A Guide to Teaching with the DIYnamics Table at the middle school level

(This could also be adapted to younger audiences, by de-emphasizing some of the technical jargon. It’s also probably suitable for older audiences that don’t have a strong science background.)

Background

Fluids are everywhere on our planet. The water in the ocean, the air in the atmosphere, and even the rock and metal deep beneath the Earth’s surface are all fluids.

It doesn’t seem like it to us, but since the planet is always rotating, these fluids are always rotating too, resulting in oceanic and atmospheric circulations and swirling liquid metal at our planet’s core that generates the Earth’s magnetic field.

As a result, it is important for us to understand the impact of rotation on fluid motions, which we call geophysical fluid dynamics! The term geophysical refers to the physics at work on a rotating planet; fluid refers to the fluids mentioned above, like air and water; and dynamics refers to motion. Together then, geophysical fluid dynamics refers to the movement of fluids on a rotating planet.

Students using a rotating tank of water to model geophysical fluid dynamics at UCLA’s Exploring Your Universe.

In this lab, we model geophysical fluid dynamics using a rotating tank of water. Models help us describe and test our scientific understanding of phenomena we see in nature. Here, the tank represents the planet and the water in the tank represents the atmosphere (or the ocean, or the planet’s core).

Schematic showing how we create a scientific model of the whole Earth in a small spinning tank of water

Models, though, are rarely perfect – and ours isn’t either. Take a moment to ask yourself:

1. What does our model properly capture about the natural world?
2. What does our model not capture about the natural world? What are the major differences between our model and the atmosphere itself?

There are many answers to these questions, but one major difference between our model and the real world is scale, or the representation of size. The actual Earth is huge, but a rotating tank is small enough to sit on a desk. So the tank can’t exactly reproduce all of our observations of nature. Think about the following:

1. The Earth’s radius is approximately 6370 km. What is the horizontal scale of our tank model?
2. The depth of the troposphere—the part of the atmosphere where weather takes place—is roughly 10 km. What is the vertical scale of our water model?

Our model also allows us to investigate the ways that fluids transport material. One way is through diffusion, where particles move from regions of high concentration to low concentration within the fluid. Another way is advection, where the fluid itself carries particles with it as it moves. We can model these modes of transport using colored food dye as a tracer for fluid motions. A tracer is simple something that gets transported by the fluid. Ocean currents, for example, are fluid motions that can be traced and are visualized in this video: https://svs.gsfc.nasa.gov/3827

Understanding the fluid motions of our world is of great importance to scientists and engineers across the world. Research institutions and governments invest substantial resources into the study of geophysical fluid dynamics. Before we go further, take another moment to ask:

1. Why do you think it would be important for us to study the behavior of geophysical fluids?
2. Can you recall a time in your life when geophysical fluids affected you?

Phytoplankton bloom from the Barents Sea in the North Atlantic Ocean captured by a satellite in 2016 (Link to image)

Fluid motions in the atmosphere and ocean have a major impact on our planet’s climate and the life that inhabits it. Winds in the atmosphere determine temperatures and precipitation by carrying heat and moisture across large distances, creating distinct ecosystems for life. Ocean currents draw up nutrients to the surface, spawning plankton blooms that sustain large marine ecosystems.

For all of Earth’s history, changes in these kinds of ocean/atmospheric currents and circulations have led to corresponding changes in climate that disrupt ecosystems and even drive biological evolution. We refer to such climates of the ancient past as paleoclimate. Earth’s modern climate is currently undergoing a period of very fast changes, so understanding paleoclimate can help us to predict where our climate system may be headed.

Beyond the atmosphere and ocean, fluid motions in the planet’s interior also impact life. Motions of super hot liquid iron in the core generate Earth’s magnetic field, which protects us from the harmful particles released by the Sun.

Our model helps us study not just Earth’s atmosphere, but also atmospheres on other planets. The massive storm systems and swirling clouds observed on the gas giants of the Solar System (Jupiter, Saturn, Uranus, Neptune), such as Jupiter’s Great Red Spot, are examples of fluid motions driven by rotation. Models similar to the ones we use in this lab can help us learn more about these features and effectively study faraway planets that we can’t physically go to ourselves.

The bands and storms of Jupiter, including the Great Red Spot (Link to image)

We focus here on geophysical fluid dynamics, but similar physics apply to more exotic fluid motions too, like rotating stars and galaxies.

Experiments

When doing rotating tank experiments, we have to let the tank rotate for some time so the water can properly spin up. When the tank first starts rotating, the water on the outside is going faster than the water near the center because friction from the tank walls is pulling it along.

But after a few minutes, all the water will be rotating at the same speed. The more water you have and the bigger your tank, the longer it takes to spin up!

Experiment 1: Rotating Columns

1. In a non-rotating tank of water, drop a single blob of dye and observe.

   1. First, before dropping it, what do you think will happen to the dye?
   2. Now after you’ve dropped it, how would you describe the movement and structure of the dye in the tank?

2. Now begin rotating a tank of fresh water, wait a couple minutes for the water to spin up and then, as before, drop in a single blob of dye and observe.

