The first cell height is decided by the model you are planning on using for your analysis. If you are planning to use a model with wall functions, your y+ value should be between 30 and 60. If you are going to resolve the viscous sub-layer, your y+ values will have to be close to 1. Depending on these y+ values and your flow Reynolds number, you have to estimate the first layer thickness (y).
- Based on the definition of y+, to evaluate the value of y, you will need U_tau.
- U_tau can be estimated if you know your shear stress value.
- Shear stress can be obtained from the knowledge of skin friction coefficient and velocity scale. Depending on internal or external flow, you should be able to get a correlation for skin friction from any standard textbook. These correlations generally are dependent of the Reynolds number.
I hope this helps.