"During the 1960s, it was shown that coherent parts of turbulent fluctuations inside jets are connected to instability wavepackets, which are linked to noise radiation," explained Joseph Nichols, assistant professor of aerospace engineering and mechanics at the University of Minnesota. "A wavepacket is an oscillation that repeats if you look at it over short time and length scales, but when you zoom out you can see it varies slowly over a long distance."
This slow variation may be caused by the jet spreading out downstream. Prof Nichols says the same basic principle applies to AM radio, where modulation of a high-frequency carrier wave communicates information about low-frequency speech patterns. "In jet noise, this carrier wave is driven by fluid instabilities that feed on the energy contained inside the jet. Instead of encoding speech patterns, however, the amplitude modulation of instability wavepackets determines the efficiency and spatial direction at which sound is released from jet turbulence," he added.
In contrast to previous research, Prof Nichols and his team looked at turbulent jets through the lens of the whole system, rather than individual components. "We treat high-speed turbulent jets as amplifiers that take turbulent fluctuations inside the jet as inputs and give back sound in the region far away from the jet as outputs," said Mihailo Jovanovi, an associate professor of electrical and computer engineering at the University of Minnesota. "We use this mathematical framework to identify modes that induce large input-output amplification and generate loud noise."
The interdisciplinary research team confirmed the existence of these new modes using high-fidelity simulations of high-speed jets. The researchers numerically solved the compressible Navier-Stokes equations by subdividing a high-speed jet flow into hundreds of millions of small pieces.
Their computations also enable an understanding of physics phenomena that are inaccessible experimentally as they happen in an extreme environment that can't be outfitted with sensors.
The team are looking to apply these simulation and analysis techniques to a broader range of problems, such as understanding instabilities in the wakes of wind turbines, investigating acoustic-flame interactions for the design of safer and more efficient combustion, and explaining shock-induced transition in hypersonic boundary layers.
