Optimizing mesh quality and Y+ layer formation on an FSAE car

[dschroeder]

@DaleKramer

If we look at the CD difference between 1 and 3 layer meshes, the CD of the 1 layer mesh is only 0.8% lower than the 3 layer CD, which is sortof OK…

BUT, the CL difference shows that the 1 Layer mesh CL is a larger magnitude by 7.1%, this is NOT OK…

I assume that you are taking these results from the numerical data of the Cd and Cl plots where both sims are in the 1% zone using the ORSI. I am not entirely convinced that the difference is not ok due to the lack of a “control” sim. We do not know what the Cd or Cl should be for this profile and we are comparing results based off of a lot of variables. Building on this I have different views on what you say here:

@DaleKramer

And in the U plot we can postulate that the higher speed air on the high pressure side of the airfoil is the cause of the significantly higher CL magnitude for the 1 layer mesh…

If we are basing assumptions on visual data, then i would like to point out the following. You have made some nice pictures of each wing comparison between the 1 and 3 layer sims where both wing profiles are in the exact same place. This allows us to compare everything much easier. Taking the U magnitude scale at the top of the picture and using it as an unofficial marker, I have placed arrows at the point in each sim where the air separates from the low pressure side of the wing. As you can see the point of separation for the 1 layer sim is at 14, while the 3 layer sim is at 13. From what i know about aerodynamics, is that the low pressure side contributes more to overall lift (in our case downforce) then the high pressure side. This difference in separation points is what i would attribute most to the increase in downforce from the 1 layer sim. HOWEVER!! based on what Retsam mentioned about flow being “alive” these assumptions from the visual data are not valid and everything we have thought could be wrong

@DaleKramer

We can further postulate that this increased speed of the air, which is significantly inside the outer 2 layers of the 3 layer mesh, was ‘allowed’ to speed up because the solvers were not treating those HEX cells with same accuracy with regard to wall function calculations as the 3 layer prisms cells in that area were treated…

In effect, the air in the 1 layer mesh was allowed to speed up there because the air did not know it was so close to a surface that should have slowed it down by shear forces…

I do not know much at all about how the solvers treat HEX vs BL cells. I would agree that this could be partially if not entirely the reason why we have a difference in CL. I hope that your new method of validating this will work and will help to clarify how accurate each result is.