Tag Archives: strouhal number

TWSB: Birds of a Feather 0.24 Together

So we all know that the peregrine falcon is the fastest bird and that the cheetah is the fastest land animal, right? Well, how do scientists compare speeds across species? A beetle,  for example, doesn’t move as fast as a cheetah (though that would be terrifying/awesome), but when size is taken into account, a slower beetle may be moving its body faster in relation to the cheetah’s. In other words, how to scientists go about judging the fastest animal on the planet?

The answer, unsurprisingly, involves math. Specifically, a number called the Strouhal Number. Invented by Vincent Strouhal in the early 1900s, the Strouhal number involves multiplying the “flap rate” f (strokes/time) of an animal by the length of the flap l (distance/stroke). This quantity is then divided by the speed of travel V (distance/time). When this is all carried out, you’re left with a bunch of unit cancellations and the final Strouhal Number fl/V as a unitless measure of comparison. This unitlessness allows for comparison across species regardless of the lengths (feet, inches, etc.) and times (seconds, minutes, etc.) used.

It turns out that almost all flapping creatures have a Strouhal number between 0.2 and 0.4, with fast birds of prey registering at about 0.24. Speedy dolphins and whales are, comparatively, almost identically fast with Strouhal numbers around 0.28. Even species with different evolutionary histories tend to cluster around specific Strouhal numbers.

More info here.

Cool, eh?

30-Day Meme – Day 25: If you have tattoos, show them. If not, talk about the tattoos you want or why you don’t think they are right for you.
I don’t have any tats, but I’d totally want to get one or two. I’d love “LEIBNIZ” scrawled across my body somewhere, ‘cause I love the man and I think we’re some level of soul mates (yes, I’m aware of how creepy I am). I’d also like to get this on my back, but I’m sure that would take a LONG time and be VERY expensive. Someday, though…