Reynolds Number in CFX

MjG · April 19, 2018, 13:54

How can i find the Re Number for a Newtonian and Non Newtonian fluid (Bird-Carreau model) in CFX?

ghorrocks · April 19, 2018, 19:04

Use a CEL expression. Reynolds number requires a definition of length, speed and material properties so you need to define these variables to be appropriate for your flow.

MjG · April 20, 2018, 04:35

Thank you for your reply! But how can I do it? In CFX-post, I go to the Expressions Tab and in Definition window I put the formula " (Density*Velocity*8e-6[m])/(Dynamic Viscosity) " and as Value output I get " <variable> " while in the Evaluate window, the expression variables (Dynamic Viscosity, Density and Velocity are blank). I thought that since I have set the material's properties and BC in the CFX-pre, these variables will be known. How can I solve it ?

Gert-Jan · April 20, 2018, 04:54

Density, velocity and Dynamic Viscosity are variables in your domain. So now you just created a new variable that you can evaluate in each numerical element. As a field variable.

But why do you want to do this in Post ?Normally you do this in a spreadsheet.
It is doable, but you need a different mindset. For Reynolds, you have to use average variables on a specific location, like an outlet:

ave_dyn=areaAve(Dynamic Viscosity)@outlet
ave_v=areaAve(Velocity)@outlet
ave_rho=areaAve(Density)@outlet
d_outlet= 8e-6[m]
Re=ave_v*d_outlet*ave_rho/ave_dyn

MjG · April 20, 2018, 18:51

I did it! I made an additional variable and I set it as scalar. Then, I choose the Algebraic Equation on the Additional Variable Model option at the Fluid Model and finally I wrote my expression.

Gert-Jan · April 22, 2018, 07:07

I really wonder what you did with the dimension d in Re?
Or are you just calculating a straight pipe or duct with fixed dimension?

MjG · April 22, 2018, 19:49

My geometry has a Π-like shape with a constant Dh.

April 19, 2018, 13:54	Reynolds Number in CFX	#1
MjG New Member macaffy Join Date: Jul 2017 Posts: 15 Rep Power: 9	How can i find the Re Number for a Newtonian and Non Newtonian fluid (Bird-Carreau model) in CFX?

April 19, 2018, 19:04		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Use a CEL expression. Reynolds number requires a definition of length, speed and material properties so you need to define these variables to be appropriate for your flow. MjG likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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April 20, 2018, 04:35		#3
MjG New Member macaffy Join Date: Jul 2017 Posts: 15 Rep Power: 9	Thank you for your reply! But how can I do it? In CFX-post, I go to the Expressions Tab and in Definition window I put the formula " (DensityVelocity8e-6[m])/(Dynamic Viscosity) " and as Value output I get " <variable> " while in the Evaluate window, the expression variables (Dynamic Viscosity, Density and Velocity are blank). I thought that since I have set the material's properties and BC in the CFX-pre, these variables will be known. How can I solve it ?

April 20, 2018, 04:54		#4
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,913 Rep Power: 28	Density, velocity and Dynamic Viscosity are variables in your domain. So now you just created a new variable that you can evaluate in each numerical element. As a field variable. But why do you want to do this in Post ?Normally you do this in a spreadsheet. It is doable, but you need a different mindset. For Reynolds, you have to use average variables on a specific location, like an outlet: ave_dyn=areaAve(Dynamic Viscosity)@outlet ave_v=areaAve(Velocity)@outlet ave_rho=areaAve(Density)@outlet d_outlet= 8e-6[m] Re=ave_vd_outletave_rho/ave_dyn

April 20, 2018, 18:51		#5
MjG New Member macaffy Join Date: Jul 2017 Posts: 15 Rep Power: 9	I did it! I made an additional variable and I set it as scalar. Then, I choose the Algebraic Equation on the Additional Variable Model option at the Fluid Model and finally I wrote my expression.

April 22, 2018, 07:07		#6
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,913 Rep Power: 28	I really wonder what you did with the dimension d in Re? Or are you just calculating a straight pipe or duct with fixed dimension?

April 22, 2018, 19:49		#7
MjG New Member macaffy Join Date: Jul 2017 Posts: 15 Rep Power: 9	My geometry has a Π-like shape with a constant Dh.