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Documentation

How to Control, Observe and Analyze Y+ During Meshing, Simulation and Post-Processing?

Objective

This article describes how to control and analyze y+ using SimScale. If you want to know what y+ is, have a look at this forum article. The article describes how to chose between full resolution and wall function.

Approach

Y+ represents the thickness of the first cell of the mesh next to the wall dimensionless as it takes physical boundary conditions as fluid viscosity and velocity into account. They correlate in the following way:

$$y^+\sim \frac{velocity * first \ layer \ thickness}{kinematik \ viscocity}$$

Let’s assume we don’t change the fluid properties and the flow velocity. The only remaining variable it the first layer thickness. So lets see how to change it.

Control Y+ during Meshing

You can control the first layer thickness with the mesh refinement inflate boundary layers: meshing->refinements->inflate boundary layers.

New Inflate Boundary Layer Setting SImScale
Create new inflate boundary layer

Depending on the mesh type there are different ways to control the first layer thickness:

Standard Mesher

With the standard mesher you can either directly define the absolute first layer thickness or you can increase the total amount of boundary layers if you want to decrease the y+.

Standard Mesher Inflate Boundary Layer Settings SimScale
Inflate boundary layer settings for standard mesher

Hex-Dominant Automatic

With the Hex Dominant Automatic mesher you can define the ratio between the first layer thickness and the cell size next to your wall.

Standart Mesher settings Inflate Boundary Layers SimScale
Inflate boundary layer settings hex-dominant mesher

Analyze Y+ during the Calculation

To observe the y+ development during the simulation you need to go to result control -> field calculations -> y+

Result Control y+ SimScale
Set y+ as field calculation under result control to observe y+ during the calculation

Once your calculation has begun, you can see the option field calculations in the run.

Check the Y+ locally in the Post-Processor

Create a new isovolume and select y+. Now you can change the values of interest. Eg. if you want to see where your y+ is higher than 1, you can set the scope to 1 or higher.

Isovolume y+ Analyzation SimScale
Isovolume for checking y+ locally

Note

If none of the above suggestions did solve your problem, then please post the issue on our forum or contact us.

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