A series of Animated GIFs excerpted from Meaning of Pi (1949). The video Explains how pi denotes the ratio of a circle to its diameter, shows the use of circles in art, industry and commerce, outlines a procedure by which the numerical value of pi can be checked and reviewed, and describes the discovery and importance of pi.
A simple cylinder in a steady flow creates a beautiful wake pattern known as a von Karman vortex street. The image above shows several examples of this pattern. Flow is from bottom to top, and the Reynolds number is increasing from left to right. In the experiment, this increasing Reynolds number corresponds to increasing the flow velocity because the cylinder size, fluid, and temperature were all fixed. As the Reynolds number first increases, the cylinder begins to shed vortices. The vortices alternate the side of the cylinder from which they are shed as well as alternating in their sense of rotation (clockwise or counterclockwise). Further increasing the Reynolds number increases the complexity of the wake, with more and more vortices being shed. The vortex street is a beautiful example of how fluid behavior is similar across a range of scales from the laboratory to our planet’s atmosphere. (Image credit: Z. Trávníček et. al)