Supercooled Large Droplet Research Articles

Facing the Challenges of Supercooled Large Droplet Icing: Results of a Flight Test Based Joint DLR-Embraer Research Project

<div class="section abstract"><div class="htmlview paragraph">Today’s airplanes are well equipped to cope with most common icing conditions. However, some atmospheric conditions consisting of supercooled large droplets (SLD) have been identified as cause of severe accidents over the last decades as existing countermeasures even on modern aircraft are not necessarily effective against SLD-ice. In 2014, the new Appendix O to the certification regulations (FAR Part 25 / CS-25) had been issued to guarantee the safe operation of future airplane when encountering SLD conditions. But as the SLD topic is quite new for the majority of aircraft manufacturers and research institutes in a same way, DLR (German Aerospace Center) and Embraer established a joint research cooperation in 2012 to obtain a better understanding of the distinct influences of SLD-ice shapes on aircraft characteristics and to evaluate proper ways for future airplane certification under App. O. Furthermore, one additional scientific goal of the cooperation was to develop and test new tools for the in-flight monitoring of aircraft characteristics as well as the on-board identification of simulation models. During the 4 years of the project, a distinct way to better understand icing-induced degradations on a specific aircraft was followed: first, data of the clean aircraft was gathered in flight test to identify a dynamic simulation model as base for the subsequent evaluations. Second, data of test flights with artificial App. C ice configurations were analyzed and used for the development of distinct modifications of the base aircraft simulation model; a first evaluation of the icing-induced changes of aircraft characteristics was conducted. Third, after the generation of SLD-ice shapes, wind tunnel testing and flight clearance, a second flight test campaign with these artificial SLD-ice shapes delivered the data for an additional model modification and identification. The results of the final data and model evaluation provide the observable degradation of SLD-ice in flight, which is well comparable to results obtained from the App. C ice configurations.</div></div>

Aerodynamic Comparison of Freezing Rain and Freezing Drizzle Conditions at the RTA Icing Wind Tunnel

<div class="section abstract"><div class="htmlview paragraph">The simulation of icing conditions in icing wind tunnels (IWTs) is a significant element in the certification of aircraft components and offers unique possibilities for research purposes. Up to 2014 only the conditions defined in Appendix C of the EASA Certification Specification 25, respectively the FAA Code of Federal Regulations Title 14 Part 25 were used for the certification processes in IWTs. In addition, Appendix O was introduced in 2014 to cover the supercooled large droplet (SLD) icing conditions of freezing drizzle and freezing rain, which pose a potential risk for flight safety. The simulation of SLD icing in IWTs is, due to the different behavior of the large droplets, very challenging and not all required conditions have successfully been recreated yet. RTA Rail Tec Arsenal Fahrzeugversuchsanlage GmbH has focused on the simulation of in-flight icing conditions since 2012 and increased effort was put in the simulation of SLDs in recent years. During several research projects funded by the Austrian government it was shown that droplet size distributions for freezing drizzle MVD > 40 μm and freezing rain MVD > 40 μm can be recreated close to the requirements. With further improvements to the spray system, a cloud uniformity within certain tolerances also has been achieved over areas large enough to cover an unscaled wing section. In order to determine the influence of the different droplet sizes on the aerodynamic performance, a NACA0012 wing section with a wingspan of 1.8 m and a chord of 1 m was exposed to different icing clouds including Appendix C conditions, freezing drizzle and freezing rain. The aerodynamic lift coefficient was recorded over a large angle of attack range and the characteristics of the different ice accretions were analyzed and documented using high resolution 3D scan technology. Furthermore, the drag increase during the icing process was measured. It was the first time that an unscaled wing was iced under artificial freezing rain conditions.</div></div>

Ice accretion and aerodynamic effects on a multi-element airfoil under SLD icing conditions

The impingement behavior of large water droplets, their interactions with the solid wall and the subsequent ice accretion and aerodynamic effects have become a key issue in in-flight aircraft icing. In this study, ice accretion and aerodynamic effects on a multi-element airfoil were investigated under the recently introduced Appendix O icing envelope. Supercooled large droplet (SLD) dynamics were taken into account by employing a unified computational approach. Ice accretion was simulated using a partial differential equation (PDE) based solver, instead of the commonly used control volume method. The numerical solver of the SLD impingement was built on the droplet deformation and droplet–wall interaction splash models. The unified solvers for clean air, large droplet impingement, ice accretion, and the aerodynamic analysis of ice effects—all of which are based on a single unstructured upwind finite volume framework—were first validated using available experimental data and then applied to investigate ice accretion and the resulting aerodynamic effects on multi-element airfoils for various flight conditions and, in particular, near-freezing SLD icing conditions. Interestingly, two counter-intuitive results were found when comparing the ice accretion and associated aerodynamic degradation for non-SLD and SLD cases. Moreover, considering runback ice was shown to be essential in the design of an ice protection system (IPS) for the multi-element wing.

