Maybe you’ve been on an airplane before, and the pilot comes on the intercom and says something to the effect of “Put on your seatbelts, I’ve lost complete control of this airplane. For the next few minutes it might feel like we’re literally just falling through the sky waiting to crash into the wheat fields of Oklahoma.”
He might also throw around the word “turbulence” and you may begin to associate this word exclusively with airplanes, or with the atmosphere. The truth is, turbulence is a very general term used in fluid dynamics. It essentially means any flow that is not laminar – flowing along constant streamlines. Turbulence is therefore chaotic fluid motion that is not predictable based on the boundary conditions associated with the environment. The definition of laminar and turbulent is fluid (ha! Get it?) based on the discipline or context of the flow, and the distinction between laminar and turbulent flow can be based on several different parameters. In many sciences, the cutoff for a flow being considered turbulent is decided using the Reynolds number, a non-dimensional parameter that determines whether viscous or inertial forces are dominant. In other words, if you were swimming through maple syrup, the flow around your arms and legs would be – in general – more laminar than a similar swim through a pool of blood (I didn’t mean for that to get morbid so quickly?). This is because the Reynolds number, and therefore the relative turbulence of these fluid flows depends on the viscosity of the fluid. Syrup is more viscous or “thicker” than blood, resulting in a smaller Reynolds number, and a less turbulent flow at the same velocity.
Turbulence is interesting to study for fluid dynamicists for many reasons. The first is that inherently interesting word up there: chaotic. Just like the old “Chaos Theory” turbulence can be incredibly complicated and unpredictable in fluid flows. Even if a flow starts out as laminar, an imperfection or disturbance of any kind (even through things you might not expect!) can cause disruptions in the flow that leads to widespread turbulence. See figure to the side depicting the eddy formation in the transition from laminar to turbulent flow. It’s that whole ‘butterfly flapping its wings causes a hurricane in the opposite hemisphere’ except it’s REAL.
Secondly, turbulence can be a valuable tool in the measurement of fluid flow. Acoustic instruments like SONAR, SODAR, or ADCP (or even their cousin, the LIDAR) depend on turbulence in the flow. The sound or light waves reflect off of this turbulence in order to return to the receiver which then records various properties about the flow based on how the waves were changed. If this is unclear, you can imagine that it works in much the same way that a bat’s echolocation works. The only difference is that the sound would not be bouncing off of a solid object, but rather it’s bouncing off of turbulent structures in the flow. The sensors in instruments like SODAR are calibrated to ignore large reflections that would be a result of birds, fish, or planes. The reflections that are recorded would be indicative of changes in the flow’s speed or direction.
Scientists may be endearingly fascinated by turbulence, but as an airplane passenger, I kind of hate it, and you should too. When you’re in a plane and experiencing turbulence, what’s really happening is that the air is not flowing laminarly under the plane’s wings like it ideally would be. The plane is playing with the idea of “stalling,” which can be due to a variety of factors, but essentially means that there’s not quite enough lift being generated on the underside of the wings. The solution? You want to get back that clean laminar flow over the wing, which is accomplished by pointing the plane downward, and speeding up until you can begin to climb again. Now you know how to fly a plane, so everyone can henceforth look to you to pilot the aircraft when the real pilot has been drowned in a sea of tribbles.*
*This is a joke. Please don’t ever fly a plane I am in.