15 In-flight Icing: Modeling, Prediction, and Uncertainty
457
and discussions within the Marie Skłodowska-Curie ITN UTOPIAE on Uncertainty
Treatment and Optimization in Aerospace Engineering can enlighten the study of
the ice accretion phenomenon by providing methods to identify and quantify the
sources of uncertainty and to quantify the uncertainty of the model output. This
can provide more robust predictions of ice shapes on aerodynamic surfaces and
consequently derive more efficient and optimized designs of ice protection systems.
The treatment of the uncertainty will help enhance the robustness of the obtained
results reducing the needs for expensive experimental and flight tests.
The first section of this chapter presents an overview of the ice accretion
phenomenon. It starts with an introduction of the icing conditions and environments,
then it describes special icing interactions. The identified uncertainties will be
introduced. Then, the diverse types of ice are described as well as the research
methods applied and the impacts on the performance of aircraft. The second
section explains the approaches for the physical and numerical modeling of the ice
accretion phenomenon. The following section describes the available ice protection
technologies. Next, the regulation in ice protection is described. Finally, the
concluding remarks regarding uncertainties in in-flight icing are included.
15.2 In-Flight Ice Accretion
Ice accretion consists of the accumulation of ice on man-made structures when
interacting with supercooled clouds, ice crystals, hoar, frost, and freezing precipitations such as snow, freezing rain, and freezing drizzle. Supercooled clouds are
made of droplets whose temperature is lower than the water freezing point but
still remain in the liquid phase. This is due to the lack of an external perturbation
causing nucleation. These droplets can freeze following an interaction (impact)
with structures causing the accretion of ice over the structures themselves. The
behavior of water and ice in the ice accretion process is still subject to large
uncertainties in the characterization and replication of icing conditions. Several
man-made structures designed to work outdoors at high altitudes and in alpine
environments are prone to experience ice accretion. Aircraft and helicopters fly
at high altitudes and probably through clouds. Wind turbines, power lines, and
antennas can be placed at high altitudes. The severity of ice accretion is dependent
on the size of the cloud droplets, the amount of liquid water and the time of the
exposure to icing conditions. In the case of aircraft, the encounter of large amounts
of supercooled large droplets in clouds was found to be the most critical scenario
[1]. Ice accretion causes the exposed structures to operate under different conditions
than the designed ones. Intuitively, ice increases their weight causing additional
loads. Furthermore, ice alters the geometry of the structures and this modifies the
interaction with the surrounding airflow and the aerodynamic forces experienced
by the structure, thus severely increasing drag and reducing lift. The consequences
might be unexpected and they depend on the severity of the icing encounter. In any
457
and discussions within the Marie Skłodowska-Curie ITN UTOPIAE on Uncertainty
Treatment and Optimization in Aerospace Engineering can enlighten the study of
the ice accretion phenomenon by providing methods to identify and quantify the
sources of uncertainty and to quantify the uncertainty of the model output. This
can provide more robust predictions of ice shapes on aerodynamic surfaces and
consequently derive more efficient and optimized designs of ice protection systems.
The treatment of the uncertainty will help enhance the robustness of the obtained
results reducing the needs for expensive experimental and flight tests.
The first section of this chapter presents an overview of the ice accretion
phenomenon. It starts with an introduction of the icing conditions and environments,
then it describes special icing interactions. The identified uncertainties will be
introduced. Then, the diverse types of ice are described as well as the research
methods applied and the impacts on the performance of aircraft. The second
section explains the approaches for the physical and numerical modeling of the ice
accretion phenomenon. The following section describes the available ice protection
technologies. Next, the regulation in ice protection is described. Finally, the
concluding remarks regarding uncertainties in in-flight icing are included.
15.2 In-Flight Ice Accretion
Ice accretion consists of the accumulation of ice on man-made structures when
interacting with supercooled clouds, ice crystals, hoar, frost, and freezing precipitations such as snow, freezing rain, and freezing drizzle. Supercooled clouds are
made of droplets whose temperature is lower than the water freezing point but
still remain in the liquid phase. This is due to the lack of an external perturbation
causing nucleation. These droplets can freeze following an interaction (impact)
with structures causing the accretion of ice over the structures themselves. The
behavior of water and ice in the ice accretion process is still subject to large
uncertainties in the characterization and replication of icing conditions. Several
man-made structures designed to work outdoors at high altitudes and in alpine
environments are prone to experience ice accretion. Aircraft and helicopters fly
at high altitudes and probably through clouds. Wind turbines, power lines, and
antennas can be placed at high altitudes. The severity of ice accretion is dependent
on the size of the cloud droplets, the amount of liquid water and the time of the
exposure to icing conditions. In the case of aircraft, the encounter of large amounts
of supercooled large droplets in clouds was found to be the most critical scenario
[1]. Ice accretion causes the exposed structures to operate under different conditions
than the designed ones. Intuitively, ice increases their weight causing additional
loads. Furthermore, ice alters the geometry of the structures and this modifies the
interaction with the surrounding airflow and the aerodynamic forces experienced
by the structure, thus severely increasing drag and reducing lift. The consequences
might be unexpected and they depend on the severity of the icing encounter. In any