reasonable result for air flow in city buildings?
 

Dear friends, i'm just using fluent for analysising the flow field in building group.Because of big space and limitation of my personal computer,i'm confronted with some problems,please help me, (1)i'm not clear what is the Re number in this situation.Surposed inlet veolecity speed is 10m/s and the whole space is 400m*300*150m. (2)hex grid size in my mesh is about 2m to 10m, i'm not sure whether it is reasonale,when iterating,fluent show message"turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 126152 cells" even continuity residual drops to 1e-4. (3)as you konw,air viscosity is very small,just 1.7894e-05,should i need using similarity theory to reduce the size of whole model for my simulation,what kinds of rule should i take? i'm appreciating for your help.

Best Regards George