Originally Posted by fluid23 (Post 717491)

Ok, you've got some things mixed around. Let me break this down for you, hopefully it will make sense.

First of all, the goal of the prism mesh is to resolve near wall flow. Without getting too technical, you basically need really small cells near the wall to resolve steep gradients in the boundary layer. Without it you run the risk of your model crashing, or best case returning bogus results.

Now the tool you mentioned, that is used for sizing cell height of a single wall bounded cell. Unless you changed it, it assumes you want a wall y+ of 1 and that whatever velocity you specify in the freestream input exists immediately outside the boundary layer at that wall location. (Actually it might even be ½ cell height that it returns, I don’t really remember). That’s great and all, but it is not very practical for a simulation like this. Your velocity distribution around the car will mean that you would technically need a different cell height at ever wall node to meet your target of y+ = 1. You simply don’t have that kind of control in commercial meshers.

By default, the prism layer thickness you set in star-ccm controls the stack up of all your prism cells. If you are setting this to be the value you got from the online tool then you are trying to cram all of your prism cells into the space you needed for one (or maybe even ½) of your wall bounded cell. Ultimately, your goal with the prism mesh is to provide cells at every location on the wall that retur y+ in your target range AND smoothly transition from your small near wall cell to your larger core mesh. The best prism layer meshes look like a smooth transition from surface to the core. The ultimate goal would be to set just one global control for everything, but often times you will need to do surface controls to set prism layer properties differently for various surfaces.

The good news is you don’t need a wall y+ of 1. Depending on your turbulence model and chosen wall treatment there are a range of acceptable values. Assuming you just went with the default selections that the software gives you, your y+ should really be between 1 and 5 or 30 and 60. Between 5 and 30 is ok, but the interpolation between low wall and high wall models can be a little hit-or-miss. You can also relax that for features that are not critical to what you are looking at. If you know that that the contribution of something toward your Cw value is negligible, then you can let that slip to 100 or more. That flexibility gives you room to refine areas of interest but still adequately capture bulk effects elsewhere.

What I suggest is setting a global total prism thickness of between 1 and 3 inches, using 5 to 7 layers and a layer ratio of 1.2. Mesh, run for about 100 – 200 iterations and check your y+ values. Then refine global settings, apply surface controls for critical components and continue refining until you get y+ values that you can live with.

I can talk on this subject for days, hopefully this helped a little.

Originally Posted by fluid23 (Post 718082)

What are your boundary conditions? How are you initializing? How aggressive is your courant number? Have you tried a courant ramp?