In-flight Icing Research Articles

UAV icing: the influence of airspeed and chord length on performance degradation

PurposeThe main purpose of this paper is to investigate the effects of icing on unmanned aerial vehicles (UAVs) at low Reynolds numbers and to highlight the differences to icing on manned aircraft at high Reynolds numbers. This paper follows existing research on low Reynolds number effects on ice accretion. This study extends the focus to how variations of airspeed and chord length affect the ice accretions, and aerodynamic performance degradation is investigated.Design/methodology/approachA parametric study with independent variations of airspeed and chord lengths was conducted on a typical UAV airfoil (RG-15) using icing computational fluid dynamic methods. FENSAP-ICE was used to simulate ice shapes and aerodynamic performance penalties. Validation was performed with two experimental ice shapes obtained from a low-speed icing wind tunnel. Three meteorological conditions were chosen to represent the icing typologies of rime, glaze and mixed ice. A parameter study with different chord lengths and airspeeds was then conducted for rime, glaze and mixed icing conditions.FindingsThe simulation results showed that the effect of airspeed variation depended on the ice accretion regime. For rime, it led to a minor increase in ice accretion. For mixed and glaze, the impact on ice geometry and penalties was substantially larger. The variation of chord length had a substantial impact on relative ice thicknesses, ice area, ice limits and performance degradation, independent from the icing regime.Research limitations/implicationsThe implications of this manuscript are relevant for highlighting the differences between icing on manned and unmanned aircraft. Unmanned aircraft are typically smaller and fly slower than manned aircraft. Although previous research has documented the influence of this on the ice accretions, this paper investigates the effect on aerodynamic performance degradation. The findings in this work show that UAVs are more sensitive to icing conditions compared to larger and faster manned aircraft. By consequence, icing conditions are more severe for UAVs.Practical implicationsAtmospheric in-flight icing is a severe risk for fixed-wing UAVs and significantly limits their operational envelope. As UAVs are typically smaller and operate at lower airspeeds compared to manned aircraft, it is important to understand how the differences in airspeed and size affect ice accretion and aerodynamic performance penalties.Originality/valueEarlier work has described the effect of Reynolds number variations on the ice accretion characteristics for UAVs. This work is expanding on those findings by investigating the effect of airspeed and chord length on ice accretion shapes separately. In addition, this study also investigates how these parameters affect aerodynamic performance penalties (lift, drag and stall).

Finite-volume Eulerian solver for simulation of particle-laden flows for icing applications

Particle-laden flows can be encountered in a plethora of engineering applications, e.g., sediment transport, fluidized beds, vehicle soiling, and aircraft icing. The latter is caused by supercooled water droplets or ice crystals impacting and adhering to an aerodynamic surface during flight. Simulating such flows requires proper application of physical models through the use of particle flow solvers, which can rely on Lagrangian or Eulerian formulations for the solution of particle dynamics. Although Lagrangian formulations present certain advantages in terms of time resources and simplicity of implementation, Eulerian solvers can easily provide complete details of the particle flow field, such as volume fraction distribution. Several Eulerian solvers exist for the particle flow, although they rely on systems of coupled transport equations, which can be further simplified in order to reduce modeling and computational effort. Therefore, the current study presents an elaborated 2D finite-volume Eulerian solver for the resolution of the dynamics of particle-laden flows through the use of a system of decoupled transport equations. Additionally, we introduced a modification into the traditional decoupled formulation in order to allow the implementation of 2nd order upwind schemes for the convective terms. Two cases are used as test benchmarks to show that the solver effectively achieves mesh independence, convergence stability, and 2nd order accuracy. Also, the solver physical modeling capabilities are evaluated by comparing results against the ones obtained through the Lagrangian formulation, the Ansys FLUENT Eulerian solver and a set of experimental results. Therefore, the elaborated Eulerian solver can be used to facilitate the modeling of in-flight icing phenomena while still retaining the accuracy of traditional formulations.

