SimScale CAE Forum

Heat transfer Wall--> Fluid (Water)


Hallo I am

having problem calculating the heat transfer from Walls to a Fluid (Water) correctly.

My first question is. What exactly is Turbulent thermal Diffusivity [kg/(s m)] in the
Start parameters. I know that I should know this but even extensive googling
didn’t help.

My second and last question is what the temperature value in the boundary condition
turbulent heat flux is used for?

The information I have about my problem are:

Geometrical shape of the floe regime.

Type of Fluid.

Flow speed. Inlet Temperature. Heat flux across one wall.

In my opinion that should be enough to calculate the temperatures of the wall in my low turbulence case.

Thanks for your suggestions.

Best regards.



Hello Agnes,

heat is transferred in fluids through molecular and turbulent mechanisms. The molecular part is represented by the thermal diffusivity as a property of the fluid. However, turbulent heat transfer is not a property of fluid but the result of intense non-uniformity in velocity field, turbulence. Therefore, turbulent thermal diffusivity is a measure of thermal energy transfer by means of turbulent mixing. It defines how turbulent heat flux and temperature gradient are related. Let me know if you would like to read more on this.

Regarding turbulent heat flux temperature BC, the temperature is used as an initial value/placeholder at the boundary.



Hallo Babak,

that the temperature in the BC is a place holder was my guess. I just set it to the same
temperature as my starting temperature of the fluid.

That the turbulent thermal diffusivity is used to specify the heat transferee, seems to me like doing it the wrong way around.

I don’t know how big the heat transfer coefficient is at any given location at the wall.

That is what I would like to compute.

And to answer your question. Yes I definitely would like to know more about the matter.

After all I want to solve the velocity, pressure and temperature field simultaneously.

Thank you very much.

Best regards.



Hi Agnes @agnesplotzke,

would the “Turbulent heat flux temperature” boundary condition do the job for your setup:

It allows you to specify heat flux either via the actual flux or the power, but it applies it homogeneously across the surface it is applied to. Is this okay for your problem at hand? If yes, you need to choose a boundary condition type “Custom” for the surface where you want to apply the heat flux. Under “Custom” you can specify for each physical quantity it’s behavior at the boundary.

Can you share what type of application your are simulating? With a better understanding of the problem, it might be possible to suggest a complete boundary condition setup.




is there any source from where i can learn and understand these things


@Ali_Arafat, do you have any learning resources to be recommended?


Hi @mehakkar123,

Let me jump in instead of Ali.
Most of the CFD analysis done on SimScale uses OpenFOAM as the solver. (in fact, the above mentioned BC’s come from OF).
If you want to deeply understand the formulation of the boundary conditions, I can refer you to OpenFOAM wiki’s and giudes.
Further, we constantly extend SimScale’s documentation on those topics. If the direct page has not enough content, try looking into Validation cases.

Good luck!


Hello Sir,

I got to know about the turbulent thermal diffusivity idea but I want to know how to calculate it for convective heat transfer simulations. And does it have an empirical formula to calculate this quantity?

Thank you.



Hi @SourabhMore,

Have you tried to look though some literature or projects that are well document that deals with turbulent thermal diffusivity?