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-   -   Reynolds and Nusselt number in DoE (http://www.cfd-online.com/Forums/cfx/119903-reynolds-nusselt-number-doe.html)

 costinruja June 26, 2013 14:27

Reynolds and Nusselt number in DoE

Good evening,

After solving my DoE simulation (DX and CFX in workbench) i realised i also have to define as output parametres the Reynolds and Nusselt number.How can i do that in CFX-Post? I know i can see them in the solver log but i need them as output parameters in DX.

Regards,
Costin

 ghorrocks June 26, 2013 18:38

You will need to manually define variables for Re and Nu. The reported Re and Nu numbers in the solver should be taken as guides only - they are not calculated based on length, fluid property and temperature scales which are much use for most engineers.

 costinruja June 26, 2013 22:33

Thanks ! I'll have to check it out first thing when i get to the office.

 JuPa June 27, 2013 06:14

Incropera and DeWitt suggest the characteristic dimension in the Nusselt number should be taken as:

Surface area divided by surface perimeter

It doesn't explain why. However I like to think it's because:

1. The Nusselt number you will be using is likely to be an average Nusselt number based on the average heat transfer coefficient of a surface. And the way you get to the average heat transfer coefficient from a local one is by integrating the local heat transfer coefficient over the entire area. So taking the characteristic length as the surface area divided by surface perimeter makes sense (to me that is!).

2. Taking the characteristic length as the surface area divided by surface perimeter adds consistency. It means you don't have to worry about which length to take as the characteristic length if you had say a triangle, square, circle or any other shape as your heat transfer surface.

I hope that helps.

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