Reynolds number

Danath · November 11, 2011, 07:57

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, 09:54

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, 06:24

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, 08:17

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, 06:07

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.

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, 02:36

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 m²/s at 20°C), 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

November 11, 2011, 07:57	Reynolds number	#1
Danath Member Evangelos Join Date: Sep 2011 Posts: 87 Rep Power: 14	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!

November 12, 2011, 09:54		#2
boger Senior Member David Boger Join Date: Mar 2009 Location: Penn State Applied Research Laboratory Posts: 146 Rep Power: 17	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 likes this. __________________ David A. Boger

November 15, 2011, 08:17		#4
Nico A. New Member Join Date: Apr 2011 Location: Magdeburg, Germany Posts: 23 Rep Power: 15	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. Danath and cesarjets like this.

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November 15, 2011, 06:24		#3
Danath Member Evangelos Join Date: Sep 2011 Posts: 87 Rep Power: 14	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 ?

January 13, 2012, 02:36		#6
Nico A. New Member Join Date: Apr 2011 Location: Magdeburg, Germany Posts: 23 Rep Power: 15	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 m²/s at 20°C), 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