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Old   December 20, 2009, 17:03
Default hydraulic diameter?
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hey, maybe I'm just confused because of the English word for it (sorry, my first language isn't English) but i need help with the "hydraulic diameter" setting in gambit.

The problem is in the attachment. I have to create a wind tunnel with a lorry inside (3D).

in the bundary conditions in fluent i can set up for both inlet and outlet the option specification method: itensity and hydraulic diameter


what is my hydraulic diameter in this case? the hydraulic diameter of the lorry? or the wind tunnel?

Is it the same like the characteristic length of the reynolds number when i want to calculate my reynolds number for this case?
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Old   December 21, 2009, 02:22
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You are in the set-up panel for your inlet.
So you need to enter the hydraulic diameter for the inlet.
Dh = 4. Area / Perimeter
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Old   December 21, 2009, 06:44
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ah okay, so then this formula




and when i want to determine the characteristic length of the lorry for the Reynolds number? I should take then Dh with the front side of the lorry?

the problem is that i (until now) just calculated flow within pipes etc.
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Old   December 21, 2009, 07:43
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Sorry, the formula I gave you is for internal flow
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Old   December 21, 2009, 07:52
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ah okay, but which formula is then for external flow?
the formula from my post is the same. im confused with this, sorry.
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Old   December 24, 2009, 11:47
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sorry for the questions, but you already helped me a lot!

now i have another question to fluent. its about the "report" -> "reference values" option. i can change the area there.

Is this area the frontal area of the lorry? (the object i wanna analyse?)

for example, when i have an airfoil with the chord = 1 and a depth = 4, the reference area is then 4, right? or is this area related to the wind tunnel?

greetings!
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Old   January 4, 2010, 01:44
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reference values are given for setting the Cd, Cl and Cx coefficients (related to yor body).
So you have to give those values for your body
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