Calculate Reynold's number in a 2d simulation

mbaldini · January 26, 2018, 09:07

Hi all! Sorry for this basic question, but I looked in the forums and I am not able to find the answer.
I am doing a 2D simulation of a cell (see Fig. 1). I got results but I don't know how to analyze the Reynold's number. Which dimension should I take as the characteristic length?

?

$R_{e} = \frac{\rho uL}{\mu} = \frac{\rho u y }{\mu}$

I am doing a 2D simulation to approximate a 3D cell, with a thickness w (see Fig.2). Does this makes sense? In the 3D simulation the Reynold's number is easily defined, taking the characteristic length as w. But this Reynold's number is comparable with the obtained for the 2D simulations? Because the reference length is orders of magnitude different.

.

$R_{e} = \frac{\rho u w}{\mu}$

Thanks!

Mauro

FMDenaro · January 26, 2018, 14:15

Any lenght you can chose define a Reynolds number, there is not a unique definition.
However, the key is that you want (I think) that in the non-dimensional equations the non-dimensional numbers characterize the problem. That is you have to use the characteristic velocity, lenght, time and so on, such that the terms are of O(1) and the non-dimensional number characterizes the relevance of each term.
In your case you can use the inlet size.

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January 26, 2018, 14:15		#2
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,777 Rep Power: 71	Any lenght you can chose define a Reynolds number, there is not a unique definition. However, the key is that you want (I think) that in the non-dimensional equations the non-dimensional numbers characterize the problem. That is you have to use the characteristic velocity, lenght, time and so on, such that the terms are of O(1) and the non-dimensional number characterizes the relevance of each term. In your case you can use the inlet size.