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In-flight icing is a hazard that continues to afflict the aviation industry despite all the research and efforts to mitigate the risks. The recurrence of these types of accidents has given renewed impetus to the development of advanced analytical predictive tools to study both the accretion of ice on aircraft components in flight, and the aerodynamic consequences of such ice accumulations. In this work, an in-depth analysis of the occurrence of in-flight icing accidents and incidents was conducted to identify high-risk flight conditions. To investigate these conditions more thoroughly, a computational fluid dynamics model of a representative airfoil was developed to recreate accidents that occurred in these flight conditions. The ice accumulations and resulting aerodynamic performance degradations of the airfoil were computed for a range of pitch angles and flight speeds. These simulations revealed substantial performance losses such as reduced maximum lift, and decreased stall angle.

Numerically predicted roughness distributions obtained in in-flight icing simulations with a beading model are used in a quasi-steady manner to compute ice shapes. This approach, called "Multishot," uses a number of steady flow and droplet solutions for computing short intervals (shots) of the total ice accretion time. The iced geometry, the grid, and the surface roughness distribution are updated after each shot, producing a better match with the unsteady ice accretion phenomena. Comparisons to multishot results with uniform roughness show that the evolution of the surface roughness distribution has a strong influence on the final ice shape. The ice horns that form are longer and thinner compared to constant roughness results. The constant roughness approach usually fails to capture the formation of the pressure side horns and under-predicts the thickness of the ice in this region.

The irregular shapes that glaze ice may grow into while accreting over the surface of an aircraft represent a major difficulty in the numerical simulation of long periods of in-flight icing. In the framework of Arbitrary Lagrangian-Eulerian (ALE) formulations, a mesh movement scheme is presented, in which frame and elasticity analogies are loosely coupled. The resulting deformed mesh preserves the quality of elements, especially in the near-wall region, where accurate prediction of heat flux and shear stresses are required. The proposed scheme handles mesh movement in a computationally efficient manner by localizing the mesh deformation. Numerical results of ice shapes and the corresponding aerodynamic coefficients are compared with the experimental results.

The high computational cost of 3-D viscous turbulent aero-icing simulations is one of the main limitations to address in order to more extensively use computational fluid dynamics to explore the wide variety of icing conditions to be tested before achieving aircraft airworthiness. In an attempt to overcome the computational burden of these simulations, a Reduced Order Modeling (ROM) approach, based on Proper Orthogonal Decomposition (POD) and Kriging interpolation techniques, is applied to the computation of the impingement pattern of supercooled large droplets (SLD) on aircraft. Relying on a suitable database of high fidelity full-order simulations, the ROM approach provides a lower-order approximation of the system in terms of a linear combination of appropriate functions. The accuracy of the resulting surrogate solution is successfully compared to experimental and CFD results for sample 2-D problems and then extended to a typical 3-D case.

CFD-Icing (CFD-I) is a powerful companion to CFD-Aero (CFD-A) in the design and certification of new aircraft, rotorcraft and jet engines. It can drastically reduce the number of tunnel and flight tests, and their associated costs, by simulating on computers the full Appendix C and beyond such as is proposed in new Appendices D and O. It can also predict performance and moment coefficients in roll, pitch and yaw. These predictions can then be used in original certification or supplemental certifications to the type design, allowing mitigating potential hazards of flight-testing. This work presents an example of the application of FENSAP-ICE to predict 45 minutes of ice accretion on a RC-26B aircraft fuselage retrofitted by the addition of a FLIR sensor and a SATCOM antenna. The predicted aerodynamic penalties are compared with recorded flight test data obtained with simulated ice shapes.

A fully CFD-based methodology for ice particle tracking based on a Monte Carlo statistical approach and a six-degrees-of-freedom particle-tracking module has been developed within the FENSAP-ICE in-flight icing system. A one-way aerodynamic coupling between the airflow and the ice particle has been adopted, such that the flowfield determines the forces and moments on the particle at each location on its track, but the particle, being much smaller, has no aerodynamic effect on the aircraft's flowfield. A complete envelope of force and moment coefficients has been computed for a representative ice shape, in order to generate a permanent database. At each time step during the integration of the particle track, the angles of the local flow velocity vector with the principal axes of the particle are determined and used to interpolate the corresponding force and moment coefficients from the particle's database. These 6-DOFs are then used to compute the next particle location.

The assessment of an unsteady approach for the simulation of in-flight electro-thermal de-icing using a Conjugate Heat Transfer (CHT) technique is presented for a NACA0012 wing and a swept wing. This approach is implemented in the FENSAP-ICE in-flight icing system, and provides simulation capabilities for the heat transfer and ice accretion phenomena occurring during in-flight de-icing with power cycling through several heater pads. At each time step, a thermodynamic balance is established between the water film, the ice layer and the solid domains. The ice shape is then modified according to ice accretion and melting rates. Numerical results show the complex interactions between the water film, the ice layer and the heating system. The NACA0012 validation test case compares well against one of the very few experimental de-icing test cases available in the open literature.

