How can i make the boundary box of a building to make the wind start flowing from it
To simulate external flow, you first have to show the software where the fluid domain is. You can do that by running an enclosure operation. Please check this documentation.
Also, to avoid problems, make sure your geometry has no intersecting bodies.
With external flows (air flowing around your object, vs internal flow which is air flowing within CAD defined boundary’s) you have the option to define a rectangular Background Mesh Box, sort-of ‘wind tunnel walls’ (it is defined in Geometry Primitives).