Using simulation during the design process maintains safety and reduces testing costs.
Aircraft performance can be compromised when ice forms on aircraft surfaces such as the wings, nacelles, sensors, and control systems. This can cause safety issues due to significant changes in the aerodynamic performance of the aircraft if larger pieces of ice detach and strike other parts of the aircraft, if they enter the engine causing damage, or if sensors begin to malfunction.
Engineers therefore design anti-icing and de-icing systems to prevent icing from occurring during the flight envelope, and should icing actually occur, to minimize the impact of the ice and enable continued safe operation of the aircraft. Examples of these onboard technologies include traditional heating systems in sensitive areas, and more recently, the use of advanced hydrophobic materials and multifunctional composite structures that can be used as both structural and thermal elements.
Aircraft icing is a highly complex phenomenon and presents many challenges to the engineers who are designing the anti-icing and de-icing systems. Physical prototype testing is expensive and timeconsuming as it requires the use of special ice tunnels that cannot always replicate the more complex metrological conditions that cause the various types of ice under consideration at full scale. Other types of testing involve mock ice shapes being attached to aircraft surfaces and then having the aircraft flown and monitored.
To reduce these time-consuming and costly tests, the industry makes extensive use of physics-based simulation to address a number of aircraft icing scenarios. These include:
Design of the Anti-Icing System: By simulating the propensity with which droplets or particles in the free stream impact the aircraft surfaces, the droplet collection efficiency on different parts of the aircraft can be determined. This information is the first step in the icing analysis, and is used in the design of the spatial thermal load requirement for the anti-icing system. More complex, coupled simulations can then be performed to study the run back of the film, which is the liquid that runs along the surface of the aircraft, and its subsequent stripping off from the aircraft surface. This is a coupled multiphase analysis due to the dependencies among the thermal load from the anti-icing system, the conjugate heat transfer, the spatially varying film thickness, and the external aerodynamic conditions. Ultimately, these simulations are used to design an anti-icing system that prevents ice forming and maintains a liquid film.
Understanding Performance Impact if Ice Forms: Engineers also need to consider the situation where ice forms on the aircraft, its impact on performance, and how to design the de-icing system to remove or limit the amount of ice that forms. This type of simulation involves predicting the thickness of the ice by determining how the incoming water freezes or runs back. As the ice thickness increases, it impacts the surrounding airflow, which in turn impacts the thermodynamic balance and feedback to the ice growth. Feedback is a phenomenon where a change in one factor will affect other factors, which in turn feed back to the original factor. To capture this, a tightly coupled transient fluid-thermal analysis is required to study this as well as the shape change in the computational model. In addition to assisting with the design of the de-icing system, this information is used to understand the implications for the aircraft’s aerodynamic performance (due to changes in weight and aerodynamic surface profiles) and identify the critical physical tests that need to be performed.
Assessment of Damage if Ice Breaks Off: In the event that larger pieces of ice form and subsequently break off, engineers need to understand the effect of any potential impact on downstream surfaces or into the engine. Specialized simulation tools are used to capture high-speed, short-duration events involving high levels of deformation and more complex material models for failure. This helps engineers design the frame work and surface of the aircraft that takes into consideration the build material, shape, and design to ensure that if an ice form does break off, there will be minimal impact on the plane and that it does not enter the engine.
Ice formation and its management on aircraft is a highly complex phenomenon and simulation does not yet have all the answers. It requires the application of the right simulation tools at the right time, backed up by critical engineering judgment and skill that is validated through experimentation. However, simulation does play a critical role in the design of aircraft under icing conditions and contributes significantly to safety, cost reduction, and innovation in the industry.
This work was done by Rob Harwood and Bala Sasanapuri of ANSYS, Inc. For more information, visit http://info.hotims.com/45608-122