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How to initialize k and epsilon values?

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Old   April 20, 2019, 10:32
Default How to initialize k and epsilon values?
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Joe lee
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I have been using OpenFoam and ANSYS Fluent simultaneously, and it seems the initial values of the k-epsilon model calculated by ANSYS are different from what I calculated, so I do not know whether I got the calculation correctly or not. Here is my settings:

I am trying to do an external flow like the motorBike tutorials, just by replacing the motorBike geometry by my own model. The parameters of my simulation:

Freestream = 12.5m/s
characteristic length (L) = 1.638m (I use the total length of my geometry as the characteristic length. Not sure if it is correct)
Fluid : air
Reynolds number = uL/v = (12.5)(1.638)/(1.5e-5) = 1365000
kinematic viscosity = 1.5e-5m^2/s
Turbulence intensity level = 0.16*(Re^(-1/8)) = 2.7637%
Turbulence length scale = 0.07L = 0.11466m

By substituting the values into the calculation tools in cfd online (https://www.cfd-online.com/Tools/turbulence.php), I got:
k = 0.17554J/kg
e = 0.057727J/kgs


However, from ANSYS Fluent:
k = 0.24954J/kg
e = 2.2404J/kgs


It seems that either set of values are desperately wrong.

May I know if I got my own calculation correctly? Help tell me if I did anything wrong! Thank you very much!!
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Old   April 22, 2019, 08:15
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Paulo Vatavuk
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Hi Joe,

You could run the simulation without the obstacle and check the values of k and e at the outlet. Doing that you will know equilibrium values of k and e. This kind of verification is recommended for CFD wind studies.

Best Regards,
Paulo
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Old   April 22, 2019, 10:10
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Joe lee
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Quote:
Originally Posted by vatavuk View Post
Hi Joe,

You could run the simulation without the obstacle and check the values of k and e at the outlet. Doing that you will know equilibrium values of k and e. This kind of verification is recommended for CFD wind studies.

Best Regards,
Paulo
Thanks for your reply. However, shouldn't the value of k and epsilon depend on the obstacle? K and epsilon could be obtained using turbulence length scale and intensity level, and these two values take the characteristic length as input. So if we remove the obstacle and capture the value of k and epilson at the outlet, how is that relevant to the case that the obstscle is presented?
I do not have a solid background on CFD, so please forgive for me ignorance. Thanks.
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Old   August 2, 2019, 03:45
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The values from k, e calculation you entered in BCs are initial guess.

And usually L is height (orthogonal to freestream direction).
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