Re: Vortex, production of.

>Message:
>Why do vortices form when fluids flow through orifices, and what determines
>the direction of rotation?

Wow, Sam! You let the cork out of the bottle with that question! I hope I don't give you more information than you wanted!

First of all, vortices do not always form when fluids flow through orifices. And the formation of a vortex does not always require an orifice, like in tornadoes for example.

When there is a drop in fluid pressure, like when liquid starts falling through a hole, or when air masses are pulled away from an area suddenly, the fluid will tend to move from the high pressure area to the low pressure area.

THE VORTEX IN A BOTTLE ---

So, say a bottle if water is turned upside down. The molecules of water closest to the opening drop through it. The molecules next to the molecules that just dropped away move into the space where the old molecules were. As the molecules above the opening continue to drop away, the molecules to the side move into the center to replace the missing molecules, but then they drop away too.

Sometimes, the water has to slow down for a second to let some air into the bottle to fill the space the water was in before. This looks like big bubbles bubbling up from the opening. (glug glug glug.) In this case, there is no spin... no vortex.

If, at any time, some portion of the mass of the water doesn't move straight out of the opening ... if it moved a little to the side first, and then out ... it might cause the water around it to move a little to the side, and might even start the whole mass of water turning a little before going out the hole.

Since a turning motion will create a vortex, and the vortex has an empty space in the middle, air can now flow right into the bottle to replace the water without having to make big bubbles, so the water doesn't have to slow down ... it will flow out of the opening faster! It builds momentum and keeps itself going. Once the vortex starts, it's not likely to stop until all the water is gone. Think of it as a kind of liquid gyroscope!

At this small scale, the direction of the turning is random. You're likely to see just as many clockwise rotations as counterclockwise rotations.

For fluids flowing through holes, a lot of vortex formation depends on the shape of the surroundings, the shape of the hole, the pressure on the fluid, etc. There exist some pretty complex theories and equations having to do with fluid dynamics, and it usually takes some of the fastest computers in the world hours, days, even weeks to calculate the simplest of fluid flow; even the slightest change in conditions can change everything. (See also: CHAOS Theory)

THE VORTEX IN THE AIR ---

On a larger scale, like say the scale of a tornado, there are different influences. Tornadoes are VERY complicated things, but in a simplified explanation, my understanding of these vortices goes something like this:

Large masses of air moving past each other along the ground with high dynamic (moving) pressure create local regions of low static (stationary) pressure. Air from higher up pours downward to replace the air being pulled aside by the moving air masses along the ground.

As the air rushes downward, it is influenced by (among other things) the Coriolis Effect.

The Coriolis Effect is a physical side effect of ballistic motion (the natural motion of objects in a gravitational field, i.e. parabolic arcs, falling, etc.) with respect to a rotating point of reference (like the Earth). In an attempt to conserve linear and angular momentum, objects moving INTO a gravity well (that is, "falling downward") of a rotating body will tend to have a deviation or a "hook" or "slice" (as in golf) to their motion.

The amount of deviation will depend on the latitude. So, in the northern hemisphere, the deviation is in the COUNTER-clockwise direction, and will appear more extreme the closer the falling object is to the north pole. In the southern hemisphere, the deviation is in the counter-clockwise direction. At the equator, there is no deviation, so no tornadoes. And while this description is not completely accurate, think if the Coriolis Effect as the Earth rotating beneath a falling object, keeping it from falling straight down. I could give you all sorts of vector algebra to demostrate the exact situation, but you could get that out of any calculus based first year college physics book.

If the falling objects are molecules of air or water, then the effect creates a vortex, i.e. tornado, hurricane, funnel... because these are the function of low pressure zones that draw the fluids downward. Notice that hurricanes never cross from the northern hemisphere to the south and vice-versa? (A high pressure zone creates fluid flow in the opposite direction.) So, airflow is like this:

        In the NORTHERN HEMISPHERE...
                The LAND moves COUNTERCLOCKWISE around the North Pole.
                Air flows COUNTERCLOCKWISE around a LOW pressure zone.
                Air flows CLOCKWISE around a HIGH pressure zone.

        In the SOUTHERN HEMISPHERE...
                The LAND moves CLOCKWISE around the South Pole.
                Air flows CLOCKWISE around a LOW pressure zone.
                Air flows COUNTERCLOCKWISE around a HIGH pressure zone.

http://www.atmos.uiuc.edu/covis/modules/forces/html/coriolis.html

(Note that this web page incorrectly refers to the Coriolis Effect as the "Coriolis Force". It isn't a force. It's an effect of the accumulation of forces, like the improperly named centrifugal "force".) Now here is an IMPORTANT point:

BEFORE you go telling all your friends about the Coriolis Effect, you need to understand that it is a LARGE SCALE effect. Even though it exists on a small scale, it barely has any influence over objects when compared to other forces. On the scale of a sink, a full sink suddenly allowed to drain might tend to create a counterclockwise whirlpool funnel, but the direction of flow is really more influenced by random fluctuations in the surrounding water, or the small bump of your hand, or the shape of the drain. It might make an interesting science fair experiment though. :) It's a little like the gravitational force between a person and a nearby building: it's so weak and so easily overwhelmed by the gravity between either object and the Earth.

Thanks for your question!