Project summary: Droplet impact on water layers
An interest in high-speed droplet impact onto a layer of water has been sparked by its important role in the growth of ice on aircraft during flight.

Aircraft icing occurs when a fixed-wing or rotorcraft flies through cloud at temperatures below freezing. Supercooled water droplets suspended in the cloud hit the body of the aircraft, most crucially on wings, nose and tail of an aeroplane and on the rotor blades of a helicopter. These droplets then freeze. There are many important parameters including the air temperature, the geometry of the aircraft and the water content of the cloud. In the worst cases significant amounts of ice can build up, leading to a severely detrimental effect on the performance of the aircraft.

Given the importance of icing to aircraft safety, being able to understand and model the icing phenomenon is very desirable. Current models capture the process fairly well for small droplet sizes (up to 40µm) but tend to greatly overpredict the size of ice growth and wrongly predict its location as the droplet size increases.

This is attributed, at least in part, to splashing. When the larger droplets impact upon the water layer they splash, ejecting droplets back out into the air flow either to escape completely or to re-impinge in a different location. The aim of this project was to investigate the impact of a single droplet onto an undisturbed water layer to try and gain some understanding of how useful splashing models could be developed to improve future icing models.

The project undertook both analysis and numerical simulation of these impacts, to discover what affects the amount and form of the splash produced. This includes the effects of surface roughness and local air flow, which are both important in practice, and also analysis of the temperature of the splash, which is important for estimating the power requirements of ice-protection systems.

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