Multi-phase smoothed particle hydrodynamics modeling of supercooled large droplet dynamics for in-flight icing conditions

Understanding the dynamics of a single large water droplet is needed for accurate simulations of the in-flight icing phenomenon. Obtaining information on the ratio of ejected to deposited water and the post-impact droplet distribution should improve the numerical modeling of the bulk of impinging droplets. In this study, a weakly compressible multi-phase Smoothed Particle Hydrodynamics (SPH) method with shifting algorithm and surface tension model is presented to simulate the single droplet dynamics. The validity of the approach has been proved by modeling the classical problems of Rayleigh–Taylor instability, dam break, and droplet formation by comparing against other numerical and experimental data in the literature. Finally, droplet impingement on a liquid film and dry solid surface has been simulated and compared against the experimental data. The effect of impact angle and film thickness on the crown formation is studied to demonstrate the importance of modeling SLD impingement for in-flight icing conditions.

Numerical simulation of ice accretion in supercooled large droplet conditions

The simulated methods for ice accretion on two-dimensional airfoil surface are established completely under the Eulerian framework in supercooled large droplet (SLD) conditions. The two-dimensional code to solve the partial differential equations (PDEs) of droplet phase is derived to simulate the impingement characteristic of SLD. Also, several semi-empirical models which explain the droplet-wall interaction are compared and discussed to show respective features when simulating the splashing phenomenon. In particular, a new boundary condition for wall called penetrable wall for splashing droplet (PWSD) is proposed to deal with the impingement of SLD on solid surface, which efficiently improves the accuracy of simulation. Then the improved impingement characteristic of SLD is input into the extended mass and heat transfer model to simulate the ice growth on airfoil surface. The multistep advanced method is carried out to better match the physical phenomenon of ice growth. At last, the simulated results of critical parameters: local droplet collection efficiency and the height of ice growth are compared with the experimental data which verify the applicability of proposed models.

An Investigation on the Characteristics of Supercooled Large Droplet Icing Accretions and Aerodynamic Effects on HighLift Configuration

An Investigation on the Characteristics of Supercooled Large Droplet Icing Accretions and Aerodynamic Effects on HighLift Configuration

Supercooled large droplet icing accretion and its unsteady aerodynamic characteristics on high-lift devices

Given the importance of understanding new FAR25, Appendix O certification, supercooled large droplet icing characteristics and its damages should be specified because of its serious hazard to in-flight safety and aircraft engineering. This study compares the icing feature and aerodynamic degradation of high-lift devices under different diameter conditions using a verified numerical method SJTUICE (a ice accretion simulating code by Shanghai Jiao Tong Univeristy). With two different flow and freezing time-steps Tf and Ts, the accuracies on fluid dynamic and icing predictions over a long period of time can be both controlled within an acceptable range. The comparisons between supercooled large droplet and non-supercooled large droplet conditions reflect that larger diameters induce more intense icing characteristics on the suction-side of each airfoil, in which the leading edge of flap is the most sensitive one to drop size. The second-accelerated flow near the gap is adverse to smaller droplet collection but benefit for impact thermodynamic enhancement of those larger collected droplets. The ice-caused gap narrowing makes flow separation earlier, which finally leads to the degradation of global high-lift performance worsen. Lift coefficient comparison also indicates the performance of high-lift devices decays more quickly under supercooled large droplet conditions. Results of this paper highlight the supercooled large droplet icing effects with respect to smaller diameter conditions.

Revisited Model for Supercooled Large Droplet Impact onto a Solid Surface

A rationally based methodology is proposed to derive a new mass loss empirical model for supercooled large droplets. The numerical results from the ONERA two-dimensional trajectory solver and the experimental results from the NASA Papadakis supercooled large droplet database are combined to get both the impinging and the deposited mass flow rates at each point of the test model. These data are used to derive a new model for the collection efficiency. It allows clearly separating the influence of the kinetic energy, which is the dominating effect close to the leading edge, from the influence of the angle of incidence, which is the most influent parameter close to the impingement limits. Moreover, the model can be used for both supercooled large droplets and small droplets.