Helicopter Rotor Ice Shedding and Trajectory Analyses in Forward Flight

Ice accretion on a helicopter’s aerodynamic surfaces during flight can lead to incidents and fatal accidents. An iced helicopter blade results in a decrease in stall angle, an increase in required torque, as well as potential damage from shed ice. While the high centrifugal forces on a rotor may serve as a natural deicing mechanism, they can also lead to uneven ice shedding, causing rotor imbalances or hitting other blades, the fuselage, or the tail rotor. Computational methods are important for predicting in-flight ice formation and are a vital tool in the design of ice protection systems. The objective of this paper is to present a computationally inexpensive methodology to determine for a helicopter in forward flight, following a period of ice accretion, rotor ice crack locations, shedding times, and potential impact zones.

Experimental Heat Loads for Electrothermal Anti-Icing and De-Icing on UAVs

Atmospheric in-flight icing on unmanned aerial vehicles (UAVs) is a significant hazard. UAVs that are not equipped with ice protection systems are usually limited to operations within visual line of sight or to weather conditions without icing risk. As many military and commercial UAV missions require flights beyond visual line of sight and into adverse weather conditions, energy-efficient ice protection systems are required. In this experimental study, two electro-thermal ice protection systems for fixed-wing UAVs were tested. One system was operated in anti-icing and de-icing mode, and the other system was designed as a parting strip de-icing system. Experiments were conducted in an icing wind tunnel facility for varying icing conditions at low Reynolds numbers. A parametric study over the ice shedding time was used to identify the most energy-efficient operation mode. The results showed that longer intercycle durations led to higher efficiencies and that de-icing with a parting strip was superior compared to anti-icing and de-icing without a parting strip. These findings are relevant for the development of energy-efficient systems in the future.

Special Issue: Deicing and Anti-Icing of Aircrafts

In-flight icing for aircrafts is a large concern for all those involved in aircraft operations [...]

A Numerical and Experimental Investigation of the Convective Heat Transfer on a Small Helicopter Rotor Test Setup

In-flight icing affects helicopter performance, limits its operations, and reduces safety. The convective heat transfer is an important parameter in numerical icing simulations and state-of-the-art icing/de-icing codes utilize important computing resources when calculating it. The BEMT–RHT and UVLM–RHT offer low- and medium-fidelity approaches to estimate the rotor heat transfer (RHT). They are based on a coupling between Blade element momentum theory (BEMT) or unsteady vortex lattice method (UVLM), and a CFD-determined heat transfer correlation. The latter relates the Frossling number (Fr) to the Reynolds number (Re) and effective angle of attack (αEff). In a series of experiments carried out at the Anti-icing Materials International Laboratory (AMIL), this paper serves as a proof of concept of the proposed correlations. The objective is to propose correlations for the experimentally measured rotor heat transfer data. Specifically, the Frx is correlated with the Re and αEff in a similar form as the proposed CFD-based correlations. A fixed-wing setup is first used as a preliminary step to verify the heat transfer measurements of the icing wind tunnel (IWT). Tests are conducted at α = 0°, for a range of 4.76 × 105 ≤ Re ≤ 1.36 × 106 and at 10 non-dimensional surface wrap locations − 0.62 ≤ (S/c) ≤ + 0.87. Later, a rotor setup is used to build the novel heat transfer correlation, tests are conducted at two pitch angles ((θ) = 0° and 6°) for a range of rotor speeds (500 RPM ≤ (Ω) ≤ 1500 RPM), three different radial positions ((r/R) = 0.6, 0.75 and 0.95), and 0 ≤ S/c ≤ + 0.58. Results indicate that the fixed-wing Frx at the stagnation point was in the range of literature experimental data, and within 8% of fully turbulent CFD simulations. The FrAvg also agrees with CFD predictions, with an average discrepancy of 1.4%. For the rotor, the Ω caused a similar increase of Frx for the tests at θ = 0° and those at θ = 6°. Moreover, the Frx behavior changed significantly with r/R, suggesting the αEff had a significant effect on the Frx. Finally, the rotor data are first correlated with Rem (at each S/c) for θ = 0° to establish the correlation parameters, and a term for the αEff is then added to also account for the tests at θ = 6°. The correlations fit the data with an error between 2.1% and 14%, thus justifying the use of a coupled approach for the BEMT–RHT and UVLM–RHT.

