User talk:J Doug McLean

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[edit] Starting Vortex

I'm intrigued to know why you find Batchelor's description of the starting vortex on a wing (and so presumably,Turner's description of the starting or head vortex on a plume) misleading. The nice thing about vorticity (idda fort) is that it has to be transported from somewhere; and the strength of Batchelor's textbook was that it presented topics like vorticity generation and transport at an undergraduate level. Bob aka Linuxlad 10:16, 5 December 2006 (UTC)

Sorry for the delayed response. I'm new to Wikipedia and wasn't thinking to check my own talk, only the article's talk. I find the starting-vortex scenario misleading because it seems to mislead some people into thinking of the starting vortex as some sort of prime mover, i.e. as the cause of the establishment of the circulation on the airfoil and thus of the lift. It seems to imply for some readers that the physical cause-and-effect flows in the same direction as the narrative, from the starting vortex to the circulation to the lift. It's true that you can't have any one of these things without the others, but the physical cause-and-effect isn't a simple one-way street. The cause-and-effect relationship between the circulation and the lift is circular. The starting vortex, on the other hand, is a byproduct of the establishment of the other two, not a cause.

I agree that vorticity provides some great shortcuts to understanding what happens in many flow situations, but it also has a tendency to lull us into wrong ways of thinking. It's just too easy to start thinking of the vorticity at one point in the field as causing what's happening somewhere else, through the Biot-Savart law. Many of the inventors of useless devices for dealing with the wingtip vortex have fallen into this trap. J Doug McLean 23:17, 3 February 2007 (UTC)

[edit] Lift

I've been trying to steal myself to write a classical theory of lift article, mainly stolen from Batchelor - it's still languishing in my sandbox, because I can't face the maths editor (and I want to be careful to unpick Batchelor's working carefully, because it's spread as bits over about 200 pages).

But one of the interesting statements I wanted to get down plausibly (which I've drawn to people's attention before) is on p407:

It follows from the calculation immediately preceding (6.4.26) that the side-force exerted by the cylinder appears in the fluid far from the body half as a momentum flux and half in the form of a pressure distribution

This is for a 2-d system of course.

Any comments? Bob aka Linuxlad (talk) 16:54, 15 January 2008 (UTC)