why whirlpool is formed?

Remember the steering wheel? When two opposite forces act on it, it turns. Now similar to that what happens when the  velocity  of fluid at two near by points is opposite? We get a vortex.

A vortex has a centre about which the fluid rotates. It is usually caused by significant difference in velocities exist over small distances. We can see examples of vortex very often in nature. Sea waves is a good example. The receding water meets the incoming water. Opposite velocities at close distances. And a vortex is formed.

Whirlpools, tornadoes are all examples of vortex. Other examples are tip vortices from the tip of an aircraft wing.

A fluid flowing at high velocities into a fluid at rest can also cause vortex. This is one case where a whirlpool can form.

Now that we understand how the rotation comes into picture we will try and understand why the fluid is being pulled down.

Remember the Bernoulli equation, which is an energy conservation equation of pressure, velocity and potential(height)? Let’s now ignore the potential term. Which means as velocity increases in a flow the pressure of the fluid or the height of the fluid will decrease and vice versa.

In a vortex the velocity near the centre is very high, which means the potential(height) at that point will be low. Now the pressure at the surface ofa fluid say water is equal to the atmospheric pressure. Hence the increase in velocity is compensated solely by the decrease in potential.

We can also use centrifugal acceleration to explain this phenomenon.

EDIT:
Bernoulli’s Equation (copied from Wikipedia)
Bernoulli’s principle

where:

is the fluid flow speed at a point on a streamline, is the value of acceleration due to gravity,

is the elevation of the point above a reference plane, with the positive z-direction pointing upward – so in the direction opposite to the gravitational acceleration,

is the pressure at the chosen point, and

is the density of the fluid at all points in the fluid.

PS: Please keep in mind that several assumptions go into the Bernoulli equation and it cant be applicable directly to a whirlpool. However it gives a  general idea of why and hence I have used it here.

Whirlpools result from the turbulent flow of water. In rivers, we often think of water as flowing smoothly, except when it comes to some object blocking its path. The water flows around the object (a submerged rock, a bridge pylon, etc.) and this may cause the water to lose its smooth- flowing properties.

As the water flows in around the object, a ‘whirlpool’ may be created. Another method that creates turbulence is to increase the flow of water through the riverbed. The faster the flow, the more likely turbulent flow will develop and the more energetic the turbulence. In the oceans the process is only a little different. We don’t usually think of ocean water as flowing, but it does.

There are huge ocean rivers that flow all around the Earth. Uneven heating of the ocean waters by the Sun, forces caused by the Earth’s rotation (Coriolis force), and uneven salt content of the various ocean waters, all contribute to the driving forces that keep these waters flowing. Sometimes, these currents run past each other or actually collide. Conflicting tidal flows can also interact.

When this happens, turbulence similar to that described for a river can result. According to the ‘Book of Popular Science’, the best known tidal-generated whirlpool in the world occurs in the Maelstrom, a strait about three miles wide in Norway’s Lofoten Islands, between Moskenesoy and Mosken Islet. One famous and fanciful, description of this whirlpool is in a story by Edger Alan Poe, called ‘Descent Into the Maelstrom”. You may find it interesting if not very factual. The whirlpool of Garofalo, in the Strait of Messina, between the island of Sicily and Italy, is produced by winds that flow against the direction tidal currents.

The destructive effects of such whirlpools have been rather exaggerated; small boats may be entrapped and wrecked in them, but not larger craft. However, even a large boat may find steering almost impossible until the whirlpool subsides. If you are interested in a book that looks at this process from the viewpoint of the Chaos Theory, read ‘Turbulent Mirror” by John Briggs and F. David Peat, Harper & Row, 1990. More recently, Paul Harvey (a U.S. radio commentator) told of a lake, I believe in Wisconsin, USA, that developed a huge whirlpool and actually sucked several boats down. It was later determined that an underground cavern developed an opening into the bottom of the lake and drained the water. In flowing water from rivers refilled the lake. The various kinds of turbulent flow can be a very interesting study. I hope this helps.

Whirlpool Facts and Mythswhirlpool

  • What is a whirlpool?
  • How do whirlpools form?
  • Which way does water rotate down a drain?
  • Hands-on experiments.

We answer these questions and many interesting facts about vortex’s and whirlpools. This information is the most comprehensive collection of whirlpool facts with graphic illustrations and pictures. As a bonus, we will include experiments that can be done in your classroom or home.

Definition of a whirlpool

A rapid circular current of liquid. [syn: vortex, maelstrom]

  • A vortex is any whirlpool with a downdraft.
  • A maelstrom is the term applied to the most powerful whirlpools.

The most powerful “natural” whirlpools are the result of tidal changes and the resulting fast-flowing water through narrow shallow straits.

But most people are more familiar with smaller less dangerous whirlpools that occur in streams or at the bottom of waterfalls. To be sure, these whirlpools can cause lots of problems for watercraft, and they can pull people down and not let them up. So they are dangerous, but not to the scale of a maelstrom whirlpool that can swallow a boat.

How do whirlpools form?

Any time water flows through a narrow path, it forms at least a partial whirlpool. As the water passes through the narrower opening, it accelerates and forms a more powerful force. If the downstream area then enlarges, it can mature into a complete whirlpool.