Multi-physics simulation of in-flight ice shedding

In-flight ice accretion may possibly jeopardise the safety of fixed-and rotary-wing aircraft. Icing can possibly occur if supercooled water droplets in clouds impinge on the aircraft surfaces and freeze upon impact. A major issue related to ice accretion is the possibility of ice shedding from the main body and impacting other parts of the aircraft or being ingested by the engines. A multi-physics framework is presented to simulate ice accretion and shedding from wings and engine nacelles due to aerodynamic forces. The aerodynamics is computed using the open-source tool-kit SU2. Cloud droplet trajectories are computed using the arbitrary-precision Lagrangian in-house solver PoliDrop. Then, the in-house ice accretion tool-kit PoliMIce is used to determine the ice layer. A FEM structural analysis is performed on the accreted ice shape by means of the open-source code MoFEM. Internal stresses within the ice geometry due to aerodynamic forces are computed. The possibility of the occurrence of cracks in the ice layer is assessed and its propagation is determined numerically. Two-dimensional ice accretion simulations are performed to check the validity of the present approach and compare fairly well with available results.

ВЛИЯНИЕ ОБЛЕДЕНЕНИЯ НА ФУНКЦИОНИРОВАНИЕ АВИАЦИОННОГО ТРАНСПОРТА: СОСТОЯНИЕ ВОПРОСА И ПРОБЛЕМЫ ПРОГНОЗИРОВАНИЯ

The review of the studies dealing with the effects of icing on airtransport operation is presented. The problems of ground-based icing leading to the deterioration of runways and complication of pre-flight aerodrome operations are discussed. The reasons for the in-flight aircraft icing, the tools and methods of icing observation, the approaches to the forecasting of icing in clouds and precipitation are considered. The methods for predicting the icing of aircraft engines in the zones with high concentration of ice crystals are analyzed.

New Roughness-Induced Transition Model for Simulating Ice Accretion on Airfoils

Appropriately modeling surface roughness is crucial for predicting in-flight icing due to its influence on convective heat transfer. Most prevalent numerical simulations to this end rely on empirical correlations, where surface roughness is assigned a constant value. This limits the accurate computation of turbulent transitions, leading to a discrepancy between the experimental and the simulated shapes of ice. To improve shape prediction, this Paper proposes an approach to simulate the roughness distribution and its effect on the transition. The roughness distribution is determined analytically for water beads on the surface, and the roughness amplification parameter is explicitly computed using the turbulence model. The results of simulations of a rough plate and airfoil with leading edge roughness yielded good agreement with experimental data for transition-related behavior. Then, the convective heat transfer coefficient on iced surface and ice shape was predicted and compared with the result of the fully turbulent model and experiments. The proposed method showed better prediction of the ice shape, especially on the suction side of the airfoil for glazed and mixed ice.

Development of nanostructured icephobic aluminium oxide surfaces for aeronautic applications

In-flight icing due to the impingement of supercooled water droplets modifies the shape of the aerodynamic surfaces of aircraft, resulting in lower stall speeds and higher fuel consumption. Icephobic coatings help reducing the adhesion strength of ice to a surface and represent a promising technology to support mechanical/thermal ice protection systems. Superhydrophobic surfaces are water-repellent and embody a straightforward solution to tackle icing: nanostructured porous aluminium oxide layers are generated with anodization and superhydrophobicity can be reached by tuning the pores size. However, not much research has been done to verify if such surfaces are generally icephobic in representative icing conditions, or if they need to have additional properties to effectively reduce ice adhesion. In this work, we investigate the ice adhesion strength on cladded Aluminium Alloy 2024 (AA2024), an alloy commonly used for aerospace components, anodized with different process parameters in sulphuric and oxalic acid and hydrophobized by a commercial fluorinated product. Upon the optimization of the two anodization processes, the micro-/nanostructures generated on the surface were effective in reducing the ice adhesion strength. From icing wind tunnel tests, the surfaces anodized in oxalic acid showed superior icephobic (i.e., lower ice adhesion) properties compared to the ones anodized in sulphuric acid because of their larger air-to-solid surface ratio. The proposed anodization process is fast, easy to perform and to implement in existing production lines, low-cost, and operator- and environmentally-friendly.

