Y Plus / Y+ Calculator for First Cell Wall Distance
Background Information
In CFD, there are two ways to capture the boundary layer physics of turbulent flow. The first method is to resolve the physics all the way to wall by capturing complete boundary layer. The second way is to use wall function which involves resolving certain part of boundary layer and using imperials relations to model the remaining part. Capturing or resolving means creating the necessary grid and solving flow and turbulence equations in the region. By modelling means, not creating the gird or solving the required equations, but to use some formula /imperial relations to extrapolate the values.
Above two approaches of boundary layer treatment are available in most of the CFD software like ANSYS FLUENT, ANSYS CFX, STAR CCM+. To use appropriate wall treatment, it is necessary to create mesh with required resolution including first cell height, the growth ratio and number of elements in boundary layer. The process creating the grid which satisfies wall Y+ is an iterative process. In one attempt, it’s hard to create a mesh satisfying Y+ at all locations on the wall as it depends on the boundary layer physics near the wall. So typical procedure followed is :
 Get approximate estimate of first cell height based on average / free stream velocity
 Create the grid with estimated first cell height, growth ration and required number of layers
 Solve for this initial grid.
 Check the resultant Y plus after solution
 Identify min, max value, region where Y+ criteria is not satisfied
 Do coarsening / refinement in the identified regions
 Rerun the simulation
This procedure is followed until Y plus criteria is satisfied for the selected turbulence model and wall treatment. The above procedure needs estimate of distance of first cell height from wall. Once this value is calculated first time, the later adjacent could be done based on CFD simulation results.
 To know more about wall Y plus and boundary layer, read the blog on “Y Plus, Wall Function and Estimation of First Cell Height”
 To know more about turbulent flow and turbulent flow modelling, read the blog on “Introduction to Turbulence Modelling”
Related Reading
Turbulence Modeling for CFD 

A First Course in Turbulence 

Turbulent Flows 

Computational Fluid Mechanics and Heat Transfer 
The Tool Creator
Sanket Dange 
Sanket is working as a CFD engineer in CCTech. He has worked on numerous CFD projects in the filed of automobile and HVAC. He has worked as a tutor at LearnCAx and taught to students through LeanrCAx online as well as offline course modules. Currently he is working in the field of CFD software development which is mainly based on ANSYS FLUENT. Sanket has written blogs on various topics in CFD. He also has interest in open source CFD technologies. Sanket holds a Bachelors degree in Mechanical engineering from University of Pune. 