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Old   November 6, 2006, 05:45
Default Reynolds number
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Fede
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I've noticed that the Reynolds number calculated by the solver is 4 times greater than the theoretical one (in the same condition). Does anyone else noticed that? Why?

Thanks

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Old   November 6, 2006, 08:53
Default Re: Reynolds number
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opaque
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Dear Fede,

What is your definition of the theoretical Reynolds number? I recall it as

Re = density * characteristic velocity * characteristic length / dynamic viscosity.

The characteristic values are problem dependent; therefore, it is up for the take..

The ANSYS CFX solver does not know anything about your particular geometry, nor the common characteristic value for your particular application. Then, it just uses a global length and a arithmetic value for the velocity magnitude in the domain.

Hope this explains your differences.

Opaque.

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Old   November 6, 2006, 10:45
Default Re: Reynolds number
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Fede
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I've a straight pipe with constant diameter, so to calculate the Reynolds number I must consider as characteristic length the diameter itself, right? Is it possible that the ANSYS CFX Solver consider another characteristic length instead of the diameter?

Thank you very much

Fede
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Old   November 6, 2006, 11:33
Default Re: Reynolds number
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opaque
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Dear Fede,

Almost everyone uses the diameter as the characteristic length in pipe flows. The ANSYS CFX solver does not know it is a pipe..

However, you can define your own Reynolds number and monitor as well. I would use the massflow version then,

Re = massFlow()@Inlet * Your diameter / (dynamic viscosity * area()@Inlet)

You should create an Algebraic Equation additional variable.

Hope this helps,

Opaque
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Old   November 6, 2006, 12:51
Default Re: Reynolds number
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Rui
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It uses the cubic root of the volume to calculate the Reynolds number. This is called "Global Length", and you can also see it on the .out file.
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