An experimental study of rain erosion effects on a hydro-/ice-phobic coating pertinent to Unmanned-Arial-System (UAS) inflight icing mitigation

An experimental investigation was conducted to evaluate the variations of the surface wettability and ice adhesion strength on a typical hydro−/ice-phobic surface before and after undergoing continuous impingement of water droplets (i.e., rain erosion effects) at relatively high speeds (i.e., up to ~100 m/s) pertinent to Unmanned-Arial-System (UAS) inflight icing mitigation. The experimental study was conducted by leveraging a specially designed rain erosion testing rig available at Iowa State University. Micro-sized water droplets carried by an air jet flow were injected normally onto a test plate coated with a typical Super-Hydrophobic Surface (SHS) coating to simulate the scenario with micro-sized water droplets in the cloud impacting onto UAS airframe surfaces. During the experiments, the surface wettability (i.e., in the terms of static, advancing and receding contact angles of water droplets) and the ice adhesion strength on the SHS coated test plate were quantified as a function of the duration of the rain erosion testing. The surface topology changes of the SHS coated surface against the duration of the rain erosion testing were also measured by using an Atomic Force Microscope (AFM) system. The characteristics of the surface wettability and ice adhesion strength on the eroded SHS surface are correlated with the AFM measurement results to elucidate the underlying physics for a better understanding about the rain erosion effects on hydro−/ice-phobic coatings in the context of UAS inflight icing mitigation.

An experimental study on different plasma actuator layouts for aircraft icing mitigation

An experimental study was conducted to examine the effects of dielectric-barrier-discharge plasma (DBD) actuator layout on the plasma-induced thermal characteristics and evaluate their effectiveness for aircraft icing mitigation. The experimental investigation was performed in the Icing Research Tunnel of Iowa State University (i.e., ISU-IRT) with an airfoil/wing model embedded with an array of DBD plasma actuators around the airfoil leading edge. The plasma actuators were arranged in different layouts (e.g., orientation, number, and width of exposed electrodes) to evaluate their effects on the anti-icing performance under a typical glaze icing condition pertinent to aircraft inflight icing phenomena. While the dynamics ice accretion or anti-icing process over the airfoil surface before and after turning on the plasma actuators was recorded by using a high-resolution imaging system, a high-speed infrared thermal imaging system was used to quantitatively map the temperature distributions over the airfoil surface. The experimental results clearly reveal that, with the same power consumption level, the plasma actuators in streamwise layout would result in higher plasma-induced surface heating and faster runback of the unfrozen water over the airfoil surface, thereby, having a noticeably better anti-icing performance, in comparison to those in spanwise layout. The plasma actuators in streamwise layout were found to not only be able to prevent ice accretion near the airfoil leading edge, but also allow the plasma-induced surface heating to convect further downstream to delay/prevent the runback ice formation near the airfoil trailing edge. A proper combination of the plasma actuation strength and the plasma discharge coverage over the airfoil surface was found to result in the optimum performance for aircraft anti-icing applications.

Effects of strain rate variation on the shear adhesion strength of impact ice

In-flight ice accretion on fixed-wing aircraft and rotorcraft can be catastrophic if not mitigated. Most modern ice protection systems are active systems, which require electrical or mechanical power to remove accreted ice, thereby increasing weight, cost, and complexity. Scientists and engineers now seek passive, erosion-resistant materials and coatings with low ice adhesion strength. Ideally, such materials, when applied to vulnerable components of an aircraft, would cause any ice to shed off the surface under normal aerodynamic loading.To aid in the development of low-ice-adhesion-strength materials, the growth and structural behavior of impact ice in a wide range of atmospheric conditions must be characterized. The structural behavior of ice has been examined under pure shear, tension, compression, and mixed-mode loading. However, one important loading consideration that has not been widely investigated on atmospheric ice is strain rate.Knowledge of the relationship between impact ice adhesion strength and strain rate is important because it can be used to design future ice protection systems, and it may dictate the appropriate course of action for a pilot flying through icing conditions—for instance, whether a helicopter pilot should increase the rotor speed rapidly or slowly to induce shedding of the ice.NASA Glenn Research Center funded the design and construction of a new centrifuge-style ice adhesion test rig (“AJ2”) by the Penn State AERTS lab. The ice is accreted dynamically by spinning flat metal test coupons at high speed inside a simulated icing cloud environment. The design and analysis of the AJ2 rig is described in detail in this paper.Experiments were performed using AJ2 to investigate how the adhesion strength of impact ice related to the strain rate applied to it. Stainless steel test coupons of known surface roughness were tested in a range of environmental temperatures. The strain rates applied to the ice ranged between 5E−8 and 5E-5 s−1. It was discovered that a similar power function exists between strain rate and adhesion strength as found in the freezer-ice studies described in the literature. Despite scatter in the data, regression analysis determined the relationship between strain rate, temperature, and adhesion strength to be statistically significant. The power “1/n” for a coupon roughness of 64 nm (Sa) was greater than that of the 80-nm coupon; this was the case for both tested temperatures. However, for the relatively smooth surfaces tested, regression analysis suggested that the surface roughness had negligible effect on adhesion strength. Lower temperatures caused a higher power “1/n” and coefficient “c” in the power function. The variation of the coefficient with temperature is consistent with Glen's power law for the creep of glacier ice in compression. However, Glen did not observe a variation of the power with temperature. The value of “n” in the current study ranged from 2.6 for the smoothest sample at the coldest temperature, to 8.8 for the roughest sample at the warmest temperature. In most cases of the current study, “n” was within the range of previously-reported values in literature (1.5 to 6).

