for the coupling process of ES and CFD, a reasonable first cell height is important for determining the convective heat transfer coefficient between wall and air . So the challenge is how to make the mesh in OpenFoam properly. I was told, it is related to a Term called Y+, so now I’m looking into it. Right now, I’m not truly cared about the meaning of this value, I just want to know how can I do it in the pre-processing.
after further reading, I think the “Layer addition” option of snappyHexMesh itself can take over this job .
 Z. Zhai, “Developing an integrated building design tool by coupling building energy simulation and computational fluid dynamics programs,” Thesis, Massachusetts Institute of Technology, 2003.