In this study, a dimensionless CFD analysis for the isothermal incompressible fluid flow in buildings is conducted to illustrate the Reynolds number similarity among different scales and the Reynolds independent phenomenon for the same scaled setup. For the similarity in different scales, the geometry similarity is required, and the Reynolds number is conserved by the boundary condition configurations to achieve the dynamic similarity, and a good match of the dimensionless results between scales can verify the dimensionless analysis procedure. For the simulations in the same scale, the dimensionless governing equations is further simplified and revealed the role of turbulence viscosity in Reynold independent phenomenon. A series of CFD simulation are conducted to study the distribution and evolution of turbulence viscosity with its relationship to achieve Reynolds independence. This paper develops a new method of justifying the Reynolds independence with CFD simulations, and this dimensionless CFD analysis method can be an important procedure for airflow pattern scaled experiments design and problem simplifications in urban and building airflows.