Effect of drop size on the impact thermodynamics for supercooled large droplet in aircraft icing

Supercooled large droplet (SLD), which can cause abnormal icing, is a well-known issue in aerospace engineering. Although efforts have been exerted to understand large droplet impact dynamics and the supercooled feature in the film/substrate interface, respectively, the thermodynamic effect during the SLD impact process has not received sufficient attention. This work conducts experimental studies to determine the effects of drop size on the thermodynamics for supercooled large droplet impingement. Through phenomenological reproduction, the rapid-freezing characteristics are observed in diameters of 400, 800, and 1300 μm. The experimental analysis provides information on the maximum spreading rate and the shrinkage rate of the drop, the supercooled diffusive rate, and the freezing time. A physical explanation of this unsteady heat transfer process is proposed theoretically, which indicates that the drop size is a critical factor influencing the supercooled heat exchange and effective heat transfer duration between the film/substrate interface. On the basis of the present experimental data and theoretical analysis, an impinging heating model is developed and applied to typical SLD cases. The model behaves as anticipated, which underlines the wide applicability to SLD icing problems in related fields.

A two-dimensional splashing model for investigating impingement characteristics of supercooled large droplets

In this article, a two-dimensional (2D) splashing model is proposed to investigate the dynamics when supercooled large droplets (SLD) impinging on a wall surface in the aircraft-icing field. Energy conservation for droplet motion and impingement is used to capture the properties of the splashed droplets. A new statistical treatment of the droplet impinging energy and angle during the droplet-wall interaction is introduced in order to calculate the average dynamics of the SLD within a micro-control volume on wall surface. Based on the LEWICE predictions of droplet collection efficiencies and the available experimental ones, a new criterion for droplet splashing/deposition as well as a new formulation for the splashed mass is suggested. Lagrangian approach is adopted to describe the movement and impingement of SLD. The proposed model together with the previously developed droplet tracking method (DTM) for calculating droplet collection efficiency with the effect of droplet reimpingement constitute a relatively complete predicting approach of SLD impingement characteristics. Comparisons between the current predictions and the experimental observations, including SLD impingement over clean and contaminated airfoil surfaces as well as shapes of ice accretion in typical icing conditions, are carried out. Further, results obtained with the LEWICE splashing model are also plotted on the same graphs in order to assess the accuracy of the current splashing model in predicting SLD impingement. Results show that good agreement is achieved between the current predictions, including SLD impingement and ice accretion shapes, and the experimental ones. The predictions of the impingement distribution over contaminated surfaces obtained with the current splashing model show a much closer agreement with the experimental results than the ones obtained with LEWICE splashing model. For further investigation of SLD impingement, the properties of the droplet splashing and reimpingement during the ice accretion process are also addressed.

Quasi-molecular modeling of a single supercooled large droplet impact

A mesoscale model for droplet dynamics based on a quasi-molecular approach is proposed. It considers the interaction between quasi-molecules within a single liquid droplet, each quasi-molecule representing an agglomeration of a large number of actual water molecules. The goal is to improve the understanding of the dynamics of large droplet collisions over dry or wet surfaces at velocities typical of aeronautical applications. This detailed analysis will eventually be used to refine the macroscopic Eulerian description of the water impingement process by providing numerical correlations for splashing and bouncing phenomena relevant for in-flight icing applications. Based on the Equipartition Theorem, approaches for extracting macroscopic quantities such as temperature and transport coefficients from the quasi-molecular method are discussed. A proper choice of the free parameters of the model that leads to accurate values of the macroscopic properties is also addressed.

Supercooled Large Droplet Modeling for Aircraft Icing Using an Eulerian–Eulerian Approach

Previous models employed in the computations of collection efficiencies for supercooled large droplets illustrate that the droplets’ interaction with an airfoil surface, such as bouncing and splashing of droplets, should be taken into account in the simulation to obtain accurate results. The previous supercooled large droplet models have been modified to compute the collection efficiencies for supercooled large droplets in such a way that the modified models can be adopted more effectively and suitably in the Eulerian–Eulerian approach. The modified supercooled large droplet models are implemented into the Eulerian–Eulerian approach within CFD++, which is the commercial computational fluid dynamics software suite from Metacomp Technologies, Inc. In addition, in order to account for aerodynamic droplet breakup in the Eulerian–Eulerian approach, a number density equation has been derived that includes a source term representing the rate of change of the number density during breakup. To validate the present supercooled large droplet modeling, simulations to compute icing collection efficiencies have been carried out for droplets of small and large diameters over clean airfoils and airfoils with glaze ice shapes.

