How to assess boundary layer quality
 

Hello guys,

I would like to know how to properly assess if the boundary layer created is correct. At the moment I am using a k-epsilon turbulence model with wall treatment, and I check the wall y+ representation over the body once the simulation is completed.

However, I have seen that you should also take a very close look at how things are modelled around the boundary layer. In my case, I am using a prism layer (5 layers) of 0.005m thickness with a 4m long body at 50m/s.

http://www.computationalfluiddynamic...oundary-layer/

To begin with, what are acceptable values for wall y+ in that case? I hear 30-300, below 5, around 1, it is confusing.

Also are there other tools to make sure my boundary layer is correctly modelled?

Cheers! :)

That depends upon what near wall treatment you are using. The most common (and default) for k-e and k-w is the All y+ wall treatment. This is the most robust option.

That being said, the y+ values you should target vary depending upon who you ask. The best answer I have heard (from CD-Adapco) is that they should be between 1-5 or 30-60 for best results and rarely beyond 120. The split in ranges is a product of using separate near wall and far wall methods and blending them together. In the blending region (5-30) you still get reasonable results, but they are not considered very accurate.

Unless you are doing a DNS analysis you aren't going to be actually modeling hte boundary layer, only approximating it. Y+ is your best bet, although checking your cell quality in the prism layer is never a bad idea. Sometimes y+ is OK but your mesh is crap and results will reflect that.

Thank you for your answer.

Yes, I am using the all y+ wall treatment. I have read (I think it was even in the Star-CCM+ help pdf file) that values should be between 30 and 300, but I agree that 300 is definitely too much.