Mad about Science: Vortex

By Brenden Bobby
Reader Columnist

One of my favorite above-ground pool activities as a child was to grab a bunch of pool noodles and start running around the perimeter of the pool with my friends. Our parents would scream “Stop! That’s how kids get killed!” and we would laugh and continue to make a giant whirlpool until it was strong enough that we could let our feet leave the bottom and allow the currents to swirl us around the pool for like 30 seconds. Then we’d start all over again and somehow not destroy the pool in the process.

The ’90s were a magical time where kids with relatively low situational awareness could manipulate the forces of nature and somehow not die in the process.

A vortex in the water. Courtesy photo.

As a juvenile buffoon, I had a modicum of understanding of what we were actually doing by “making a whirlpool.” It turns out that creating vortices is a pretty significant mathematical marvel that my young brain was able to puzzle out by grabbing objects with a lot of surface area and pushing them around the edge of the pool. 

The large surface area was creating resistance when the object was pushed against the water, which meant my body needed to exert more force in order to move it. This became easier to do the more energy I exerted over time, because that energy was being transferred into the surrounding water with nowhere to go but the direction in which I was pushing. 

In a way, I was effectively banking energy in the surrounding fluid that I could exploit when jumping onto an inflatable.

The reason this doesn’t work when you try it in the lake is because the energy has so much room to disperse. Your body simply can’t output the amount of energy needed to trigger a vortex in such a vast body of water.

There are three major forces at play to create vortices in huge bodies of water: gravity, temperature and wind. Whirlpools in the ocean are caused by water currents traveling in two different directions, which is most often influenced by a differentiation in temperature. Fluid matter doesn’t impact in the same manner that solid matter does. 

Two cars colliding on the freeway will smash together, scatter their pieces and fling or push the intact bits around based on the forces involved. Water currents won’t collide and stop, but will instead move around each other, which then causes a change in pressure and a deformation of the water’s shape.

It appears that the center of the whirlpool is sinking to create a void in the center. What is actually happening is that the change in pressure is pulling the neighboring air down into the whirlpool. If you want to see this in action with your own eyes, simply fill up your tub and pull the drain — just be sure to actually take a bath first so you’re not senselessly wasting water.

The force involved with draining your bathtub is gravity. The drain of your tub acts as a limiter for the amount of water that can leave the tub at once. As more water drains, the overall amount in the tub diminishes, which reduces the amount of area in which the energy is being dispersed. The water begins rushing toward the drain but is forced to spin in a vortex, as the drain is limiting the amount of fluid that leaves the tub. The water acting in this fashion begins to pull air down as well create the identifiable whirlpool pattern.

Fluid dynamics are useful for a number of applications in human society. Whirlpools aren’t simply churning features of oceanic destruction — we use them regularly in a number of environments.

Mixing paint is one of the most vital uses of vortices in human society. Paint is often composed of three materials: pigment, binder and medium. In the case of acrylic paint, the pigment is bound by acrylic suspended in water as a medium. Over time, gravity pulls these components apart and the heavier pigments begin to settle at the bottom. Solving this problem is simple: create a vortex.

A vortex mixer is a motor with an angled head and rubber cap attached to it. When you press down on the rubber cap, it engages the head with the motor, which then causes it to spin. Spinning an item on a single axis creates centrifugal force, which will actually push the pigments to the sides and keep most of the mixture separated. 

Centrifugal force is fantastic for separating materials, but not for mixing them together. The key of the vortex mixer is that the head is angled, effectively swirling the paint. This motor will run up to 3,200 revolutions per minute, or about 53 times per second. This is a lot of energy being transferred into a very small amount of liquid, which intensifies the vortex and mixes up the pigment, binder and medium.

Another interesting way people create vortices for mixing is by using magnets. A magnetic mixer works in two parts. The first is a magnetic stir stick placed inside of a vial that is suspended by clamps over a magnetic disc. Using a dial, you adjust the amount of electricity that travels through the base, which alters the strength of the magnetic field. This field will cause the stir stick to move around rapidly inside of the vial. By tuning this field to a precisely desired level, the stirrer will create a perfect vortex for mixing chemicals.

Stay curious, 7B.