[Golaselo]

Hello everyone!

Could someone help me how to calculate turbulent heat flux in my case? For example plot turbulent heat flux on some wall? I know in post cfd there is no output like "turbulent heat flux", so what value I have to have to calculate it. Could anybody help me how to do it?

Best regards, WG

[LuckyTran]

Are you sure that's what you want to do?

At walls you have no slip and no turbulent fluctuations. The turbulent heat flux is zero.

[Golaselo]

Hello LuckyTran, maybe you are right. I am bit beginner in CFD world. So could I calculate turbulent heat flux in whole domain? Or for example in straight line of flow? If yes, how to do that?

Thank you for your reply.

[LoGaL]

Originally Posted by LuckyTran

Are you sure that's what you want to do?

At walls you have no slip and no turbulent fluctuations. The turbulent heat flux is zero.

I am not sure, the no slip condition is applied at the cell face, not at the cell center, where the velocity is actually going to be non zero. When Fluent calculates the heat flux through the wall, it uses cell centered values to reconstruct the gradient, so the conductivity is going to have a turbulent contribution.

[LuckyTran]

Originally Posted by LoGaL

I am not sure, the no slip condition is applied at the cell face, not at the cell center, where the velocity is actually going to be non zero. When Fluent calculates the heat flux through the wall, it uses cell centered values to reconstruct the gradient, so the conductivity is going to have a turbulent contribution.

Hence the need for clarification because otherwise we end up calculating things that should not even be calculated. No need to even mention flux reconstruction because that would involve properly calculating things the correct way which should be zero regardless of what the unlimited or limited reconstruction gradients even are.




To get the turbulent heat flux field (which is a vector field) you need to fetch the temperature gradient then multiply it by the turbulent viscosity and divide by the turbulent Prandtl number. Depending on your definition of turbulent heat flux (you may/may not need to divide also by the density). You can create custom field functions or a UDF.

If done using a custom field function, the temperature gradients aren't available by default. You must enable the option to retain them in memory during calculation. There are three components of the temperature gradient dT/dx, dT/dy, and dT/dz so you will need 3 custom fields. Here's a cute YT video on how to retain the temporary storage and includes the temporary gradient as an example.

[Golaselo]

Hello LuckyTran,

I got ma dT/dx and dT/dy. I also got turbulent Prandtl Number and Turbulent Viscosity. So if I have dwo gradients, dT/dx and dT/dy which gradient should I calculate?

[LuckyTran]

Both. Or whatever you need. Each is a component of the turbulent heat flux. Remember, heat flux is a vector.