As water is pulled into an opening by gravity, it begins to spin. The direction it spins is discussed below. Once this begins, it intensifies and forms a cavity in the center of the drain. The cavity creates a vacuum into which objects such as bubbles, water molecules, and other floating objects are pulled. As these objects are “sucked” into the vortex, the centrifugal (outward) force maintains the hole in the middle through which air passes.

What makes whirlpools spin?

As the water is pulled down into the opening, the water particles fight for the smaller space and push each other to the side. This pushing and nudging by itself would not necessarily cause the water to spin, especially in a perfectly-shaped funnel and no other directional influence on the water. But a perfectly-shaped funnel never exists in nature. There are inconsistently-shaped rocks or other obstructions that force the water away from them. This initiates a spinning motion that accelerates as the water is pulled by gravity.

What gives whirlpools their vortex shape?

As the water spins and accelerates, the centrifugal force tries to force it to the outside. Of course, it is contained by the rocks or other naturally-occurring obstructions, so it cannot “fly out” of the natural funnel. Water is heavier than air, so the center of the vortex creates a column of air which is simply the vacancy caused by the water being forced to the side.

Can a whirlpool suck a boat or ship into it?

It is merely a matter of size. If the maelstrom is large enough and the boat or other object is small enough, the object will be drawn down through the vortex along with the water. It is understandable then, that fables exist about large ships being sucked down and eaten up by giant whirlpools, but actual documented cases that we would consider trustworthy do not exist. On the other hand, you wouldn’t want to try to paddle through a large whirlpool in a row boat. Even small whirlpools are VERY powerful. While it may not suck a human being down into it, it is very likely that even an Olympic swimmer would soon tire trying to avoid it, and then risk drowning. So NEVER try to challenge a naturally occurring whirlpool in a stream or river.

What direction does the water spin?

Have you ever heard that water goes down a drain in different directions in the northern and southern hemisphere? Is it truth or myth?

Technically, it is true. If there is no other outside force present such as the direction of the inflowing water, and the drain (hole) is perfectly level, water will rotate counterclockwise north of the equator and clockwise south of the equator. This is called the “Coriolis Effect” named after Gaspard-Gustave Coriolis, a French scientist, who described it in 1835.

The Coriolis Effect is extremely slight, similar to the fact that we can stand on the earth without being knocked over by the 1,000 mile per hour speed of the rotating earth. There are many forces that are stronger on a small body of water such as the angle of the drain, the slightest hand movement as you remove the plug, etc.

In a natural setting such as a stream, there are usually other obstructions that create the spinning direction.

If you were to stir water in the opposite direction of your hemispheric location, that is sufficient energy to create a vortex whirlpool since that stirring is a stronger localized influence than the slight rotational influence of the earth.

Some people even believe that water swirls down a toilet according to hemispheric influence. But that is NOT true. The direction of rotation in a toilet is caused by the direction of the water flowing into the toilet bowl from the outlets around the inside of the rim.

What about a bathtub drain? Will the Coriolis-Effect be sufficient to influence the direction of the vortex? Only if the tub and drain are perfectly level, and the water is not disturbed when the drain plug is removed. But that is nearly impossible.

Try it.

Fill a tub with 3” or 4” of water, let it settle for at least 5 minutes, then slowly remove the plug and watch the whirlpool form. It is best to pull the plug with a chain rather than by reaching in with your hand. Your hand will cause slight currents as you put it into the water and then remove it, and that might be enough to influence the direction of the water.

Pop Bottle Experiment

One of the most fun whirlpool experiments is with a pop bottle. You will probably spill water with this experiment, so it is best to do it outside or over a tub.

Fill a 1-liter plastic pop bottle with water and turn it upside down. Watch as the water fights to get out of the small opening. The fight is between water and air. As water leaves the bottle, it ABSOLUTELY MUST BE REPLACED WITH AIR. If air doesn’t replace the space occupied by the water, a vacuum will form and slow down the water while sucking the sides of the bottle in. Notice the large bubbles of air climbing to the top of the water level. It is very uncoordinated.

You can also demonstrate the power of this vacuum with a straw. Stick a straw down into a glass of water, place your finger over the top end of it, and then remove the straw from the glass of water. Notice that the water stays in the straw. It can’t fall out because you have your finger over the top end so no air can get into the straw to replace the water. Now remove your finger, and the water falls out freely.

Similarly, if you punch or drill a hole in the bottom of the pop bottle, put your finger over it, fill it with water, turn it upside down, and then remove your finger from the hole, you would be able to see how freely the water falls out of the bottle.

tornado in a bottleCreate your own WhirlpoolBut there is an even better way to let air into the bottle, and it demonstrates a whirlpool vortex at the same time.Fill the bottle with water, turn it upside, down, then move it in a circular motion to get the water spinning, then stop making the circular motion and let the water continue to spin.

This will start a vortex action, and it will increase as gravity pulls the water down. Notice the hollow opening in the water that allows air back up into the bottle.Compare how much faster the water leaves the bottle when you create a vortex. It is much faster with the vortex action because the water and air are not fighting as they pass through the same small space.Build a Whirlpool in a BottleClick here or on the picture to see illustrated instructions about how you can build your own whirlpool in a bottle or “tornado tube” like the one pictured here. It is easy to build with two plastic pop bottles, and is lots of fun to play with.

 

Water SculpturesA sculptor in London by the name of William Pye builds large water whirlpools which are located in tourist destinations and other locations. You can see pictures and videos of them on his website.

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