# Just go with the flow: The math of fluid

## The Coffee + Math Series

We have  a team full of mathematicians, math teachers, and math enthusiasts. We  want to share our passion for math with everyone, so we’ve decided to  launch a new series called Coffee + Math.

Coffee + Math is as simple as it sounds. We have a coffee and chat about our favorite math equations, math facts and other math tidbits.

This  week we grabbed a coffee with Jake Horder, resident mathematician and  qualified environmental engineer, to find out more about his favorite  math equation.

#### Coffee of choice?

Single origin, black, no sugar.

#### What’s your favorite math equation?

My personal favorite is the Navier-Stokes equation.

#### Ok… Can you translate that into English?

Sure! Imagine that your coffee is made up of lots of tiny parcels of fluid.

What are the ways each parcel can change over time?

Hint: A parcel can move, stretch, twist or spin depending on the forces applied to it.

The  left hand side of the Navier-Stokes equation is the inertia; a measure  of how difficult it is to change the speed and direction of the parcel.

The inertia is made up of terms describing how the velocity changes in time and space.

The  right-hand side of the equation contains the forces that act on the  parcel, the things that might make it move and twist and deform. These  include the surface forces like pressure and friction from the  neighboring parcels of fluid as well as the force on the whole parcel  due to gravity.

If  we add up the effect all the parcels have on each other as they move  around inside the mug then we get a description of how the coffee as a  whole behaves.

The Navier-Stokes equation is essentially Newton’s Second Law, F=ma, but for fluids.

#### Why are you so interested in this equation?

I  started sailing when I was quite young. I picked up a lot of my skills  from fellow sailors, yet I often felt like I was following rules without  really understanding why.

Why did the boat have to point this certain way? Why do the tell-tales need to be straight? Why is there a secondary sail in front of the mainsail (the biggest sail)?

Years later I realized that a lot of my sailing skills could be explained by the study of physics and mathematics.

It  was genuinely exciting to find connections between what I was learning  during my university fluid mechanics classes, and what I had always  wondered about in my everyday life.

A  plane wing acts to turn the direction of a flow field, constantly  throwing air downwards, which in turn lifts the plane upward (see  Newton’s third law of motion). Of course this is the same as the  mainsail on my Hobie catamaran, which curves the wind around it to pull  me along the water.

I think about a flag fluttering in the breeze, the periodic back and  forth as turbulent vortices tumble along each side. Clearly the very  same phenomenon rustles up my mainsail if I point too high into the  wind, the air can’t stick to the sail anymore and begins to trip over  itself.

These  and many other everyday encounters with fluids all descend from the  ideas captured by the Navier-Stokes equation. I feel that a deeper  understanding of the nature of fluids emerges from being able to read  the mathematics in the flow. And so far it has been a very satisfying  journey.

#### How is this equation useful outside of sailing?!

Fluids are everywhere. Knowing about the equation of fluid motion can give us a greater understanding of a lot of cool natural phenomena.

Is oobleck a solid or a liquid? What does a year of global weather look like? How does a spiral galaxy form?

We  can also use the Navier-Stokes equation to guide us in designing  efficient structures, machines and devices that are immersed in or  contain fluids. We can consider the flow of air past a wind turbine,  build a faster bullet train, or even model how blood might move through  an artificial heart.

Without  knowledge of the Navier-Stokes equation, the only other way we could  design these things is by guess-and-check, and that gets very expensive  (not to mention time-consuming)!

#### Can you tell us about the history of this equation?

Our very existence is dependent on water. So the study of how water, and other fluids behave, dates back thousands of years.

Archimedes  and other ancient Greek thinkers made many important observations using  the mathematics and technology of their era, but with Newton’s  invention of calculus in the 1600s, a more sophisticated mathematical  toolbox was available to scientists like Claude-Louis Navier and Sir  George Stokes to fine tune our understanding.

Navier  was a French engineer who pioneered the study of how structures bend,  twist and deform and he went on to apply the same ideas of stress and  deformation to fluids. In the 1840s it was Stokes who, among many other  achievements, formalized this relation mathematically into the familiar  Navier-Stokes equation.