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Fede November 6, 2006 05:45

Reynolds number
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?


opaque November 6, 2006 08:53

Re: Reynolds number
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.


Fede November 6, 2006 10:45

Re: Reynolds number
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


opaque November 6, 2006 11:33

Re: Reynolds number
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,


Rui November 6, 2006 12:51

Re: Reynolds number
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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