Imagine air flow through an open door. Does anyone know, even roughly, how to determine the distance required to allow the jet to lose velocity and break up?

I know this is less a CFD problem than a general fluid dynamics problem, but I know there are a lot of qualified people out here.

(1). Take a look at the book by Schlichting, "Boundary layer theory". (2). Roughly, for laminar or turbulent round jet, the spreading is proportional to X, and the centerline decay is proportional to 1/X. (3). Also there will be a inviscid potential core region of about 6D before the jet decay starts.

Kyle,

To some extent I have to disagree with you that this is not a CFD problem. CFD is all about gaining insight into physical situations. Some physical situations have analytical solutions that you can look up in a book, and others don't. John's suggested reference is a good place to begin, and it will give you a little insight, but it is clear that you are not dealing with a round jet in free air. You are dealing with a rectangular opening that is bounded on one side (the floor), and likely bounded at some distance by walls and a ceiling.

The amount of detail that is required to gain the insight that you need is problem dependent. For example: Say your boss likes his office cold and brings in a large fan that happens to blow out through the door. If you want to know where to locate your desk such that you don't feel the breeze, then a round jet approximation is good enough.

However, let's say that you hear that one of your disgruntled co-workers is planning to come into the office on Friday evening, put a dead skunk in your boss' office and close the door. You want to know where you should not be standing on Monday morning where the door is opened. For that I'd use CFD, because the additional insight would be crucial.

Regards, Alton

Disclaimer: I'm in no way suggesting that it is a good idea to put a dead skunk in your boss' office. There could be negative consequences, and none of us would want that.