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.
buoyantPressure Sets fixedGradient pressure based on the atmospheric pressure gradient
due to the nature of the PISO-algorithm used in interFoam, it is necessary to have a very "good" pressure field, as the pressure field is required in the velocity correction step of the scheme in order to reach mass conservation (i.e.div(U)=0).
If you look at the PISO-algorithm you will see that this means if the flux is zero (as it is orthogonal to the wall) it is necessary that the second derivative of pressure (orthogonal to the wall) must be zero, too (in areas where the density is constant).
buoyantPressure fulfills this requirement in areas of constant VOF-function / density. I'm not sure about mass conservation if sloped walls touch areas with VOF-values 0<VOF<1.
I see it,
if I give k & epsilon the initial value of 0;
if there is not at least one patch with buoyantPressure