An experimental and theoretical investigation into ice ribbing instabilities

Ice formations on aircraft wings or wind turbines both reduce performance and present significant hazard. In typical UK conditions, where temperatures rarely go much below freezing, ice usually forms as hard, smooth glaze ice with water present in sheets sheared by the airflow over the wing or blade. On a wind turbine the freezing of the radial water flow can form shards some metres long at the tips, which represent lethal projectiles when dislodged. The change in geometric presentation due to the glaze ice can also cause aerodynamic instabilities. On an aerofoil, the sheared water film can itself be unstable and freeze as ribs that change the surface roughness and performance. However, the mechanics of icing in these transitional regimes is not well studied, and without this understanding it remains challenging to develop effective anti- and de-icing strategies for structures to safeguard performance and safety.

This project seeks to develop that understanding of the complex interplay between the fluid mechanic and thermodynamic processes in freezing water sheets with geometries relevant to wind turbine blades and aerofoils. Through this understanding we can identify interventions that could delay the formation of ice ribs, or remove the possibility of turbine tip formations through e.g. smart coatings.

The project will first explore the stability of water films accelerated on a spinning disk, mimicking the sheared boundary layer over an aerofoil or wind turbine blade, using linear stability analysis of the governing equations and verified though laboratory experiments. In the second phase, freezing through a glaze ice boundary condition will be incorporated into the stability analysis to gain insight into the origin of frozen ribs, and their growth into three-dimensional ice formations. The laboratory experiment will be developed alongside this theory, to verify the predictions under tightly controlled fluid mechanic and thermodynamic boundary conditions.