Influence of solidification on the impact of supercooled water drops onto cold surfaces

This study presents an experimental investigation of the impact of a supercooled drop onto hydrophilic and superhydrophobic substrates. The aim is to better understand the process of airframe icing caused by supercooled large droplets, which has been recently identified as a severe hazard in aviation. The Weber number and Reynolds number of the impinging drop ranged from 200 to 300 and from 2600 to 5800, respectively. Drop impact, spreading, and rebound were observed using a high-speed video system. The maximum spreading diameter of an impacting drop on hydrophilic surfaces was measured. The temperature effect on this parameter was only minor for a wide range of the drop and substrate temperatures. However, ice/water mixtures emerged when both the drop and substrate temperatures were below 0 °C. Similarly, drop rebound on superhydrophobic substrates was significantly hindered by solidification when supercooled drop impacted onto substrates below the freezing point. The minimum receding diameter and the speed of ice accretion on the substrate were measured for various wall temperatures. Both parameters increased almost linearly with decreasing wall temperature, but eventually leveled off beyond a certain substrate temperature. The rate of ice formation on the substrate was significantly higher than the growth rate of free ice dendrites, implying that multiple nucleation sites were present.

Eulerian Modeling of Supercooled Large Droplet Splashing and Bouncing

A conservative Eulerian numerical approach for modeling postimpact Supercooled Large Droplets undergoing splashing and bouncing on aircraft surfaces is presented. The approach introduces the effect of the postimpact droplets by successive solutions of the conservation equations. Two models have been selected to identify the droplet splashing and bouncing conditions, and to provide initial conditions for the reinjected water. The method has been applied to droplet impingement in Supercooled Large Droplet conditions on clean and iced NACA 23012 geometries, as well as the MS(1)-0317 airfoil, and the results have been compared to experimental data. Good agreement is observed for both impingement limits and collection efficiency. Additionally, the method has been applied to a three-element high-lift configuration operating in one of the proposed Appendix O Supercooled Large Droplet environments to demonstrate the danger posed by the re-impingement of splashing and bouncing droplets on complex interacting aerodynamic components.

Simulation and validation of slat de-icing by an electromechanical system

The Electro-Impulse De-Icing system is an alternative process of de-icing wing slat structures made of carbon fiber reinforced plastics or aluminum. It is especially qualified for no-bleed systems that are used in modern and future aircraft. Due to the time-dependent interactions between the induced magnetic field and the structural deformation it can be necessary to couple these physical fields in analyses. This paper presents a three-dimensional simulation as well as experiments of flat plates. The simulation is characterized by electrical and structural finite element calculations, which are two-way coupled in each time step. The current progress is based on real tests which are executed at a special test rig. A coil, which is connected to an impulse generator, is used to induce magnetic forces. Flat plates of aluminum or carbon fiber reinforced plastics (with an additional aluminum doubler) were tested at room temperature and deformation results were used to validate numerical simulations. Further research deals with the simulation of the de-icing process itself with a stress criterion for ice adhesion. Therefore, the test rig is mounted in a cooling chamber. The ice layer is produced by spraying cooled water through a nozzle with a droplet size of supercooled large droplets (SLD). The deformation progress with and without ice is analyzed at different impulse forces and ice thicknesses. The coupled finite element analysis gives the opportunity to simulate the process of de-icing in situ to the time-dependent loading of the plate by magnetic forces. Furthermore, the complex dynamic behavior of the structure can be simulated with excellent agreement to real tests.

