For a rectangular domain, use this equation:
Note that is the hydraulic diameter and so depending on your shape of domain, there will be a different equation for calculating the equivalent circular section. Next we need to calculate the turbulent length scale of the flow, which can be done crudely (yes, I am aware that the k- model has it's own definition of the length scale utilizing the relationship between k, epsilon and ) by:
Taking " " as being equal to 0.000001458 and "C" as being equal to 110.4 (these are commonly used values of dynamic viscosity of air at a specific temperature within CFD literature), you can then calculate back through to get your actual at your given temperature in. Assuming you know the air Pressure and Temperature [K}, then you can use the ideal gas law equation to calculate your air density:įrom there, we need to use Sutherland's Law for calculating the fluid dynamic viscosity of air as it relates to temperature: If that is the case, do the same thing but use this equation below:įor Omega, its a little more involved. From there, you divide by 2, but since we only looked into one direction, a crude way of approximating this throughout the domain is to multiply that final result by 3 one for each axis, since there will be fluctuations in velocity in all directions.Īlternatively, you can split them out into each separate axis if you have a vector with a non-zero second or third component. So a rough way to calculate this would be to take your domain's inlet velocity, multiply it by your turbulence intensity (i.e. This basically reads as "k" being equal to the time average of the velocity fluctuations in one direction squared, all divided by 2. One equation for turbulent kinetic energy (k), in J/kg, is: I will go over all the "major" variables that people may or may wish to calculate here - there are ways to skip to the ones you want, but this way will give you all that you need, I hope. As always, any questions, please do feel free to post here or DM me I posted this within the CFD main thread stickied up the top of this sub-forum, but figured that I would put it here in its own thread so that the information wouldnt be lost within the pages of it.
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