boundary layer mesh resolution
 

hi everyone;
i am trying to simulate conjugate, transient external supersonic flow around a blunt body. is there any rule of thumb for boundary layer mesh parameters. is y+ only parameter to control. any suggestions and experiences are welcome.

the help has a best practise guide called Transonic/Supersonic External Aerodynamics: Steady State RANS Approach which has lots of good tips on all aspects of model setup

# of points across the boundary layer is much more important than y+
y+ affects which regime the wall models and wall boundary conditions fall under but # of points across the boundary layer is, quite literally, the boundary layer mesh resolution.

Target 8 or 10 points in the boundary layer, or 30, etc.

it is far less simple than that and some of that advice is incorrect. if you chose the all y+ wall method the code will choose the best method based on the local y+ but you should still obey the mesh requirements for each. to use wall functions y+ should be between about 30 and 150 and the first cell is thus large enough to cover the viscous sublayer and boundary layer and cells beyond this should grow at a reasonable rate to then match the local core mesh size so if the prism layer is about the size of the core mesh then you only really need one prism layerbut for a low reynolds number mesh where y+ is <=1 you need to integrate across the viscous sub layer with 10-20 layers growing slowly and ensuring these capture the full boundary layer and also meet the core mesh at about the same size which sometimes takes 50 layers

As mentioned above; Good resolution of boundary layer. Do a test 2D sim/ hand calculations to determine height of your boundary layer and layers needed to have good representation of the velocity profile. I would estimate anything from 10 - 30 layers with standard air density and dynamic viscosity.

I also agree with ping, wall y+ should not be above 300, you will get unreliable results in shear forces. If you can afford it, try to adjust your first prism cell size to a height that gives you y+ approximately 1. otherwise y+ 30-50 is fine.

Important, y+ values and resolution of boundary layer velocity profile are not the same thing, if you do a good job on both of them you will get reliable results.

Roman

thank you for your replies. i will take into account your valuable advices.