Where Are My Teeth?

Leafblowing as a Model for Pyroclastic Flow

Some of you may think that this is just along the same lines as my Heisenberg Raking Theory, but it's not. For one thing, it's not particularly useful (whereas Heisenberg Raking Theory provides an elegant justification for doing a really half-assed job).

The oak tree in the back yard had finally finished shedding, and the mind wanders when blowing leaves. I noticed that there's a trick to it: if you get under a pile, you can levitate whole bagsful off the ground in a mass and shift them gently downwind. The air reduces the leaves' sliding friction (which is considerable) and the matted leaves hold in the air air underneath them.

As I waved the leaf-blower back and forth, the analogy to

In particular, you get some very interesting feedback loops among airspeed, flow direction and behavior of the leaf mass. When the leaves mat together, they can direct and contain the air very effectively, but too much airspeed and you get blowouts, or individual leaves pinned against tiny obstacles in the lawn that would mean nothing in a lighter, bouncier breeze. Meanwhile, leaves at the edge of a pile will peel off into the airstream (not away from it) so you get a sort of bernoulli-induced increase in the number of leaves where the air is moving fastest.

How this translates to huge piles of sharp-edged rocks bounding downhill with masses of incandescent air entrained and new hot air generated from the ground underneath them, I don't exactly know. But I was also struck by the way that I could sometimes peel the leaf cover off the ground with the blower and bend it up and back so that it looked a little like a breaking wave. You could imagine that if the situation were reversed -- leaves and their trapped air moving, still air providing the resistance, the same kind of thing might happen to the leading edge there.

The whole phenomenon also appears to depend strongly on the shape of the leaves -- oak (and maple) leaves seem almost designed to interlock and mat together, as opposed to beech or birch. A few flat skipping-boulders on the front of a pyroclastic flow could do as well, I expect.

Ah well. Obviously what I need is access to a virtual-reality wind tunnel to check all of these ideas out. And remember, it's all still quite useless.

(The only useful thing I did learn was how to avoid vorticity traps -- those embarassing puffs where not only do the blow leaves end up all over the place, but thanks to little cyclones shed by the main air blast, bunches of them end up moving toward your rather than away. But that would be telling.)

Posted by wallich at November 11, 2003 03:55 PM