Learning Forum

[Dennis Chen]

Is there any meaning or purpose behind looking at Y+ for walls that are directly perpendicular to the flow direction?

[Rob]

Yes, as flow will also be along those walls too. However you also need a much lower aspect ratio at the impingement point to maintain accuracy. 

Remember the benefit of inflation meshes is an economic way to improve wall boundary layer resolution without adding too many cells into the streamwise direction. However this only works well when the flow isn't changing too much along the walls. Far too many courses focus entirely on y+ without covering the reasons we may need to be more careful in the near wall region and completely forget to explain mesh resolution in the bulk flow regions around jets etc. 

[Dennis Chen]

Hi Rob, thank you!   I have one more quick question that may seem a little silly.    I am mostly a structural person and I've only really done CFD projects focused on conjugate heat transfer and for FSI.  I really appreciate your understanding. 

For the FSI case, when looking at static pressure on a wall perpendicular to the flow direction (see image below), what Y+ value should I even target?  I understand this depends on the turbulence model I want to use as well.   My goal is to get accurate pressure calculation on these walls to get the correct forces to be transmitted to the FEA side. 

[Rob]

If you're wanting a very accurate result aim for y+ = 1  as that's generally a good sign you've fully resolved the boundary layer gradients. If you have a y+ > 30 then you're using wall functions, they're still accurate but not quite as accurate. It's the same for heat transfer, with y+ = 1 being slightly more important.  Don't forget to resolve the flow separation and wake as that may also affect the result on the wall: the suction behind the wall may be important too. You may want to look up Ferrybridge (UK) power station in 1965 as cooling towers blew over due to dynamic wind effects. 

My definition of silly questions has been significantly revised following a few years of AIS support......