Learning Forum

[Fran4campi]

Hello,nnI am working with K-w SST turbulence model. Till now I have been resolving the boundary layer using y+ below 1. However now I want to use wall functions to speed up the process. Once I coarse the mesh, how can I activate wall functions?nAdditionally, is it okey y+ around 30? What if in some regions it becomes smaller than 30 so it gets in the buffer region? Is the model able to handle that somehow?.Many thanks in advance.nnKind regards,nnFrann

[Rob]

In k-w you don't, have a look at the documentation explaining how the model works. Selectable wall functions are for k-e and RSM models amongst others. n

[Fran4campi]

Hi Rob, thanks.nI have been checking the documentation but I haven?t managed to find how k-w wall function handle it. It seems to be using a continuous function but no information on y+ I have found.n

[Rob]

https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v211/en/flu_th/flu_th_komega_wall_bcs.htmln

[Fran4campi]

I cannot access that since I don?t have customer accountn

[Rob]

You can:n

Occasionally people will send links to the Help system rather than give a reference. These trigger a password check on the Ansys system, but..... For anyone who is having the same problem as I have, i.e., not being able to access the URL (shows Customer Portal Login). You can access these links by:First open the help section of Ansys through the software ( ? button on the upper-right side of the Fluent Solver User Interface: This opens help in a browser)Copy the link you want to access Past it in browser URL (new tab) where the help documentation had been opened.You can access these links. Just in case this helps someone.Thanks to @smalbro93 for the hint.https://forum.ansys.com/discussion/22563/using-help-with-links

n

[Fran4campi]

Thanks Rob, that was useful!!nSo now I know that when using this turbulence model depending on the grid it switches from the viscous sublayer formulation (resolve the boundary layer) to the wall function formulation (model de boundary layer). However I would like to know the switch y+ value, so I can make sure in my grid that always wall function formulation is used. That value is not reported in the guide, do you have any insight?nMany thanks! n

[Rob]

I can't comment beyond what's in the manual, and we recommend that k-w is run with a well refined near wall mesh. If you want to coarsen the near wall mesh I'd tend to use k-e and choose one of RNG or Realisable depending on the application: you can then choose one of the several wall functions. Enhanced Wall Treatment us usually a good option. n