The applicability of FENSAP-ICE-Unsteady to solve ice accretion on rotating helicopter blades is investigated using a two-bladed rotor and a generic cylinder, to represent a fuselage, for a forward flight test case. The unsteady rime ice accretion is simulated by coupling, at each time step, flow and water drop equations to the Messinger icing model. Mesh displacement effects are taken into account by an Arbitrary Lagrangian-Eulerian method. This new icing model is applied to rotor/fuselage flows by considering two grid domains: the first being fixed around the fuselage, and the second rotating with the blades. The gap region is stitched with tetrahedral elements to fully guarantee flow conservation.

Ice accretion is a purely unsteady phenomenon that is presently approximated by most icing codes using quasi-steady modeling. The accuracy of ice prediction is thus directly related to the prescribed time step, or the time span during which the impact of ice growth on both flow and droplets can be neglected. Such approximation is removed by FENSAP-ICE-Unsteady which fully couples in time a diphasic flow (interacting air and droplet particles) with ice accretion. The two-phase flow is solved using the Navier-Stokes and Eulerian droplet equations, while the water film characteristics and ice shape are obtained from the conservation of mass and energy within a thin film layer. The iced surface being constantly displaced in time, Arbitrary Lagrangian-Eulerian terms are added to the governing equations to account for mesh movement. For rime ice, numerical results show that full unsteady modeling improves the accuracy of ice prediction when compared to one-shot ice accretion.

In order to ensure flight safety in icing conditions and meet FAA or other national aircraft certification regulations, which require an aircraft to be able to safely operate throughout the icing envelope of Part 25 Appendix C, ice protection mechanisms have to be employed on critical locations of an aircraft. Among different anti-icing mechanisms, hot bleed air systems are the most reliable and efficient ones, and are widely used on commercial aircraft to protect critical surfaces such as leading edge wing panels and high lift devices, empennage surfaces and engine nacelle lip. Due to the complexity of anti-icing experiments and flight tests, advanced numerical simulation of complex thermal anti-icing systems has been highly anticipated as a supplementary design and certification tool. CHT3D [1], the new 3-D Conjugate Heat transfer module of FENSAP-ICE [2] for the simulation of hot air and electrothermal ice protection, will be presented in this paper.

A numerical approach to the trajectory simulation of break-up ice in the in-flight icing code FENSAP-ICE is investigated. At each time step, the displacement and rotation of the moving domains is computed from a 6-DOFs analysis of the forces and moments. The moving domains are amalgamated into the fixed background mesh by a hole-cutting and stitching algorithm, producing a continuous unstructured hybrid mesh, eliminating interpolation between domains and ensuring that the fluxes are fully conserved across the entire mesh. To maintain good load balancing on parallel computers, the finite element CFD flow solver uses an efficient parallel iterative matrix solver and domain decomposition to partition the computational domain into equal-sized subdomains. Test cases used to validate and demonstrate the features of the computational algorithm are shown.

Numerical in-flight icing simulations have become a powerful tool in the type-certification process of commercial aircraft, helping to focus and reduce the number of flights in natural icing conditions and icing wind tunnel tests. Despite the numerical software's sophistication, applicants should always validate it for various icing conditions, and show good predictability to the aviation authorities. Icing being a key certification issue for the design of such aircraft because of its direct link to flight safety, MHI (Mitsubishi Heavy Industries) has early on adopted FENSAP-ICE and integrated this 3-D icing software into its Regional Jet program. The objective of this paper is to validate, on geometries of significance to MHI, this numerical tool for accuracy in predicting the flow field, droplet impingement and ice shapes. This step is required prior to integrating the software in the actual design process.

The FENSAP-ICE system was first conceived for fixed-wing aircraft and air induction system icing, but major developments are underway to augment its capabilities for icing simulation of rotorcraft and tiltrotor aircraft. A project is underway, under the auspices of the RITA (Rotorcraft Industry Technology Association) to reduce icing certification flight tests through use of second-generation three-dimensional (3D) Computational Fluid Dynamics (CFD) based technologies. The plan is to supplement traditional icing tunnel and flight-testing with modern 3D inflight icing simulation tools that facilitate the development and certification of all-weather operations rotorcraft and tilt-rotor aircraft. A viscous fully 3D ice accretion and runback modeling capability is being developed and initial correlation studies to both 2D and 3D icing test cases have produced very good results.

FENSAP-ICE is a second generation CFD-based in-flight icing simulation system, bringing to the icing field simulation advances widely used by the aircraft and turbo machinery industries. It is built in a modular and interlinked fashion to successively solve each of flow, impingement, accretion, heat loads and performance degradation via field models based on the Euler/Navier-Stokes equations for the clean and degraded flow, and new partial differential equations for the other three icing processes. This paper presents the FENSAP-ICE system and shows examples of its use to calculate impingement and ice shapes on a 3D helicopter rotor blade tip and on a nacelle inlet.