New icing model finds leading edge

August 04, 2016 — Ottawa, Ontario

"Lobster tail" ice accretion

When aircraft experience a buildup of ice on exposed surfaces, this has the potential to degrade performance and increase risk to continued safe operation. To mitigate this threat, the National Research Council of Canada's (NRC) aviation icing researchers are investigating the characteristics and formation of very small (0.02-0.04 mm) supercooled (below 0 °C) water droplets that can be present in clouds. When these droplets hit the leading edge of an aircraft wing or other parts of the aircraft, they freeze upon impact, building up ice and posing safety threats.

Recently, Dr. Krzysztof Szilder of NRC's Aviation Aerodynamics group has developed original icing modelling capabilities that exceed those of the best around the world (e.g. national aerospace labs in the USA, UK, France and Italy). The traditional icing models used by these labs consider continuous fluxes of impinging droplets and continuous water flow along the ice surface. NRC's innovative approach to ice accretion modelling, called morphogenetic modelling, considers the three-dimensional behaviour of an ensemble of fluid elements, which impinge, move along the icing surface and freeze.

"This approach is capable of predicting simultaneous rime and glaze ice accretion, accretions with variable density, and complex 3D features such as "lobster tails", rime feathers, ice ridges and rivulets," said Dr. Szilder. "No other models are able to predict these important features on swept wings, the morphology of which has a large impact on airflow over the wing, and thus safety."

Now the team is preparing to validate their 3D code in wind tunnel tests. In August they will replicate specific computer simulated conditions in the NRC Altitude Icing Wind Tunnel using a swept wing at 30 degrees and 45 degrees to the oncoming flow. Using 3D scans of the ice shapes and image processing techniques, they will compare the two sets of results. If the tunnel test data validates the simulation data, it would prove the accuracy of the 3D computer modelling, giving aircraft manufacturers and safety regulators confidence in the model to accept its results when certifying aircraft. The wind tunnel validation would also allow the modelling to be used for other engineering applications such as ice accretion on wind turbines, transmission lines and bridge cables resulting from freezing rain or drizzle.

With the experience gained from these tests, the team will then extend their modelling to supercooled large droplets (SLD), which the FAA and EASA have recently included in their regulations. These droplets can be from 0.1 mm to over 1 mm in diameter and can freeze on sections of the aircraft that are not covered by existing ice protection systems. As a result, manufacturers will require access to simulations to assess the impact of SLD conditions on continuing safe operation. In order to certify aircraft to the SLD environment, computer simulation must be validated through flight testing in SLD conditions or wind tunnel tests. To meet these new regulatory demands, NRC will be upgrading its wind tunnels to ensure that original equipment manufacturers can cost effectively certify their components.

Definitive validation data obtained from the upcoming tests would put the team's modelling in the forefront of this field. From there they will be looking to license it to a Canadian company that can develop the concept into a commercial product.

The 3D icing code development and validation is a project supported by NRC's Reducing Aviation Icing Risk program.

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