Any physicists in the house? This film at SciCast shows a neat trick with dropping playing cards, where if you drop them end-on they flutter, but if you drop them side-on they parachute down and hence can be aimed rather accurately. Now, my explanation for this has long gone thus:
The playing card’s terminal velocity when dropped flat is lower than the laminar/turbulent transition speed. So the airflow around stays laminar, and the cards fall smoothly. When dropped end-on they rapidly start to flutter — like the opposite of a flag, with the air staying still and the flag moving through it. You’d still expect the flag to flutter, and that flutter is what sets the cards tumbling.
However, my colleague who’s writing up my notes isn’t convinced. Which is fair enough, because, hey, it’s 15 years since I didn’t do any fluid dynamics. She replies:
What I’ve been looking at is that low Reynolds number gives you laminar flow and high R number gives you turbulence, but the R number is also related to the scale length of the object moving through the fluid. I’d like to say that only the speed of the object matters but I’m not sure how the horizontal card can suffer from air resistance which causes it to fall slowly and for this air resistance to not have an effect on the turbulence it feels. Suddenly I regret not paying more attention in second year.
My thinking on that:
The swept volume for the horizontal card is still larger. The flow might be laminar, but there’s more air involved which implies more work done to displace it.
Anyone else like to weigh in?