Multiphase SPH Modelling of Supercooled Large Droplets Freezing on Aircraft Surfaces

Solidification is an important issue that needs to be addressed when modelling supercooled large droplets (SLD) impacting aircraft surfaces. This work develops a 3D triple-phase smoothed particle hydrodynamics (SPH) solver that can simultaneously simulate SLD impingement and solidification on cold surfaces by solving the Euler equations for fluid dynamics, the energy equation for heat transfer, and a latent heat model for phase changes. The solver is validated against a Stefan problem in which a liquid is cooled and solidified by a cold boundary, and against a still droplet freezing on a cold plate. It is then applied to droplets impacting on cold surfaces at aeronautical speeds, analysing the freezing time and ice particle distributions as the droplet velocity and impact angle are varied. This is an important step towards a macroscopic numerical model of SLD post-impact behaviour for in-flight icing simulations.

Numerical prediction of dropwise condensation performances on hybrid hydrophobic-hydrophilic surfaces

The use of hybrid surfaces, alternating hydrophobic and hydrophilic (or less-hydrophobic) areas, has been recently proposed as a valuable tool for dropwise condensation performances enhancement. The hydrophilic region, in particular, is designed in order to promote the removal of the droplet from the hydrophobic region, reducing their average size and thus increasing the heat transfer performance. Here, dropwise condensation on heterogeneous surfaces composed by a texture of alternating hydrophobic and hydrophilic (or less-hydrophobic) vertical strips is numerically investigated through a Lagrangian-based phenomenological model, originally developed to study droplet evolution in the framework of in-flight icing phenomenon. The effects of droplet nucleation, growth, coalescence and motion are implemented. The model is extended to the case of curved surface. In order to properly assess the hybrid surface, however, a better knowledge of the interaction between the droplet and the film is required. Thus, additional numerical simulations are carried out using an in-house Eulerian full solver of the liquid film layer, in order to understand the migration mechanism of a drop between two different wettability regions. This will give useful information for a proper implementation of the effect of hydrophilic strips in the phenomenological Lagrangian model. A parametric analysis allows to find the optimal geometric configuration of the hybrid pattern, leading to maximum heat transfer performance, for a given set of wettability properties, nucleation density and condensation rate per unit area. Comparison with experimental data from the available literature ensures the capability of the current Lagrangian model to properly catch the physics behind dropwise condensation process on both planar and curved hybrid surfaces. The uncertainty of the estimate of some model input parameter and their influence on the computed solution are be also discussed.

