What is the significance of Y-Plus values?

Y plus values indicate the distance up to which the boundary layer effects will be pronounced.(as much as i know

It can also be used as a grid independence criterion. Rather than refining your domain to the point where the results converge to a limit, y+ can be used instead.

Let's say one has a base domain which resulted in a y+ of 2.2 max. A modification to geometry in that domain must also be meshed such that same y+ of 2.2 is achieved. Thereby, a comparison between before and after geometry modification is valid. By merely changing geometry but not ensuring same y+ value (by means of meshing), a comparison may not be valid. Alternatively, one can keep on refining until reach a true grid independence. Disagree?

I am trying to solve flow over a cylinder in FLUENT at Re=0.68E05 and Mach No=0.1. What is the range of Y plus which I have to use in my grid for K-E turbulence modal with standard or enhanced wall functions.

Dimesionless wall distance. There are 3 areas of flow field (e.g. straight tube):

1. near wall layer => almost linear velocity profile, (provided by the turbulence model)

2. next layer => logarithmic velocity profile (provided by the turbulence model)

3. far away layer => constant velocity profile

It govers the production of kinetic energy. If y+ is to high (more than 100) there is to much kinetic energy. As a effect of this, the pressure drop increases unrealistic, and possibly (extremely) you can get swirl, where no swirl is in reality.

To get realistic results for the near wall layer and the layer next to it, it is very important to be in a range von 30 < y+ < 100 (in transient calculation 120 is OK, too)

Hi, thanks for your very informative reply. I already understand the velocity profiles near and away from the wall. However, I am still confused about how I can modify the y-plus values near the wall, are they mesh dependent?

Also, on a more general note, why not just have a very high density mesh throughtout the whole domain, instead of localised regions? Is the computational effort the only reason, or is there a more technical reason?

Can you recommend me a text where I can find information on the y-plus values in specific regions, that would be most helpfull.

Thank you very much M Patel

The node of the first cell-layer away from the wall must have a certain distance. This distance is normalized (dimensionless). The needed distance is INDEPENDEND from the mesh, and belongs only to the nature of the flow (in first instants the velocity). You HAVE TO create a first layer of a certain thickniss. E.G. in my high compresible flows (mach about 0.7) I need a cell-layer thickness of 0.3 mm (!!!), but it depends on the flow. The rest of the mesh is up to you. Only with this cell thickness to k-eps turb.-model is able to calculate the right kinetic energy.

A more technical reason is the convergence-rate of the several matrix-solver-concepts. E.G. in a only about 1 Mio cell mesh the Gauss-Seidel relaxation reacts only to the changes of the neighbour cells, and don't gives a real solution progress. It can't reach the right solution. Nearly each solver has a boarder of this kind. Multigrid-solver can be advantagous in very large meshes.

The y+ value belongs only to the first cell layer, and not in the internal of the mesh. Take a look in any CFD-literature (StarCD Methodology Guid incl.). Is is always the same discribtion.

Again, this link below from same parent website (http://www.cfd-online.com/Links/onlinedocs.html) contains recommendations for reading. Of interest is this one below in which u can find more references at the end of the literature.

https://pronet.wsatkins.co.uk/marnet...nes/guide.html

Schlichting's and Wilcox's are typically referred.

hi m patel.as far as i know y plus is imp in following mattres. In the near wall region you reuire higher resolution of gradients,because u have set up no slip condition i.e. no velocity at wall.So for that u will rewuire refined mesh to capture BL.It is not necessry that y plus shoulkd be lower.It may alos higher depending upon size upon problem.If u are takinf laminar flow then obviously y plus is important. u can first simulate ur problem and in psot u can check y pluis value .If it is higher then u van go for refineing mesh andaginn solve it.This is what i know for cfx.I do not know for star cd but mmore or less it will remain same. hope it helps u. regards mehul pandya

Hi Mehul,Patel Dont u think turbulence Model is not required for laminar, since u dont have any Reynolds Stress terms for the lamonar flow and u will exact solution for NS equation.

Ya Regarding yplus value for High-Re model it should be >11 ideally but we can realx Y+>30 above in Star CD.But when u come across Low-Re the Y plus value<1 so for which very dense mesh is required since u need to capture all turbulence terms and Wall shear stress.

If anything more deatails u need feel free to contact me

lol

Alex Muthaiah

[mechkween]

Hi,

I am studying the flow over a truck using the k-e turb model and my wall y plus for the truck range from 17 to 2500! I am not able to understand how to refine the mesh to get a better value for wall y plus. I have tried to refine the mesh but after a certain point the solver takes an excruciating time to get through one iteration. The mesh is unstructured tetrahedral and it is a 3 d simulation.
Kindly advise how to get the wall y plus between 30 to 300. Am using Fluent as solver.

Thanks

[maziar.alasvand@gmail.com]

you use Ansys for meshing or Gambit?
you have to create boundary layer mesh to decrease the y+.in Gambit it's difficult but in Ansys you use (infation) that is very simple.

[ettehadi]

I have two question related to Y plus.

1- I am trying to have y+<5 while using K-epsilon method (enhanced wall) in a 5 m pipe. My y+ results give a value of less than 5 but the earlier values are about 8 and then it downs to less than 3 severely. Is this ok or not? Y plus value must be less than 5 throughout the geometry wall?

2- My goal is to find pressure drop throughout a pipe. The amount of pressure drop changes with quality of mesh even for the Y plus value less than 5. For example for the y+=3 I have dp=3600 Pa and for y+=0.5 I have dp=3400 Pa. which one is true?
Thank you for your attention to my request

[aqua]

hi, does anyone know how to calculate X+ and Z+ for LES in Starccm+ ? To perform LES, I need to consider X+ and z+...

Aqua

[DaveyBaby]

This might be really complicated! Searching "A User-Coded Region Profile" in the help brings up an example where a loop is used to iterate through from cell to cell. Maybe by doing this you could extract a distance with something like:
centroid[i][0] -centroid[i-1][0] and normalise it.
If you find out, could you please reply to this post? I can't see an easy way to do this!

[m zahid]

hi , anybody know the difference between log of y+ and simply y+ .
thanks

[aja1345]

Originally Posted by Alex Muthaiah
;170395

Hi Mehul,Patel Dont u think turbulence Model is not required for laminar, since u dont have any Reynolds Stress terms for the lamonar flow and u will exact solution for NS equation.

Ya Regarding yplus value for High-Re model it should be >11 ideally but we can realx Y+>30 above in Star CD.But when u come across Low-Re the Y plus value<1 so for which very dense mesh is required since u need to capture all turbulence terms and Wall shear stress.

If anything more deatails u need feel free to contact me

lol

Alex Muthaiah

Thanks sir,

You said that for High-Re model it should be >11 But I dont know range of these values of Re. I am grateful that these range of Re tell and explain me.