By increasing the element height near the wall. Approximate x2

Ya while giving the mesh details, you will get that in mesh Topology

when using the enhanced wall function，it will be better when Y+ is less than 1.

“should be between 30 to 300” is for the standard wall function. So if you really use the enhanced wall function, Decrease Y+ not increase it.

if you want to decrease Y+,decreas the distance of the fist layer of mesh. if increase, increase the distance.

Hi,

In the log-layer (turbulence equilibrium), one can say that:

    y+ = (Cmu^1/4 k^1/2 y) / (nu_Wall)

where Cmu = 0.09 (typically), k is from your k-epsilon model, and nu_Wall is the kinematic viscosity at wall condition.  For gases, nu decreases with cooling. Thus, if you are allowed to set the wall temperature at a much lower value, then nu_Wall decreases and y+ increases but not much.  Another more effective way would be to use a coarser mesh (as Ilya Evdokimov suggested).

Houjian Zhao gave you the right answer. The y+ value should be near 1 (near the wall), which means you will have a good resolution of the mesh in that region. I recommend you to study the law of the wall before you go to the simulations.

http://www.software.ricardo.com/support/manuals/vectis/html/2013.2/vectismax/dns_wall_region.png

Good luck!

Your y+ looks good. You might not change it!

Houjian Zhao has the answer right on spot to your question.

y+ value is directly related to the physical distance of the first cell from wall.

If you want higher y+, make your first cell or the first grid point further away from the wall and if you want lower y+, decrease that distance.

As per your problem description, I suggest used y+ less than 1 when for enhanced wall function. Also, for the range of 30 to 300” is suitable for standard wall function or scalable wall functions. So if you are using enhanced wall function then decrease Y+ instead of increas it. You also tried k-w sst model if you dont want to use wall function approach.

Good Luck

Thanks alot.but i read in ansys fluent tutrial that for enhanced wall treatment dont need check y plus and enhance wall treatment use for all y plus.is right?

@seyede sahar Hosseini Ansys-Fluent uses wall functions as the default option and that's why you usually don't need to check this value because there is no need of a high resolution of your mesh in that regions (these functions "predict" the flow near the wall). That's the purpose of wall functions, have a coarser mesh at the wall boundaries which means less computation time.

In your case, if you use the enhanced wall treatment you must have a mesh with a good resolution near the wall (y+=1). So you need to check it.

How? In your mesh, the wall boundaries should have an inflation with a first layer height determined assuming a y+ near 1. The formula is given by the law of the wall.

 @joao oliveria: thanks alot for your answer,when i see countour of y plus for one wall in channel,near the other wall that under heat flux is 1,but middle of the wall y plus increase,should i refine mesh?in ansys tutrial didnt check y plus for enhanced wall treartment.

@seyede sahar Hosseini let me see if understood your question. you should refine the mesh near the walls where you have a flow since there will be present a viscous sublayer. So the y+ should be always near 1 near these walls. Far from the wall there are other values of y+ but these will depend on the growth rate of the inflation.

Answer: the y+ should be always near 1  near the walls where there is the existence of a flow. Since the y+ is always assumed to be 1, the mesh refinement near these boundaries depends on the velocity and temperature of the fluid in that region (once again, law of the wall formula, check the link bellow). Now you have two options:

- You have different walls and you calculate a different first layer height for each wall (different inflations) in function of the fluid velocity and properties in that wall;

- since it is a natural convection problem (small velocities), I guess you could use average values of velocity and fluid properties since there wouldn't exist large variations. So, you can use the same first layer height for all the wall boundaries.

https://people.rit.edu/pnveme/EMEM851n/Turbulence2/wall_treatments.html

Good luck!