wind speed-up over structures

huangyhg

wind speed-up over structures
suppose you take a structure with dome roof, and put a 1' cube on the roof.  what's the wind load on that cube?
from asce 7-05, fig. 6-21, i come up with a cf factor of 1.3 in that case.
on the other hand, if i take a 1' square surface and just suspend it in the air, from fig. 6-20, i get a cf factor of 1.8 (for b/s=1, s/h<0.16).  so being on that dome roof reduces the wind force considerably.
meanwhile, my fluids textbook shows that in a 2-d potential flow around a cylinder, velocity at the cylinder at right angles to the wind is double the velocity at infinity.  so it seems like the force on the cube on the roof ought to be higher, rather than lower.
any thoughts?  am i missing something here?
for the case of the cube on the roof, did you multiply by the factor in 6.5.15.1?  if applicable, that should bring your wind pressure on the roof above that of the sign you're comparing to.
that is exactly what i was missing, thank you!