The purpose of the Wind Field Diastrophy is to progress slowly from an undisturbed flow in the model area to a flow around the structure.
To obtain this the buildings are made “solid” in 3 steps: In the first step (100%), the wind speed at the facades equals the free flow velocity, in the second (50%) it is half the free stream velocity and in the final step (0%) the wind speed is zero at the building walls which is the normal condition.
If the model are is very complex, the model might have troubles to solve the wind field.
Possible solutions: Small modifications in the model, increase the border grids or increase the model height.
See also Knowledge Base 10: Flow Problems