ANSYS FENSAP-ICE Capabilities
FENSAP-ICE can calculate 3-D glaze, rime or mixed-type ice shapes on any aircraft surface, for any icing condition. It has a built-in graphical interface to facilitate selection of icing conditions from Appendix C, D, and O. The ice shapes on wings, stabilizers, control surfaces, air data probes, rotors and propellers, turbofan blades and passages, radomes, cameras, etc. can be used to assess performance degradation. Icing on intake screens can be used to calculate blockage effects. Various roughness models are provided, including an analytical model that eliminates dependence on empirical correlations. Large glaze horns can be simulated in 2-D and 3-D with Multishot. The Extended Icing Data option enables simulation of “beak” ice shapes that grow when the stagnation temperature is above freezing.
Aerodynamic Performance Penalty Analysis
Ice build-up on wings, stabilizers and rotors is detrimental to the aerodynamic performance of these components. It reduces lift, increases stall speed, reduces stall angle of attack, significantly increases drag which could be difficult to overcome for small aircraft, causes strong vibrations on helicopter rotors, increases the blockage between turbomachine compressor rotor blades, etc. Ice roughness alone due to mere millimeters of ice is enough to cause noticeable changes in aerodynamic performance. Severe ice build-up on the suction side of wings can lead to loss in aileron control, while ice on horizontal stabilizers can cause tail stall. Accidents do happen due to in-flight icing and unfortunately some of them have been fatal. FENSAP-ICE can calculate airflow on iced aircraft with a morphed and resurfaced grid, accounting for analytically calculated ice roughness distribution as well. Results can then be used to assess the losses in performance of iced components.
Ice Crystal and Supercooled Large Droplets
The FAA expanded certification requirements in 2014 by introducing Appendix O to characterize two main supercooled large droplet (SLD) environments: Freezing drizzle and freezing rain. SLDs have the potential to bypass ice protection systems designed to comply with Appendix C, and are therefore very dangerous. FENSAP-ICE provides comprehensive models and a user-friendly environment to support SLDs.
A similar graphical environment is provided for Appendix D, also introduced in 2014 by the FAA to ensure protection against hard-to-detect ice crystals occurring at high altitudes over strong convective cells. FENSAP-ICE allows the user to define total water content and liquid water content, provides all the models to simulate the complex physics of crystal ice accretion and covers the entire ice crystal environment.
The FENSAP-ICE Turbo capability can accurately predict the very complex, 3-D icing environment in multistage compressors. Many turbofan engine incidents characterized by flameout or rollback events in the presence of ice crystals at high altitudes have been reported by flight safety agencies. Recent studies have indicated that ice buildup can occur in the low-pressure compressor in mixed-phase environments, containing little or no droplets, but a large concentration of ice crystals. Due to their complex kinematic properties, ice crystals can penetrate the compressor core, where they begin to melt due to the increasing temperature along the gas path, eventually sticking to the surfaces and causing ice build-up. The accreting ice causes airflow distortion and blockage, resulting in compressor rollback and even surge.
Ice Protection System Analysis
The performance of bleed-air or electro-thermal ice protection systems (IPS) can be studied with FENSAP-ICE through the conjugate heat transfer (CHT) between air, water, ice and the solid materials that compose the IPS. The airflow domains are separated from the metal or composite skin of the aircraft component;, therefore, all the ANSYS airflow solvers can be used. For bleed-air systems, a steady-state thermal equilibrium between domains is computed to verify that the protected region is free of ice. For electro-thermal systems, the IPS response time to the cyclic activation of heater pads is analyzed in a time-dependent CHT simulation encompassing phase change, heat conduction and water runback to accurately predict the amount of ice that forms, melts and refreezes. A wide variety of IPS configurations can be analyzed.
Mesh Optimization with OptiGrid
One of the major difficulties of CFD is that computational meshes must be generated before the location and details of the solution can be known, an empirical process that requires user experience but invariably leads to less-than-ideal results.
OptiGrid removes this difficulty by estimating and then equi-distributing the truncation error of the solution variables with four operations: coarsening, refinement, swapping and node movement. The first three operations accelerate the optimization, but only node movement ensures that the node locations, edge lengths, directions and aspect-ratio of the cells are truly optimized and meet the desired requirements.
The benefits are superior accuracy on the most computationally-efficient meshes possible, and a mesh independence study every time cycles of solution and mesh optimization are performed.