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-   -   Reynolds number (http://www.cfd-online.com/Forums/openfoam-pre-processing/94304-reynolds-number.html)

Danath November 11, 2011 08:57

Reynolds number
 
Hi

i have run my case with "nu nu [ 0 2 -1 0 0 0 0 ] 0.01"

Now i want to change Reynolds number
for example i want to rum my case with reynolds number 60 what can i do?

i am a little bit confused with the tutorial!

boger November 12, 2011 10:54

Check the definition of Reynolds number. Your flow field is characterized by the kinematic viscosity (nu) but also some characteristic velocity (probably through your boundary conditions) and length (through some aspect of your mesh). You'll need all three to calculate (or change) your Reynolds number.

Danath November 15, 2011 07:24

Re = (4 Rh U)/ν

because i know U =1m/s and i know Re for example Re= 50
i know Rh =1^2/4 (non-cylindrical conductors)
so
ν= 1*1/50 = 0.02

"nu nu [ 0 2 -1 0 0 0 0 ] 0.01" i suppose if i want to run my case with Re= 50 i will change 0.01 with 0.02 is this right ?

Nico A. November 15, 2011 09:17

Hello Danath,
you are right with the new kinematic viscosity. generally you can either change the viscosity or the velocity (for a constant geometry) to change the Reynolds Number. I often prefered changing the velocity for demonstrative purpose, because it is direct proportional.

kid January 6, 2012 07:07

Hi,
Sorry if i case any confusion here but can you through light on this.
Suppose my fluid is air and knowing the input boundary condition i extract V and nu for a given Re.
So, what i have done is calculate and place both V and nu for a given Re as my boundary condition.
But does it agree with what ypu said above?

Regards
CFDkid
Quote:

Originally Posted by Nico A. (Post 332192)
Hello Danath,
you are right with the new kinematic viscosity. generally you can either change the viscosity or the velocity (for a constant geometry) to change the Reynolds Number. I often prefered changing the velocity for demonstrative purpose, because it is direct proportional.


Nico A. January 13, 2012 03:36

Hey kid,

I think there is a conflict in what you said. Before starting the simulation it should be clear if you want to a flow for a specific Re, velocity or
viscosity.
Example: you consider an air flow, nu is given (nu=1.54*E-5 m/s at 20C), for one given geometry, the hydraulic diameter is constant and to receive Reynolds Number of e.g. Re=60, -> adjust the velocity in the boundary condition.

Best regards
Nico


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