Technion researchers reveal mini-tornados in fluid jets in world first
The phenomenon in practice can be seen in the way a jet of water from a faucet bends when run across the surface of a spoon.
By DONNA RACHEL EDMUNDSCoandă jet (blue) flowing over a curved surface.(photo credit: COURTESY TECHNION)
Researchers at Technion have, for the first time ever, found a way to observe instabilities in jets of fluid which had long been hypothesized but never seen. The finding has practical applications anywhere a fluid moves across a curved surface, and could lead to improvements in ventilator technology, among other things.Almost everyone has seen the Coandă effect, described by Romanian inventor Henri Coandă as "the tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to entrain fluid from the surroundings so that a region of lower pressure develops." What this looks like in practice can be seen in the way a jet of water from a faucet bends when run across the surface of a spoon.Coandă exploited the effect in aerodynamics as it governs the way air flows over airplane wings, but less well understood is a phenomenon first proved theoretically in 1870: that as the liquid flows it can be centrifugally unstable - that is, mini tornados form in the stream, lose their shape, become wavy, and ultimately break down into incoherent turbulence.Front view – Mini-tornadoes (red steaks) embedded in the Coandă jet. (Courtesy Technion)Now Technion PhD student Lev Dunaevich, under the supervision of Professor David Greenblatt of the faculty of Mechanical Engineering has shown how this takes place, by finding a way to observe the process.Dunaevich and Prof. Greenblatt propelled an effectively two-dimensional jet-stream over a circular cylinder to visualize the stationary tornadoes, and ultimately show their breakdown. In doing so, they determined the critical conditions that create the tornadoes, found to correspond remarkably closely to theoretical models, and have therefore allowed the phenomenon to be further studied to determine real-world applications.“The Coandă effect has long been suspected as the reason for unequal ventilation of the lungs in intubated patients and, with the prevalence of COVID-19, the discovery of this instability can play a decisive role in the design of more effective ventilators,” Prof. Greenblatt said.The research, published in Physics of Fluids, could also have implications for blood flow, the design of micro-fluid mixers, micro air vehicles and electronics cooling systems.Funding recently secured from the Israel Science Foundation has allowed Dunaevich and Prof. Greenblatt to investigate ways to manipulate the tornadoes with the aim of controlling the Coandă effect to benefit medical and industrial technologies.
