The placement of ultrasonic transducers beneath an aircraft skin has been shown to prevent ice formation on leading edges during flight. This technology is more energy efficient than traditional ice prevention through manual heating of the affected areas. In order to further develop such technology, the physical mechanism through which this behaviour occurs is of interest.
Sponsor
This project is funded by Boeing Research & Technology. 
The challenge
The multi-physics modelling allowed us to understand why thermal contact was reduced
We are interested in understanding the behaviour of a supercooled water droplet as it impacts an ultrasonically excited substrate. The droplet freezes very rapidly, with a time scale of 10-100 microseconds, thus the effects of the ultrasonic excitation suggests that by this time the droplet is no longer in contact with the substrate. In order to understand how this occurs, we need to simulate the impact of an individual droplet on a metallic substrate under the influence of ultrasonic excitation.
The research
The impact of a droplet on an ultrasonically excited substrate is a complex multiphysics problem. The transducers are placed beneath the aircraft skin, thus the waves must travel through this medium, typically an elastic-plastic solid. The water droplet impacts in excess of 100 m/s, and requires a fully compressible model, incorporating the surrounding air, to account for cushioning and vaporisation effects.
The water and air/vapour are described by a two-phase model, and coupled with the elastic-plastic substrate through a sharp interface ghost fluid method. This model was validated against experimental results for water impact against metallic substrates, and against numerical results for the cavitation behaviour. We then investigated the effects of the ultrasonic excitation against an impacting water droplet.
Results
We considered three possible mechanisms for preventing the lasting contact of the droplet with the substrate; bouncing (due to surface tension effects), break-up (due to the vibration), and cavitation-induced lift-off. We found that impact speeds were too high for surface tension to have a significant effect on the behaviour. Vibration-induced break-up of the droplet was possible for sufficiently energetic vibration, but this was not a feasible mechanism for the transducers which would be used in aerospace applications.
It was a cavitation-induced lift-off of the droplet from the surface that could account for the droplet not adhering to the substrate. The vibrations passing through the substrate resulted in a large expansion wave being transmitted into the droplet. This generated a thin layer of water vapour between the droplet and the substrate, which dissipated further compression effects. As a result, we would expect ultrasonic vibration to lead to droplets being unable to make lasting contact with the aircraft substrate, and thus to either freeze away from the substrate, or be carried away due to the airflow around the aircraft. We performed a parameter study over a wide range of transducer properties, and found that we could consistently achieve lift-off of the impacting droplets.
Publications
- Validation of water impact against elastic-plastic substrates (in preparation) Millmore S.T. and Nikiforakis N. (in preparation)
- The importance of the substrate model for water droplet impact Millmore S.T. and Nikiforakis N. (in preparation)
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Millmore S.T. and Nikiforakis, N. 2019. High-speed droplet impingement on ultrasonically-excited substrates (in preparation).