In the transonic speed regime the overall speed of an airplane is less than Mach 1 but some parts of the flow around the aircraft break the speed of sound. The photo above shows a schlieren photograph of flow over an airfoil at transonic speeds. The nearly vertical lines are shock waves on the upper and lower surfaces of the airfoil. Although the freestream speed in the tunnel is less than Mach 1 upstream of the airfoil, air accelerates over the curved surface of airfoil and locally exceeds the speed of sound. When that supersonic flow cannot be sustained, a shock wave occurs; flow to the right of the shock wave is once again subsonic. It’s also worth noting the bright white turbulent flow along the upper surface of the airfoil after the shock. This is the boundary layer, which can often separate from the wing in transonic flows, causing a marked increase in drag and decrease in lift. Most commercial airliners operate at transonic Mach numbers and their geometry is specifically designed to mitigate some of the challenges of this speed regime. (Image credit: NASA; via D. Baals and W. Corliss)
On today’s date, September 18th, Leon Foucault was born. Well-noted for his demonstration of the Foucault Pendulum, which was conceived as a simple experiment to show the rotation of the Earth, Foucault also devised an early measurement of the speed of light, discovered eddy currents, and is credited with naming the gyroscope.
"The image is assembled from several hundred 30-second exposures made while attempting to record Perseid meteors in August 2006. The unusual-looking building in the foreground is the famous Porter Turret telescope at Stellafane. Moonlight illuminated the foreground during the exposures" - Dennis di Cicco