[thomas.]

Hello all,

I am solving a 2D natural convection driven laminar flow case with some bubble generation due to boiling. In order to solve the velocity field properly, I have to solve the boundary and thermal layer, i.e., do some refinement of the mesh near the walls.

I had a thought: I could use either way a wall function to approximate the boundary layer effects on the flow in order to reduce computational cost (quite limited in my case), in spite of having a laminar flow.

I programmed a UDF function to keep the eddy viscosity at the same value as the molecular viscosity, I activated the k-e turbulence model, chose the enhanced wall treatment, I set up the boundary conditions (turbulence intensity=0 and turbulent viscosity ratio = 1) and ran the simulation. Result: even more computational cost.

So I got a step back an disabled the turbulence equation solver (solution controls > equations) and ran the simulation again. Result: a very similar solution to the solved-boundary-layer-case with a bit less of computational cost.

My question then is: as I disabled the turbulence equation solver, did I also disabled the use of the wall functions, or did I keep them?

I have gone through every user´s-guide and theory-guide manual and I haven´t been able to find an answer.

Thank you very much for your help in advance!

Bye.

[LuckyTran]

By freezing the turbulence equations (say k and epsilon) you may freeze the computation of the k and epsilon equations, but you do not prevent other parts of the computation. For example, the velocity wall boundary condition still has wall functions. The wall functions are still active on the energy equation if that wasn't also disabled. The calculation of y* is also not disabled.

I don't get why you're using a turbulence model if the flow is laminar. Shouldn't you just use the laminar solver rather than trying to bootstrap a turbulence model and trying to make it to act laminar? I don't get why if the flow is laminar, you're also trying to use (turbulent) wall functions to apply the boundary conditions, you should be applying the laminar wall boundary conditions.

[thomas.]

Hi,

first of all, thank you very much for your answer!

Regarding your questions your post, I have further questions and I will answer yours:

The wall functions are still active on the energy equation if that wasn't also disabled. The calculation of y* is also not disabled.

Q:
Where did you find that information? I have been searching for that everywhere!

I don't get why you're using a turbulence model if the flow is laminar. Shouldn't you just use the laminar solver rather than trying to bootstrap a turbulence model and trying to make it to act laminar? I don't get why if the flow is laminar, you're also trying to use (turbulent) wall functions to apply the boundary conditions, you should be applying the laminar wall boundary conditions.

A:
Because of two reasons. (1) The RPI wall boiling model needs a wall function, in order to calculate the quenching heat flux. (2) In stead of solving the laminar boundary layer - which has higher computational cost as a consequence, due to the mesh refinement near the walls - I could approximate it with a wall function, in order - as said before - to reduce computational cost.
I know that Fluent´s wall functions are for turbulent boundary layers, but according to my results (I have ran the same simulation with a proper cell refinement near the walls and with a coarser mesh using wall functions), the results are VERY similar.

Thanks very much in advance!