how can I improve the y+ value?

Madeliz · February 22, 2014, 17:22

Hi, I'm working on a project that involves wind generation with a diffuser augmented wind turbine. I would like to know how can I improve the y+ value on the diffuser. It presents a value of 533 aprox on the y+.

JuPa · February 22, 2014, 18:25

There is a lot of literature which explains the definition of Y+. Here's a start:
http://www.cfd-online.com/Wiki/Dimen...tance_(y_plus)

It is a dimensionless wall distance. It is particularly important in turbulence modeling close to walls. High speed flows close to a wall produce higher Y+ values. The Y+ value you need depends on which turbulence model you use (if you're using one).

You can estimate how big or small your first cell height should be to obtain your desired Y+ by using tools such as this:
http://www.cfd-online.com/Tools/yplus.php

In your case if you want to reduce your Y+ you need to refine the mesh closer to the walls. After you've done this re-run the simulation on the refined mesh, and check the Y+ values again.

Just remember if as you refine you also increase the number of elements in your mesh and greatly increase the simulation time. Hence don't refine unless you have to. This goes in hand with a mesh sensitivity study.

Madeliz · March 30, 2014, 10:56

Thank's a lot it works but I have a question now as you can see at the picture it presents an irregular result around the rotor section is that ok?

y+ difusor global.jpg

ghorrocks · March 30, 2014, 18:05

This generally indicates a coarse mesh. Higher than intended y+ also results from coarse meshes. The solution should be obvious.

But as for whether your current mesh is OK - do a sensitivity check and find out. You cannot tell from just an image.

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February 22, 2014, 18:25		#2
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 14	There is a lot of literature which explains the definition of Y+. Here's a start: http://www.cfd-online.com/Wiki/Dimen...tance_(y_plus) It is a dimensionless wall distance. It is particularly important in turbulence modeling close to walls. High speed flows close to a wall produce higher Y+ values. The Y+ value you need depends on which turbulence model you use (if you're using one). You can estimate how big or small your first cell height should be to obtain your desired Y+ by using tools such as this: http://www.cfd-online.com/Tools/yplus.php In your case if you want to reduce your Y+ you need to refine the mesh closer to the walls. After you've done this re-run the simulation on the refined mesh, and check the Y+ values again. Just remember if as you refine you also increase the number of elements in your mesh and greatly increase the simulation time. Hence don't refine unless you have to. This goes in hand with a mesh sensitivity study.

March 30, 2014, 10:56		#3
Madeliz New Member Madelaine Join Date: Oct 2013 Posts: 6 Rep Power: 12	Thank's a lot it works but I have a question now as you can see at the picture it presents an irregular result around the rotor section is that ok? y+ difusor global.jpg

March 30, 2014, 18:05		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	This generally indicates a coarse mesh. Higher than intended y+ also results from coarse meshes. The solution should be obvious. But as for whether your current mesh is OK - do a sensitivity check and find out. You cannot tell from just an image.