[ndev]

Hi All,

I have a 3D blade/airfoil mesh that has a fine looking prism boundary along the front 80% of the blade, but the prism layer hits a point where it just starts shrinking to almost nothing...

The blade technically has 4 surface patches along the chord, one for the leading 10%, bottom, top, and an internal baffle surface coming off the trailing edge (to maintain the sharp edge and keep prisms growing out into the fluid).

When I was using wall functions (and therefore my first prism layer cell height corresponded to ~ 200 > y+ > 30) the prism layer was fine, but now that I am asking icem for a much lower first layer height (~ y+ = 5), I'm getting this behavior.

Has anyone seen this before? Does anyone know which settings might be causing this and how I could get a constant height prism layer all the way out to the end of the baffle?

Thanks!!

[kad]

Have a look at Simon's tips and tricks documentation. It is linked in the sticky thread. It has lots of information on growing prism layers. Make sure to lower min prism quality and keep prism angle at 180 degrees. You did not just forget to check the baffle surface before computing prism mesh? It somehow looks this way.

[ndev]

Thanks for the reply kad!

I checked out Simon's pdf and it definitely has some good advice!

The baffle is definitely checked for prism growth, and it's hard to see in the picture, but there are approximately 6 to 8 (very very thin) prism layers that do make it all the way out onto the baffle...

Prior to first attempt and this posting, I had lowered min prism quality to 0.001 and my max prism angle was set to 176 (I think I had initially set this in an attempt to prevent the prisms from wrapping around the baffle surface). I ran the mesh construction last night with a min prism quality of 1e-6 and max prism angle of 180, but ended up with a boundary layer that started shrinking even sooner than the first attempt!

I'm going to experiment some more today and post some more thoughts/questions later on.

[kad]

Okay, I thought you wanted the prism layers to wrap around the baffle surface. As you do not, try set the angle around 90 degrees or play a little with its value.

There is also a little bit more robust method for generating prism. You can create less but thicker layers and split them afterwards with the split prism tool. Try floating height "0" for all surfaces and create 2-4 times less layers. After that split the layers until you reach your desired number and resolution. Avoid big jumps in cell sizes when doing this.

[ndev]

I'm not sure exactly how these factors are related, but it seems as though the boundary layer stays at the same level for longer if I minimize the wall surface element/first layer tetra size... I generated a mesh with over 30 million cells, specifying a much smaller max size on the blade surface, and the prism layer is almost what I want (see picture below)...unfortunately I'd really like to be in the 10 - 15 million cell range...but maybe that's unrealistic given a rotor diameter of 10 meters and a boundary layer first cell height of 0.00005 meters.

The prism splitting method seems like a good idea...especially as I'm getting a decent prism layer at larger sizes already. Thanks for the tip!

[kad]

Do you use floating height for prism? When using floating heigt the thickness of the prism layer is somehow determined by the surface element size. So a huge jump in element size can lead to problems.

0.00005 meters seems very low, but of course it depends on your boundary conditions. Maybe you make some test runs with different grids and look at the range of y+. Blocking is not an option in your case?