February 8, 2012 - Happy Birthday, Daniel Bernoulli



How did studying the speed of blood flow and blood pressure help us design airplanes?

Dutch-Swiss mathematician and scientist Daniel Bernoulli (born on this date in 1700) observed and tested the flow of fluids, that is, liquids and gases. He tried to understand how the speed of blood flow in veins and arteries related to the pressure it exerts. To find out, he punched holes in pipes and inserted vertical straws, and then he measured the height to which the fluid rose up the straw. Here's what he found:


The quicker the motion of fluid in the pipe, the less the pressure exerted, and the lower the level of fluid in the vertical straw. When fluid moves at lower speeds, it exerts more pressure and pushes up into the straw to a higher level. Bernoulli was able to devise mathematical equations to match his experimental findings.

Here is an animation you can manipulate to see the difference in speed-of-motion (velocity) and pressure, depending on the shape of a pipe. Be sure to “grab” and “drag” the little yellow boxes to change the shape—then check out the graphs below that show the relation of the speed to the pressure. Make really exaggerated shapes with a really narrow mid-portion, for example, and one very wide end, to see the results most clearly. 

Apparently, for more than a century, based on Bernoulli's findings, doctors measured blood pressure by sticking point-ended glass tubes directly into people's arteries. Eeek! I feel glad that a less invasive and painfully method was eventually invented!

Later on, airplane wings were designed to take advantage of Bernoulli's principle—with the air moving more quickly over the top of the wing, creating less pressure—and therefore providing lift as the greater pressure under the wing pushes the wing up.

Bernoulli's fluid mechanics have been used in many other ways, of course; also, Bernoulli made other contributions to mathematics, statistics, and science. So hooray for Bernoulli!


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