In January 1800 Thomas Young wrote to the secretary of the Royal Society, Edward Whitaker Gray, to outline his recent "experiments and inquiries respecting sound and light," as he titled the letter.
Among the findings that Young reported was the generic deflection of fluids near a boundary. Just as a stream of air blowing over water will raise a dimple, he wrote, a candle’s flame will be drawn toward a stream of air. In both cases, and in others, proximity to a surface reduces the local pressure, leading to a net force on the stream.
The effect is not named after Young, however. That honor was bestowed by the aeronautical engineer Theodore von Kármán on his fellow engineer and near contemporary, Henri Coandă. Born in 1886 in Bucharest, Romania, Coandă started off as an artillery officer in the Romanian army. But after six years of military service, during which he was able to pursue his passion for aeronautics, he left Romania for Paris in 1909 to enroll at the École Nationale Supérieure d’Ingenieurs de Construction Aéronautique.
Coandă’s first job after graduation was with Gianni Caproni’s aircraft factory in Milan, Italy. There, he designed and built an aircraft, the Coandă-1910, that made use of his namesake effect. Depicted on the two stamps above, the Coandă-1910 resembled other aircraft of the time, but with a crucial difference: Instead of spinning a propeller, its four-cylinder piston engine powered a fan that expelled air back along the fuselage. Whether the Coandă-1910 ever flew or was even capable of flight has not been established.
Coandă’s effect has had a more successful life. Its natural manifestations include the diversion of air streams over mountainous terrain and the squirting of blood from the heart’s left ventricle into the left atrium during a disorder known as mitral regurgitation. Among the effect’s practical applications are boundary-layer control systems, which blow engine exhaust over wings to boost lift at low air speeds, and HVAC diffusers, which extend the reach of cooled or heated air.
My favorite application of the Coandă effect is the Avro Canada VZ-9 Avrocar, a flying car developed in the 1950s for the US Air Force and US Army. The VZ-9 looked more like a flying saucer than a car with wings. A central, upward-facing intake drew air into a centrifugal compressor, which directed the flow into three jet engines that were arranged symmetrically around the outside of the compressor like the fireworks in a Catherine wheel. Ducts directed the exhaust down (for lift) and out (for control).
As you can tell from the video, the VZ-9 could actually fly, albeit in somewhat wobbly way. It flew at a maximum speed of 119 mph, but never lifted more than a few feet off the ground. The pitching evident in the video was never eliminated. The project was canceled in December 1961.
By contrast, the Coandă effect remains in good health. Using Google Scholar, I came across several recent papers that evoked it, including
- "Aerodynamic design optimization of rear body shapes of a sedan for drag reduction."
- "PIV measurements and analysis of transitional flow in a reduced-scale model: Ventilation by a free plane jet with Coandă effect."
- "On some recent applications of the Coandă effect to acoustics."
- "Application of Coandă effect in robots—A review."
I also found a paper posted last year to the arXiv e-print server by Teresa López-Arias of the University of Trento in Italy. Having discovered Young’s letter to Gray, she posed the question whether the Coandă effect should really be called the Young effect.
