Wind Hitting a Building


If you have a square building that had wind hitting it at a 90 degree angle, would the wind acually hit the building,and go around? Or just before reaching the building would it start to split and go different directions? Please help me out with this concept.


The answer to your question depends on the scale on which you look at the problem. At the smallest scale, the atomic size, nothing ever hits anything. Objects just get close enough that their electric fields have significant interaction. If we take as our definition of "hitting" to be that the interaction between the objects causes a noticeable change in the momentum of the objects involved then we could agree that the wind "hits" the building. Buildings may actually move under the influence of moderately strong wind.

If you look at the details of the interaction between moving air and an object approximately stationary in that air stream you will find that there is a rather thin layer of air all around the object in which there is no movement of air towards or away from the object. Rather the air in this layer moves along the object surface. So in fact the air goes around the building. Because there is friction between the object surface and the gas molecules that make up the air mixture there is even a very thin layer of air approximately one molecule thick which does not move at all relative to the object.

Because the air moving toward the building is prevented from passing through it, there is a pressure increase that backs up the incoming molecules and this effect, quite separate from the quite small scale things we have been talking about, diverts some of the air flow out to the sides so the air stream does split. If we take "split" to mean that there is noticeable sideways motion of the air then the split may occur half a building width or so away from the building.

This has been a rather long winded way of saying that the air stream splits, it still "hits" the building enough to have an effect on it and at the molecular level there is some air near the building that is not disturbed.

This information is brought to you by M. Casco Associates, a company dedicated to helping humankind reach the stars through understanding how the universe works. My name is James D. Jones. If I can be of more help, please let me know. JDJ