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john June 12, 2007 22:45

Why Re is different for external and internal flow
We know that Reynold's no is different for internal flow and external flow? I wonder why does it happen? Is it the charecteristic length, which is different in both and so Re is different in both?

Robin June 13, 2007 11:50

Re: Why Re is different for external and internal
Are you referring to the Reynolds number output by the solver or Reynolds numbers in general? Please be more specific.

The solver is only crudely estimating the Reynolds number based on the RMS averaged velocity and a length scale equal to the cube root of the volume. This is only provided as a sanity check and has nothing to do with the device Reynolds number. The solver cannot determine what the proper length scale is to use for the Reynolds number as it is ambiguous and depends on what you are referencing it to.

Regards, Robin

Robin June 13, 2007 14:22

Re: Why Re is different for external and internal
Hi John,

Given that you have posted this on multiple forums, I can safely assume that you are not interested in what the CFX solver is outputting, but rather the nature of the Reynolds number. It is actually explained quite nicely on Wikipedia at and you'll find similar explanations in many text books.

In simple terms, the Reynolds number represents the ratio of advective transport (or inertial forces) to diffusive transport (viscous forces). Viscous forces tend to damp out fluctuations in the flow field, preventing large scale turbulent structures from developing. If the Reynolds number is large, the viscous forces are insufficient to keep the flow stable and turbulent structures develop. The problem is analogous to a beam buckling; short and fat beams don't buckle because the shear stresses overcome the inertial forces due to gravity, long and narrow beams will buckle because the shear stresses are insufficient to keep it straight.

External flows usually concern the flow around vehicles such as cars, aircraft or ships. These are very large in comparison to most internal flow devices and therefore the Reynolds numbers are larger. Of course density and viscosity play a role too, so if you have a dense fluid with a very low viscosity it is also possible to have a large Re in a small device.

Regards, Robin

john June 13, 2007 22:31

Re: Why Re is different for external and internal
Thank you very much Robin

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