# Caracteristic length used for Peclet number

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 June 2, 2015, 10:00 Caracteristic length used for Peclet number #1 Member   Laurent DASTUGUE Join Date: May 2014 Location: Paris, France Posts: 69 Rep Power: 3 Hi foamers, Does anybody know what is the caracteristic length used when OpenFOAM calculate the Peclet number ? I don't have access these days to OpenFOAM and i have to answer this question. Thanks a lot. Laurent

 June 2, 2015, 10:38 #2 Senior Member   Alexey Matveichev Join Date: Aug 2011 Location: Nancy, France Posts: 1,305 Rep Power: 23 Hi, It seems you have access to the Internet. https://github.com/OpenFOAM/OpenFOAM...d/Pe/Pe.C#L118 https://github.com/OpenFOAM/OpenFOAM...olation.C#L136 So, Pe is calculated in assumption Sct = 1 and length scale is cell size.

 June 2, 2015, 10:51 Sct ? #3 Member   Laurent DASTUGUE Join Date: May 2014 Location: Paris, France Posts: 69 Rep Power: 3 Hi Alexey, thank you for the links, and for your answer concerning the length scale. Just one question : What is Sct ? Laurent

 June 2, 2015, 11:00 #4 Senior Member   Alexey Matveichev Join Date: Aug 2011 Location: Nancy, France Posts: 1,305 Rep Power: 23 Sct is turbulent Schmidt number (https://en.wikipedia.org/wiki/Schmidt_number). As you can see, in Pe.C everything is divided by viscosity, yet for Peclet one has to divide by diffusivity. If we assume that Sc = 1, we can just divide by viscosity (for turbulent case we have to assume that Sc = 1 and Sct = 1, so nuEff( = nu + nut) == Deff( = nu/Sc + nut/Sct)).

 June 3, 2015, 04:56 #5 Member   Laurent DASTUGUE Join Date: May 2014 Location: Paris, France Posts: 69 Rep Power: 3 Ok i understand now. Is the Peclet number calculated here a massic one ? (Pe=Re*Sc) or a thermic one (Pe=Re*Pr). If this is a massic one, can i say that Pe = Re, since Sc = 1 ? Laurent

 June 3, 2015, 05:08 #6 Senior Member   Alexey Matveichev Join Date: Aug 2011 Location: Nancy, France Posts: 1,305 Rep Power: 23 Hi, If you assume Pr = Prt (turbulent Prandtl number) = 1, then it becomes "termic" And yes, you can call the value calculated by Pe utility Reynolds number.

 June 3, 2015, 05:18 #7 Member   Laurent DASTUGUE Join Date: May 2014 Location: Paris, France Posts: 69 Rep Power: 3 In fact what i want to do is the calculation of the convection coefficient h. I know that there is a relation between h and the Nusselt number, and i know a relation letting me to have Nusselt, by the use of Re and Pr values. Since i have the value of Prandtl number easily, knowing the fluid caracteristics, i just need the value of Re to have Nu and to have finally the value of h. So reading your posts, i understand the following : using Pe utility, i can do the operation Re = Pe/Pr to have Reynolds number. Am i right ? Or must i apply some operations on Pe utility results before having the Peclet number value which will let me calculate Re ? Laurent

 June 3, 2015, 07:54 #8 Senior Member   Alexey Matveichev Join Date: Aug 2011 Location: Nancy, France Posts: 1,305 Rep Power: 23 Hi, Short answer: it depends. Pe field produced by Pe utility is surfaceScalarField and its value is where L is of order of cell size, u is in fact where S is face normal vector with module equal to the surface of the face. So, in fact Pe is Re. If you are happy with h as a surfaceScalarField, then yes, you just manipulate result of Pe utility; if, for example, you need convection coefficient as volume field, you need additional steps.

 June 3, 2015, 08:07 #9 Member   Laurent DASTUGUE Join Date: May 2014 Location: Paris, France Posts: 69 Rep Power: 3 Hi, thank you very much for your help. So what we have is : Pe = Pe / Pr, isn't it ? Why is this utility not called Re ??? Laurent

June 3, 2015, 09:37
#10
Senior Member

Alexey Matveichev
Join Date: Aug 2011
Location: Nancy, France
Posts: 1,305
Rep Power: 23
Hi,

Quote:
 Originally Posted by laurentD So what we have is : Pe = Pe / Pr, isn't it ?
Yes, though not sure if I get the notation right.

Quote:
 Why is this utility not called Re ???
I don't know. Due to certain historical reasons?

 June 3, 2015, 10:14 #11 Member   Laurent DASTUGUE Join Date: May 2014 Location: Paris, France Posts: 69 Rep Power: 3 Thank you very much Alexey for your time. Have a good day. Maybe this utility will be called Re in a next future thanks to your explanations ;-) Laurent

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