FYFD

As a child, I loved to ride in the car while it was raining. The raindrops on the window slid around in ways that fascinated and confused me. The idea that the raindrops ran up the window when the car moved made sense if the wind was pushing them, but why didn’t they just fly off instantly? I could not understand why they moved so slowly. I did not know it at the time, but this was my early introduction to boundary layers, the area of flow near a wall. Here, friction is a major force, causing the flow velocity to be zero at the wall and much faster -- in this case roughly equal to the car’s speed -- just a few millimeters away. This pushes different parts of large droplets unevenly. Notice how the thicker parts of the droplets move faster and more unsteadily than those right on the window. This is because the wind speed felt by the taller parts of the droplet is larger. Gravity and the water’s willingness to stick to the window surface help oppose the push of the wind, but at least with large drops at highway speeds, the wind’s force eventually wins out. (Image credit: A. Davidhazy, source; via Flow Viz)

Inside each of us is a remarkable and constant flow, driven by a muscle that’s always at work. As blood circulates through our bodies, it goes through a surprisingly varied journey. In the heart, as seen above, blood flow is very unsteady and quite turbulent, due to the beating pulse of the heart. As valves open and close and the muscle walls constrict and relax, the rushing blood moves in eddy-filled spurts. In the outer reaches of our capillaries, however, the nature of the flow is quite different. Thanks to smaller vessel sizes and other factors, capillary blood flow is much steadier and more laminar. Viscosity becomes more important, as do the non-Newtonian properties of components in our blood. (Image credit: mushin111/YouTube, source; via Science; submitted by Gary N.)