diameters long then velocity must be diameters per unit time.

Thus one variable cannot be changed independently of the others! Remember pressure is proportional to 1/Area => 1/length^2. This must be changed to compensate for a change in length units.
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Old   May 14, 2009, 16:45
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Iaroslav
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Thanks Daniel,

What you said makes a lot of sense. However, is there an option in Fluent to make my boundary values relative to some other length (i.e in terms of diameter lengths) or would I have to calculate these manually?

Thank you,
Iaroslav
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Old   May 15, 2009, 04:03
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Daniel Tanner
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Irrespective of any automatic options to do this in fluent (I am not aware of any) I would always advise that you work it out. It gives you some insight into the problem and avoids any ambiguity in setting the boundary conditions.
Good Luck!
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