There wasn’t much Werner Heisenberg didn’t know by the time he was in his 70s. He was a world-renowned theoretical physicist who had won numerous awards, including the Nobel Prize. Even as he lay dying in February 1976, he yearned for more information. “I’m going to ask God two questions when I meet him: why relativity? “And why the turbulence?” he was said to have remarked. “I’m confident he’ll have an answer for the first.”
It’s a great line to close on, not least because it’s unexpected, at least to the layperson. After all, it’s unusual to see relativity emerge as the superior option in the face of… well, anything, let alone a phenomenon you last heard about on a bumpy airline ride. However, nearly half a century later, we’re still trying to figure out what causes turbulence. So, what exactly is this phenomenon? What is it that makes it so perplexing? And, more to the point, what is it about it that is so enthralling?
WHAT DOES TURBULENCE MEAN? The question “what is turbulence?” has two possible solutions. Or maybe none at all. It all depends on your perspective. “In the scientific world, we don’t actually have a universally agreed-upon definition for turbulence,” James Beattie, a PhD student in theoretical physics at the Australian National University, told IFLScience.
“It’s one of those things where you just know it when you see it.” Take a step back, and we’re all familiar with the term “turbulence.” If you drink your coffee with milk, you’ve probably noticed the clouds of white blooming up and spreading throughout the drink, creating intricate swirls in the liquid before settling down into a flawlessly blended cup of java.
That’s turbulence for you. When a fluid moves as a collection of eddies – tiny whirlpools and the reverse current they form – its size, speed, and orientation constantly change as they interact and influence one another. It’s simply how the cosmos blends itself together. “Turbulence is chaos — an unexpected, rapidly changing flow,” Blair Johnson, Assistant Professor at the University of Texas at Austin’s Johnson Environmental Turbulence (JET) Laboratory, told IFLScience. “It’s why boats make wakes and volcanic ash clouds move into the atmosphere, [mixing with] their surroundings,” says the author.
But delve a bit further, and things become a lot murkier — and far more perplexing. “A sort of hydrodynamic instability is turbulence,” Beattie explains. “What do I mean by ‘instability,’ then?” If I were to disturb (think of a gently prod) a turbulent fluid, the disturbance would spread throughout space and time (exponentially rapidly!!!).” Consider inserting a twig into a muddy puddle: the silt and dirt swooshing around in the raindrops reacts to the incursion by forming those psychedelic-looking swirls all around it. But aren’t we a touch begging the question here? Are we really any closer to an answer if we define turbulence as something that happens when fluid is turbulent?
“All well, but what causes the fluid to become turbulent in the first place?” According to Beattie. “This takes us to turbulence’s type of fluid instability: turbulence is a high-Reynolds number instability.” The Reynolds number in a fluid is found by dividing the fluid’s inertial force (the force resulting from the flow’s velocity) by its viscous forces (essentially how “gloopy” it is). You’re working with a fluid with a lot of momentum and low viscosity — a high Reynolds number flow – if the inertial force is substantially greater than the viscous force. The Reynolds number would be low in the case when the inertial force is low and the viscous force is strong.
Consider the difference between running a bath (high Reynolds number) and delicately putting money into a basin (low Reynolds number). The fluid dynamics change from calm (or laminar) to turbulent at some point along this honey-to-bathwater spectrum. Reynolds himself provided one of the most eloquent examples of this. He dyed the center of a clear pipe filled with flowing water in a now-classic experiment. The dye stayed as a clear visible layer throughout the pipe when the water had a low velocity and thus a smaller inertial force. However, when the water’s velocity grew, turbulence developed, causing the two liquids to mix.
When the Reynolds number is high, “the forces connected with moving the fluid about are significantly stronger than the forces responsible for dissipating the energy,” as Beattie describes, the changeover occurs. For example, if the Reynolds number is less than 2,000, the flow is considered laminar; if it is greater than 4,000, the flow is considered turbulent. “We don’t necessarily know how huge is large enough” in individual circumstances, Beattie adds. “As far as I know,” he continues, “knowing in detail the exact Re [Reynolds number] transition for many different fluid systems is still an unresolved subject in turbulence theory, so you can see that we don’t even know when turbulence occurs!”