Numerical investigation on super-cooled large droplet icing of fan rotor blade in jet engine

Icing (or ice accretion) is a phenomenon in which super-cooled water droplets impinge and accrete on a body. It is well known that ice accretion on blades and vanes leads to performance degradation and has caused severe accidents. Although various anti-icing and deicing systems have been developed, such accidents still occur. Therefore, it is important to clarify the phenomenon of ice accretion on an aircraft and in a jet engine. However, flight tests for ice accretion are very expensive, and in the wind tunnel it is difficult to reproduce all climate conditions where ice accretion can occur. Therefore, it is expected that computational fluid dynamics (CFD), which can estimate ice accretion in various climate conditions, will be a useful way to predict and understand the ice accretion phenomenon. On the other hand, although the icing caused by super-cooled large droplets (SLD) is very dangerous, the numerical method has not been established yet. This is why SLD icing is characterized by splash and bounce phenomena of droplets and they are very complex in nature. In the present study, we develop an ice accretion code considering the splash and bounce phenomena to predict SLD icing, and the code is applied to a fan rotor blade. The numerical results with and without the SLD icing model are compared. Through this study, the influence of the SLD icing model is numerically clarified.

Lagrangian Approach for Simulating Supercooled Large Droplets’ Impingement Effect

In this paper, a droplet tracking method and a splashing model have been developed to calculate the droplet collection efficiency in a supercooled-large-droplets regime using the Lagrangian computational method. In the droplet tracking method, the droplet deformation and the droplet-wall effects (for example, splashing, bouncing, and reimpingement), which are the typical cases in a supercooled-large-droplets regime, are incorporated by introducing the mass residual ratio. The effects of the transition from the conventional-small-droplets impingement to a supercooled-large-droplets impingement, as well as the splashed secondary droplets, on the droplet collection are considered in the current splashing model. The performance and capacities of the droplet tracking method and the supercooled-large-droplets splashing model are validated against the alternative experimental reference data. The mass loss ratio and the mass back ratio are introduced in order to explore the distribution and the quantity of the mass loss and mass back caused by droplet splashing and reimpingement. The predicted results show that the quantity and the distribution range of the mass back ratio on airfoil surfaces are relatively lower than those of the mass loss ratio. A significant mass back is observed when the airfoil is contaminated with ice. No mass loss or mass back is observed beyond the impinging region for the given conditions.

MPS法を用いたNACA翼におけるSLD着氷シミュレーション

Icing simulations are very important in the development phase of aircraft and jet engines. Currently, icing simulation.s are performed with the grid-based method such as the finite difference method and the finite volume method. However, these methods cannot completely reproduce the characteristics of accreted ice because of the limitation of grid systems which must be used in the simulations. On the other hand, particle-based methods are now popular in industries, because it is a mesh-free method and thus grid generation can be skipped. In addition, it can predict liquid interface sand the large deformation satisfactorily. In the present study, we perform the Super-cooled Large Droplet (SLD) icing simulation on the NACA0012 airfoil using the MPS method which is one of the particle-based method. We expect that the icing simulation by the MPS method might be able to predict the complex ice shapes which cannot be reproduced by the grid-based simulation. The obtained result indicates that the MPS icing simulation can reproduce the ice shape like a feather and the ice thickness reasonably. However, it is confirmed that the ice volume is underestimated due to the improper droplet model and the icing judgement.

G0504 スプラッシュおよびバウンドを考慮した三次元着氷現象の数値シミュレーション(GS5 混合・物質伝達)

Ice accretion is the phenomenon that super-cooled water droplets impinge and accrete on a body. It is well known that ice accretion on blades and airfoils leads to performance degradation and has caused severe accidents. One major cause of accidents due to ice accretion is supercooled large droplets (SLD). In this study, we apply a SLD model to a rotor blade of a jet engine under an icing condition, and the numerical results with/without the model are compared. We confirm that with the model, the volume of ice near the hub is larger than the result without the model. Through this study, the influence of the SLD icing model in a jet engine is numerically clarified.

과냉각대형액적 충돌예측을 위한 오일러리안 기반 수치 모델링

Supercooled large droplet issues in aircraft icing have been continually reported due to the important safety considerations. In order to simulate the impingement behavior of large droplets, a two-dimensional and compressible Navier-Stokes code was developed to determine the flow field around the test model. Also, the Eulerian-based droplet impingement model including a semi-empirical approach for the droplet-wall interaction process and droplet break-up was developed. In particular, the droplet-wall interactions were considered as numerical boundary conditions for the droplet impingement simulation in the supercooled large droplet conditions. Finally, the present results were compared with the experimental test data and the LEWICE results. The droplet impingement area and maximum collection efficiency values between present results and wind tunnel data were in good agreements. Otherwise, the inclination of collection efficiency of the present result is over-predicted than the wind tunnel data around a lower surface of the NACA 23012 airfoil.