[tom.opt]

Hi,
I'm trying to refine a mesh to fulfil the following criteria:
y+ <=1
Dx+ <=50
Dy+ <50

This is for a SAS/LES simulation.
It's the first time I'm dabbling into higher order methods,p reviously I've only used RANS.
Whilist I know how to get y+ , I'm confused about how I can get DX+/x+ , Dz+/z+?
Could someone explain this for me in simple terms please?
and is there a way to get the Dx+ and Dz+ from y+?

Many Thanks,
Tom

[FMDenaro]

Originally Posted by tom.opt

Hi,
I'm trying to refine a mesh to fulfil the following criteria:
y+ <=1
Dx+ <=50
Dy+ <50

This is for a SAS/LES simulation.
It's the first time I'm dabbling into higher order methods,p reviously I've only used RANS.
Whilist I know how to get y+ , I'm confused about how I can get DX+/x+ , Dz+/z+?
Could someone explain this for me in simple terms please?
and is there a way to get the Dx+ and Dz+ from y+?

Many Thanks,
Tom

From the definition of y+ = u_tau*y/ni you can deduce as counterpart a general expression


h+=u_tau*h/ni=(u_tau*L/ni)*h/L = Re_tau*h/L

[tom.opt]

Originally Posted by FMDenaro

From the definition of y+ = u_tau*y/ni you can deduce as counterpart a general expression


h+=u_tau*h/ni=(u_tau*L/ni)*h/L = Re_tau*h/L

Thanks for your answer.

what's h/h+ ? I'm confused
or do you mean h would be x/z?

and if that's the case I guesss my question is how do you calculate x, z?
I know that y is the near wall distance but I don't understand how x or z would be calculated from my solution.

[FMDenaro]

Originally Posted by tom.opt

Thanks for your answer.

what's h/h+ ? I'm confused
or do you mean h would be x/z?

and if that's the case I guesss my question is how do you calculate x, z?
I know that y is the near wall distance but I don't understand how x or z would be calculated from my solution.

Yes, h is for a generic step size in x or z direction. Therefore, given you computational (dimensional) steps dx and dz, knowing the value of Re_tau, using the reference lenght L you get dx+ and dz+.

[tom.opt]

Originally Posted by FMDenaro

Yes, h is for a generic step size in x or z direction. Therefore, given you computational (dimensional) steps dx and dz, knowing the value of Re_tau, using the reference lenght L you get dx+ and dz+.

Could you explain how do I get dx, dz ? I'm still a bit lost

[FMDenaro]

Originally Posted by tom.opt

Could you explain how do I get dx, dz ? I'm still a bit lost

For example, if you want dx+=30

Re_tau*dx/L=30 -> dx= ... in meters

[LuckyTran]

When you run RANS, your first cell thickness is some length (say 0.01 mm) and that gives you a certain wall y+ of let's say 2. Btw, y+ is a global coordinate system (not just next to walls). x+ and z+ work the same way. x+, y+, and z+ are special Reynolds numbers.

So in order to achieve a wall y+ of 1 in LES, you would want a near wall spacing of 0.01mm / 2 = 0.005 mm.

Therefore to achieve Dx+ <= 50, you would need a spacing in x direction of 50*0.005 mm = 0.25 mm. And the same for y-direction. Difference is in y you have two criteria, a core mesh criteria for your LES (Dy+ < 50) and a near wall criteria for your wall models (which you call y+ < 50 but really it's another Dy+ < 1 because you're just talking about cell sizes)

Btw a nearly identical question was asked here with the same confusion. Is there some epidemic going around?

[tom.opt]

Originally Posted by LuckyTran

When you run RANS, your first cell thickness is some length (say 0.01 mm) and that gives you a certain wall y+ of let's say 2. Btw, y+ is a global coordinate system (not just next to walls). x+ and z+ work the same way. x+, y+, and z+ are special Reynolds numbers.

So in order to achieve a wall y+ of 1 in LES, you would want a near wall spacing of 0.01mm / 2 = 0.005 mm.

Therefore to achieve Dx+ <= 50, you would need a spacing in x direction of 50*0.005 mm = 0.25 mm. And the same for y-direction. Difference is in y you have two criteria, a core mesh criteria for your LES (Dy+ < 50) and a near wall criteria for your wall models (which you call y+ < 50 but really it's another Dy+ < 1 because you're just talking about cell sizes)

Btw a nearly identical question was asked here with the same confusion. Is there some epidemic going around?

Perfect thanks.
This is the best explanation I got so far. I get it now.
I think the problem is that all the books and links I visited are unable to explain concepts in simple terms and make LES hardly accessible for a novice.

[FMDenaro]

Originally Posted by tom.opt

Perfect thanks.
This is the best explanation I got so far. I get it now.
I think the problem is that all the books and links I visited are unable to explain concepts in simple terms and make LES hardly accessible for a novice.

Maybe your confusion is due to the fact that dx+ and dz+ are just two numbers resulting from the adopted grid size while y+ is a function of the distance from the wall, not a single value.