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-   -   calculate values for eps and k from Re or u????? (https://www.cfd-online.com/Forums/openfoam-pre-processing/78256-calculate-values-eps-k-re-u.html)

 sbar July 16, 2010 05:09

calculate values for eps and k from Re or u?????

Hi Foamers!

I'm simulation an Flow around a cylinder to calculate the drag coefficent at different Reynold numbers. Im validating against thoeretical values.

My problem ist how to get the inlet conditions for k and epsilon. As inlet value i have just the Reynolds number. I can calculate the velocity from Re but is there an possibility to calculate k and epsilon from Re or the velocity? Or is there an possibility to tell Of to calculate the values like setting the inlet type to zeroGradient or calculated (i got errors when i tried).

 boger July 17, 2010 09:10

Hi Sebastian,

You'll need to specify the values in some way, but this topic has been discussed before in the forum. Ryan had suggested this guide from the Fluent manual. I just checked, and that link is still active. Also see the follow-up post from Mark on that same thread.

 sbar July 20, 2010 03:49

Hi David,
Thanks a lot! that helped!

 sbar July 20, 2010 05:54

ok now another question appeared :)

when the turbulence intensity is calculated by formula 6.2-1 of the fluent help:

I= 0.16(Re_Dh)^(-1/8)

I decreases at high Re numbers. This confuses me bit. doesn't high Reynoldnumbers mean "more turbulence" in general?

Is it because I is u'/u_avg and u avg increases "faster" at high Re?? (sorry for the sloppy description I'm quite unexperienced at cfd)

thanks a lot for the help!!

 kiran August 15, 2010 07:42

hi sebastian,

its a general confusion for any one who is new to tubulence initially when i was new.

tubulence generally increases with increase in Re(at low Re numbers less than 10^5) number but when the flow Re is further increased(more than 10^6) then its effect on the flow decreases.

I means turbulent intensity Re_Dh is the Re number of flow.
generally I is very less percent for high compressible flows.

do not think of any u`/u avg terms to avoid your confusion

all the best

 sbar August 16, 2010 04:10

ok! thanks for the reply kiran!

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