# Wall function k-omega SST model

 Register Blogs Members List Search Today's Posts Mark Forums Read

 March 12, 2018, 09:23 Wall function k-omega SST model #1 Senior Member   Join Date: Oct 2014 Posts: 124 Rep Power: 11 Hello members. One of the benefits of the k-w SST model is that it will automatically use the low-Re formulation in the viscous sublayer and will use the wall function if the cell height is in the log-law layer. From the manual: "The wall boundary conditions for the equation in the - models are treated in the same way as the equation is treated when enhanced wall treatments are used with the - models. This means that all boundary conditions for wall-function meshes will correspond to the wall function approach, while for the fine meshes, the appropriate low-Reynolds number boundary conditions will be applied." Yes, it is clear, but: which wall function? Standard, scalable or non equilibrium? Then, which is the difference between enhanced wall treatment and enhanced wall fucntion? Thanks Regards

March 12, 2018, 11:11
#2
Senior Member

Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,668
Rep Power: 65
You need to be careful.

Quote:
 Originally Posted by Ema40 One of the benefits of the k-w SST model is that it will automatically use the low-Re formulation in the viscous sublayer and will use the wall function if the cell height is in the log-law layer.
This part is talking about the automatic wall function for the omega model. It uses the linear law for omega and log law for omega

Quote:
 Originally Posted by Ema40 From the manual: "The wall boundary conditions for the equation in the - models are treated in the same way as the equation is treated when enhanced wall treatments are used with the - models. This means that all boundary conditions for wall-function meshes will correspond to the wall function approach, while for the fine meshes, the appropriate low-Reynolds number boundary conditions will be applied."
This part is talking about the BC's for the k-equation.

Enhanced Wall Treatment and Enhanced Wall Function mean almost the same thing. Enhanced wall functions are the blended linear & log standard laws. Enhanced wall treatment uses these enhanced wall functions. Btw, the scalable laws are just standard laws with limiters. Where the EWT is really special is when you are talking about the epsilon equation (because there you switch to a different epsilon equation and it is not a simple wall BC exchange), but that isn't relevant when you're talking about the k-w SST model.

March 13, 2018, 04:40
#3
Senior Member

Join Date: Oct 2014
Posts: 124
Rep Power: 11
Quote:
 Originally Posted by LuckyTran You need to be careful. This part is talking about the automatic wall function for the omega model. It uses the linear law for omega and log law for omega This part is talking about the BC's for the k-equation. Enhanced Wall Treatment and Enhanced Wall Function mean almost the same thing. Enhanced wall functions are the blended linear & log standard laws. Enhanced wall treatment uses these enhanced wall functions. Btw, the scalable laws are just standard laws with limiters. Where the EWT is really special is when you are talking about the epsilon equation (because there you switch to a different epsilon equation and it is not a simple wall BC exchange), but that isn't relevant when you're talking about the k-w SST model.

Thanks!

1) Therefore. From the manual and literature: turbulent viscosity, omega production and k dissipation are damped toward low Re numbers (i.e. toward the wall).

Now, consider the case of very fine mesh (y+<1). In the first grid in contact with the wall (that, because the mesh is fine enough, is within the laminar layer): the k-omega model will apply its damped values and stop, or, anyway, some fixed value will be imposed like in wall functions?

Indeed, in the manual there is written "This means that all boundary conditions for wall-function meshes will correspond to the wall function approach, while for the fine meshes, the appropriate low-Reynolds number boundary conditions will be applied."

QUESTION. By saying "the appropriate low-Reynolds number boundary conditions will be applied" means that the k-omega will simply continue to apply its damped values, without specified values like wall fucntions?

2) Instead, if the grid is NOT fine enough, special wall functions will be applied both in the viscous and in the log layer ? Or,
these special wall fucntions are applied also in the case of fine mesh? (https://www.sharcnet.ca/Software/Ans..._wall_bcs.html).

Thanks!

 Tags enhanced wall treatment, k-omega model, wall functions