Durability of superhydrophobic duplex coating systems for aerospace applications

In-flight icing, caused by the collision of supercooled water droplets with exposed aircraft surfaces, is an important safety hazard and a major issue in aviation. Superhydrophobic surfaces (SHS) have been shown to offer improvements to heating-based anti-icing and de-icing systems, reducing energy requirements for ice prevention and facilitating ice removal. A common SHS fabrication technique is to develop a surface with hierarchical roughness, and to then coat this surface with a hydrophobic topcoat. From this arises an issue: the durability of the entire system is fundamentally limited to the durability of this topcoat. In the present study, we develop a superhydrophobic duplex coating system with an emphasis on the environmental durability of the thin hydrophobic layer. The system consists of a thick TiO2 coating deposited by suspension plasma spraying, and a thin coating stack deposited by plasma enhanced chemical vapor deposition. The thin coating stack is based on DLC:SiOx—diamond-like carbon networked with silicon oxide—which exhibits a water contact angle of up to 95° and a hardness of up to 11 GPa, while the full coating system offers a contact angle of 159° and a contact angle hysteresis of 3.8°. The coating system is exposed to icing/deicing cycling, as well as rain erosion and accelerated aging tests; the results are compared with TiO2 coatings using stearic acid and fluoropolymer hydrophobic layers, as well as a commercially available superhydrophobic spray. The duplex coating system is shown to maintain water droplet mobility after 170 icing/deicing cycles, is resistant to prolonged UV and high-temperature exposure and offers a 300-fold improvement over the stearic acid in rain erosion tests.

Hybrid System Combining Ice-Phobic Coating and Electrothermal Heating for Wing Ice Protection

In-flight icing for aircraft is a large concern for all those involved in aircraft operations. Generally, an electric heater has been used to prevent in-flight icing. A hybrid anti-icing system combining ice-phobic coating and electrothermal heating (ICE-WIPS) has been proposed by the Japan Aerospace Exploration Agency (JAXA) to reduce the power consumption in the heating unit. In order to validate the effectiveness of ICE-WIPS, validation and demonstration tests are conducted using icing wind tunnels at the Kanagawa Institute of Technology (KAIT) and at the Icing Research Tunnel in the NASA Glenn Research Center. Using a NACA0012 airfoil as a test model, ICE-WIPS demonstrates substantial reduction in power consumption as compared to the existing heating system. The reduction depends on the in-flight icing conditions; more than a 70% reduction is achieved at a liquid-water content (LWC) of 0.6 g/m3 and a median-volume diameter (MVD) of 15 μm at 75 m/s with zero angle of attack. In wet-icing conditions, more than a 30% reduction in power is achieved.

Comparison of turbulent Prandtl number correction models for the Stanton evaluation over rough surfaces

ABSTRACT In-flight ice accretion code predictions depend on heat loss over rough surfaces. The equivalent sand grain roughness models the friction coefficient, but an additional model is needed for heat transfer predictions. In this paper, a two parameters model based on a turbulent Prandtl number correction is derived from the sublayer Stanton-based model. For flow over rough surfaces, the new model predictions will be compared to a three parameters model from the literature. First, the models are coupled with the Spalart–Allmaras turbulence model to predict heat transfers over rough surfaces. Thereafter, the two models are implemented in the open-source software, SU2, and then verified and validated for flow over rough flat plates. Lastly, the predictions of the two models are shown to be similar for ice accretion roughness on flat plates, aerofoils and a wing. Model discrepancies are always within the experimental 15% error margin.

In-flight icing simulation for two-dimensional configurations

This paper presents a comprehensive aircraft icing simulation tool implemented in an in-house Navier–Stokes parallel multi-block solver. In detail, the droplet flow field is solved by Eulerian approach, and a Partial Differential Equation (PDE)-based ice accretion model is adopted to determine the runback water flow and icing rate. Numerical validations are performed on the two-dimensional (2D) NACA 0012 airfoil, where good agreements with the literature are observed. Additionally, the paper investigates the influence of droplet size on the final ice shape. Results show that droplets with greater Median Volume Diameter (MVD) are more likely to impact on the wall, which results in larger droplet impingement limit and icing limit.

Combining CFD-EFD-FFD data via Gappy Proper Orthogonal Decomposition

ABSTRACT This paper demonstrates how, by means of Reduced Order Modelling (ROM), CFD cost can be drastically reduced and, in addition, a more complete investigation of a continuous design space obtained by adding experimental fluid dynamics (EFD) and flight fluid dynamics (FFD) data. The ‘Gappy’ Proper Orthogonal Decomposition (GPOD) method is used to enrich the EFD and FFD data to the size of their companion CFD. All three are then combined in a multidimensional database providing a much finer real-time exploration of a design space. Examples from aerodynamics and in-flight icing are given.