[palermarc]

Dear all,

I am simulating the flow past an airfoil in transonic conditions. I am using the SST turbulence model, as first part of my task there is simulating the flow using a y+ value between 10 and 500.

I have found that increasing the y+ value the result matches better and better with the experimental data I have. As you can see in the below figures.

[IMG][/IMG]

[IMG][/IMG]

As you can see, as y+ decreases the result becomes worse. What I expected was better results for smaller y+ values.

The second part of my task is to " to refine the mesh
around the shock location and by achieving y+ ≈ 1 at the airfoil boundary". Does anyone know how to do that? Because, using the y+ calculator with y+ distance equal to 1 , after the simulation I have found y+ = 1 at the trailing edge of the airfoil while inside it went smaller than 1.

My mesh is a C-grid.
The software I am using is CFX.

Thank you very much for your help

[robo]

CFX should be able to output local y+, so that you don't need to try and back calculate it afterwards. That'll probably get you a better result.

In the Cp vs position graph: for the majority of the airfloil, it looks to me like all y+ values are close enough; likely within the experimental accuracy, which ought to be reported for a study like this. The major difference is around the shock, which could be attributed to mesh spacing; ie for the same mesh size in number of cells, you might be getting better longitudinal spacing by relaxing the y+ requirement. I'd need to know more about how you meshed to really say much about that though. Also, Cp is going to have a largely inviscid character in unseperated flow, so I wouldn't expect much y+ dependence anyway. I imagine that if you looked at viscous drag, there would be significantly more y+ dependent.

I'm unclear on what the theta vs speed graph is showing me, and I'm suspicious that its mislabeled, since the the graph shape is exactly the same as the other, and the speed value goes from 0 to 1.

[flotus1]

Have you made sure that y+ is the only source of error in your simulations or at least significantly larger than the other error sources?

[palermarc]

Originally Posted by robo

I'm unclear on what the theta vs speed graph is showing me, and I'm suspicious that its mislabeled, since the the graph shape is exactly the same as the other, and the speed value goes from 0 to 1.

Yes, I forgot to change the labels. Can you explain me how to extrapolate y+ from CFX? I can do he Y+ plots over the airfoil and from them I have obtained that it is equal to one only on the trailing edge while near the shock location it is larger. Should I increase the Y+ value I want when I evaluate it in the tool? In such a way I might obtain a Y+ almost equal to 1 in the central part of the airfoil.

Anyway I have done some modifications to the mesh grid changing the spacing downstream the airfoil from equispaced to exponential as you can see below.

[IMG][/IMG]

And I also did a small error in the Reynolds, the new plots are for y+ equal to 1 and 250.

[IMG][/IMG]

The same trends seem to appear, the max expansion is reduced and the shock location is more upwind. Do you have any suggestion about the spacing on the airfoil? I am using ICEM.

Originally Posted by flotus1

Have you made sure that y+ is the only source of error in your simulations or at least significantly larger than the other error sources?

I don't get what you say. Can you be more precise?

Thank you