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Name: Paul B.
Status: Student
Age: 30s
Location: N/A
Country: N/A
Date: May 2004


Question:
I am a volunteer fire fighter and a science teacher. We were told to always apply our solid stream water (fire stream) in a clock-wise circular direction to keep the fire and hot gases away from us. If we were to move our fire stream in a counter-clockwise direction it cause the hot gases and fire to move around us and "bite" us from behind. When I asked my instructors why this would happen they said because it just did or because we are in the northern hemisphere. When I asked my fellow science teachers, they were unable to answer my question. I have actually done this in fire training situations, but why does this work the way it does?



Replies:
Assuming that the stream of water from the fire hose is not rotating as it leaves the nozzle, I cannot see any reason why it matters which direction you spray a vertical flame. If the water stream is rotating with respect to the direction of the nozzle, I suppose the idea is to move against that direction, but I think that would be a very small effect compared to the convection of a fire. It could be that the fire hose nozzles are designed so that the stream does rotate in the same way that a rifle bore is designed to cause the bullet to spin so that the bullet is more stable in flight, but I do not know that first hand. The idea that is in the northern hemisphere is a reference to Coriolis forces, but that effect is negligible except for large masses like the atomosphere.

Vince Calder


Another variation of a science myth that keeps going around.

In various forms, it declares:

- for no apparent reason, your tub always drains with a clockwise (cw) or counter-clockwise (ccw) swirl.
- toilets flushing supposedly do it too.
- tornadoes and hurricanes rotate according to their hemisphere
- and now your letter.

These all have to do with gravity-driven vortices, which are well known to amplify tiny a mounts of initial rotation until they are intense swirls which saturate their local environment.

Because of this amplification, it is always vaguely possible that your local environment is so quiet, that largest determiner of the initial direction of rotation, is the earth's slow rotation.

For huge objects like hurricanes this is almost always true.
For small objects like bathtubs it's almost never true.

I really doubt that earth rotation can matter for an air vortex in a burning building. It is much more likely to be initiated by an asymmetry in the shape of the room or the locations of the doors and open windows, the starting location of the fire, the direction of pre-existing drafts as the fire was starting, or the direction of the wind outside the nearest open window.

Do you actually observe swirling motion inside buildings in the fires you fight? Do they have a predominant direction? That is, before your stream is started? If not, the best circular sweep direction is unrelated to hot air's pre-existing swirl, and earth's rotation is doubly irrelevant.

It could be that hot air spurts are caused by reversing your own direction of sweep, or by having two hoses doing opposite directions and their wet-spots colliding periodically. Training everybody to a habit of circulating one way all the time will make a smoother air mass on the average. It wouldn't matter which way, as long as it gets used by every hose on the scene, for as long as any hose is flowing. But if you have actually experienced bite caused by doing [ccw] all by yourself, I have not addressed your issue yet.

If the hose is slung under your right arm, distinct asymmetries arise, which are pretty likely to matter. Then the opposite circulation would be correct for left-handed hose carries. Ever discussed?

Your stream probably injects a substantial rotational thrust into the fire. Suppose the impact spot of your stream creates lots of expanding, rising steam. Your point of vulnerability, your self, is a bit to the left of the nozzle. If the air is rising as the impact plume from your nozzle drives it in bursting swirls, a swirl is more likely to pass over your head and miss you, if you are cycling clockwise.

Does a large steam plume arise from the stream's point of impact? If so, imagine it as a giant hand stirring the pot of air in the room. You would probably prefer that the direction of that stirring is away from you on your side of the hall, or on your side undefended by the stream.

How much forwards-running cool air draft is induced on your right side by the water stream? It could subtly function as an air screen, deflecting hot air flows which come at you on your right side, back into the room. Conversely, such a driven draft must pull in air from behind you, and if hot air is allowed out through a large hole on your left, it could easily be pulled around behind you and into your back.

If you then stand in the middle of a hallway and aim your stream parallel to a wall, you are creating some net swirl in the burning room because your stream is closer to one wall than the other.

A stream near and parallel to one wall would probably be deflected by far walls, to form a hot air draft back at you. But if you are cycling clockwise, immediately after each thrust you draw a line of water on the floor in front of you, which by rising could deflect the backdraft upwards. Also, next your stream is in a defensive "parry" position, upwards and across your front, cooling and pushing against this draft. Matching the typical time delay for a return burst would matter then. Cycling too slowly would be undesirable. Is that familiar?

It might be easier to see the spot where your arcing stream impacts, depending on direction.

All told, "It just does" begins to sound about right.

I'm just guessing, and I don't presume that my guesses were very correct. Since you have some fire experience, you may be able to feel out dynamics like that. 3-D Computer models of convection might only need about 10,000 volume-points to play with this, so perhaps some amateur programmer will try it soon. I am not sure that a reduced-scale experiment would be representative. Scale is likely to matter here.

Jim Swenson



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