   1. Before doing so, what do you think will happen this time?
   2. Now how would you describe the movement and structure of the dye?
   3. Do you notice a change from the non-rotating experiment? If so, how would you describe it?

3. Pump up the speed of the tank and wait a couple minutes for it to spin up. Drop in a single blob of another color dye and observe.

   1. Before you drop it, how do you think the dye will behave?
   2. Now after you’ve dropped it, do you notice a change in the movement and structure of the dye as compared to the more slowly rotating experiment?
   3. What can you conclude about the effect of rotation on the movement and structure of the dye?

In non-rotating experiments, the dye moves fairly quickly in all directions and has little structure. The transport is dominated by diffusion. However, rotating dye moves more slowly and forms vertical columns – the faster the rotation, the smaller the radius of the columns. In this case, diffusion is limited, and the rotation organizes the flow into columns.

Physical analog: This experiment provides an effective model for planetary interiors. In planetary interiors of rotating planets, fluids form columns that are aligned with the axis of rotation. These columns can create circulating electric currents that generate magnetic fields.

Illustration of Earth’s core. Planetary rotation forces the liquid iron outer core into columns that help generate Earth’s magnetic field (Link to image)

Experiment 2: Vortices

1. In a non-rotating tank, spray in fine dye patches with spray bottles if available. Then with differently colored dyes, add in a couple more blobs. Using one or two strokes of a pen or a finger, mix the colors.

   1. Before doing so, what do you think will happen to the colors?
   2. Now after you’ve mixed the colors, what do you observe?
   3. How would you explain your observation?

2. Refill the tank with clean water now. Begin rotating the tank and wait a few minutes for the water to spin up completely.

3. As before, spray in fine dye patches with spray bottles if available and a couple more blobs with differently colored dyes. Now mix the colors with one or two strokes of a pen or finger.

   1. Again, before mixing, ask yourself what you think will happen?
   2. Now after you’ve mixed the colors, what do you observe?
   3. What do you notice when you observe from the side of the tank?
   4. Can you draw connections between this experiment and Experiment 1?
   5. What might your observations represent in the natural world?

You may observe swirling patterns from the colored dye. These are called vortices (plural of vortex), or eddies (plural of eddy). They show up readily in rotating fluids. In this model, we generate vortices mechanically, meaning we physically stir the fluid to create the turbulence. But vortices can also be generated by other sources of turbulence – hurricanes, for example, are large weather systems in the tropics that are the result of rising hot air in a rotating atmosphere.

Vortices also occur in the ocean, where they are commonly called gyres if they’re large. Ocean gyres concentrate nutrients to promote plankton blooms, but they can also concentrate trash: The Great Pacific garbage patch is the result of a gyre in the Pacific Ocean that spans from Asia to the American west coast and is known for the high concentrations of plastics and chemical sludge gathered at its center by ocean currents.

The North Pacific Gyre and other ocean currents stirred mechanically by winds and continental boundaries (Link to image)

Physical analog: Vortices are typical in stirred fluids and are a fundamental component of turbulent flow in the atmosphere and oceans. Major storms and ocean gyres are examples of vortices driven both thermally and mechanically.

Note: Tornadoes develop from large storms, but their rotation is not due to planetary rotation.

Hurricane Douglas moving away from the Baja California Peninsula in 2002 (Link to image)

Experiment 3: Spinning Dye Curtain

1. Begin rotating the tank.

2. After a minute, before it’s spun up, rather than a single blob in one place, drop in a streak of dye from the center of the tank to the outside edge.

   1. Before doing so, what do you think will happen to the streak of dye? How do you think it will behave differently from just a single blob?
   2. What do you observe happening to the streak?
   3. How would you explain your observation?

Looking from the side, you can see the dye forms curtains, or sheets. Since rotation aligns fluids with the rotation axis, a streak of dye forms a curtain, whereas a blob of dye forms only a column. The vortices from Experiment 2 looked like curtains from the side as well, and were unable to mix well because of that structure.

Physical analog: Relevant to oceanic circulations. Rotation creates what’s called an Ekman boundary layer near the surface and bottom floor of the ocean where friction affects the velocities of fluid motion.

Bonus Experiment: Convection

In doing the previous experiments, you might have noticed small instabilities at the surface, tiny swirls forming a honeycomb type of pattern.

1. If you missed them, perform Experiment 2 in the sun or shine a flashlight onto the surface of the spinning water.
   1. How would you describe the surface as compared to the same experiment run without sunlight or a flashlight?
   2. What do you think is causing the observations that you see?
   3. What do you think your observations might represent in the natural world?

These observations are the result of convection. Heat, from the sun or the flashlight, is responsible for the tiny vortices.

The heat evaporates the water at the surface, which cools the surface water. Because colder material is denser, the surface water sinks while the water beneath rises. This convection is a form of stirring, so it too forms vortices under the influence of rotation.

Physical analog: Convection is enormously important for transferring heat in the atmosphere and ocean, as well as within the Earth’s interior where it helps drive plate tectonics. This experiment shows tiny, upside-down versions of hurricanes!

Terms

geophysical fluid dynamics, model, scale, diffusion, advection, friction, vortex/eddy, gyre, convection