You are probably most familiar with turbulence when you are flying in an airplane because turbulence in the air plays a bigger role in passenger comfort and there is nothing the pilot can really do about it. It’s pretty much a function of the environment and its winds and not the pilot’s skill. But, the shape and design of the aircraft also plays a role in this. Cars are the same, where aerodynamics play a role and the shape and design of the car affects things from gas mileage to the sound the wind makes when the car is traveling at 60 miles per hour.

Cascadilla Gorge, in Ithaca

Let’s take a quick look at what a turbulent flow actually is. It is typically described as the state of of the fluid of being chaotic or random. Picture a waterfall. While we know the general direction of the water, we cannot predict how individual streams will travel. As the water falls, it mixes with air and through the magic of physics (by way of air drag and gravity), we get a random flow of water.

On the other hand, if the fluid flow is smooth, it is described as laminar. Imagine someone pouring a nice hot cup of coffee in the morning. It’s smooth, steady, predictable, and really calming to look at (generally).

Fluid flows can be described as laminar, turbulent, or something in between. So far, though, we have described flows as turbulent, or laminar in very qualitative terms. Because we’re engineers, we’ll need to introduce a little bit of numbers and math, and we use the Reynold’s number to describe the state of the flow. It is calculated by the following formula, where a low Reynold’s number is related to laminar flow and a high Reynold’s number is related to turbulent flow. It’s all just relative and there isn’t an absolute boundary when a flow switches to the other. Usually, though, it is convention to taken Re = 2100 to 2300 to be the condition when the flow transitions into turbulence.

Reynold's Number

Let’s think about this a little bit. (I’d recommend using your bathroom sink and and bathtub as visual aids.) As we increase the velocity, Reynold’s number increases and we get a more turbulent flow. If we increase the diameter of the pipe (say, bathtub), at the same velocity, we also get more turbulent flow. Finally, a more viscous fluid, using the classic example of molasses, the Reynold’s number is smaller because of the increased viscosity, and we get a slow, steady, and laminar flow. This makes sense!

One final note: while laminar flows are nice to predict and more pleasing to look at, it’s terrible when we want to design things to flow through water or air, for example. The laminar state is a direct consequence of friction, and something called the boundary layer. Let’s consider this. The flow is laminar because the “flow particles” stick well with each other and the surface it shears with (like inside a pipe or the top of your car). A highly laminar flow is able to stick to the body, and the boundary layer can grow for as long as the laminar conditions are met. This is bad because once this relatively huge boundary layer transitions into turbulence (because we know it will eventually), we’re faced with significant wake turbulence. To counter this effect, we just “trip” the flow into turbulence at the very beginning.

Check out this little bit about golf ball dimples.

And then see if you can follow what this article is saying about applying the same golf ball idea to cars (via Autoblog.com).

Categories: In-Depth Articles
Tags: Flow, Laminar, Turblent
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