Random Explained

I still don't really get turbulence. I know it can be quantified with average velocities, but why does it happen? While we may not have all the answers, the below video narrated by Robert W. Stewart, of the University of British Columbia, does a great job of explaining this perceived randomness. Stewart smokes his pipe, and lets us know that Turbulent flow is not necessarily random as we think.

Like in my transport class, he goes over the Reynolds number and why turbulent flow is about high Reynolds numbers (much greater than 2000). He also defines turbulent flow has having disorder, vorticity, and efficient mixing. For a video that seems to have been made half a century ago, the illustrations and experimentation were very clear. As viscosity was decreased, the flow turned turbulent and the pressure gradient increased. He also explains that this increase in pressure is due to the Reynolds stress, or the additional stress in turbulent flow compared to laminar flow.

But the part that I found cool was that if a funnel was put in at the entrance, the flow was laminar for a greater Reynolds number (around 8000). The video says that the funnel makes the flow smoother and less mixing happens, making more laminar flow. Thus, once again, turbulent flow is not random, and the number 2000 is not random either; it applies to specific geometries. Further, as Stewart says, random implies some sort of Gaussian distribution, which is not the case with turbulent flow. So the idea of randomness is not accurate--its more like we don't know.

The last cool part of this video is the dye illustration part, and how basically turbulent flow makes a blob bend and thin until it has enough of a surface area increase to go through molecular diffusion very fast. This mixing would really make sense for chemical reactions that require mixing in order to go to completion. As a chemical engineering major, this makes a lot of sense to me.

The video really helped clear up my concepts about turbulence, so definitely give it a watch! While I cannot promise the lips will be synced to the dubbing in the video, Stewart does an excellent job defining turbulence as best as possible (50 years ago).