Numerical Computation of Aircraft Icing and Study on Icing Test Scaling Law
|School||China Aerodynamics Research and Development Center|
|Keywords||Aircraft icing Leading edge ice accretion Ridge ice Rime ice Glaze ice Mixed ice Aerodynamic characteristics Icing test scaling law Anti/deing Droplet impingement characteristics|
Aircraft can experience icing when encountering a cloud that contains supercooled water droplets. Ice accretion is a common phenomenon in flight. It is one of the main hazards to flying. Investigation on icing mechanism and effects is the basis of anti/deicing equipments design and the establishment of flight and operation rules in icing condition.Predicting leading edge ice, forecasting ridge ice, and evaluating effects of ice on aerodynamic characteristics are fundamental aspects in icing study. Icing scaling law is the theoretic basis for icing wind tunnel test and selection of test parameters. Unfortunately, the importance of them has just attracted attention recently in China. Few researchers pursue the studies on ice accretion, and no one has touched upon ridge ice and icing scaling law .There is a great lag between China and Europe or America in the field of ice accretion research.In order to understand ice accretion deeply and fill up the gap, the formation and effects of leading edge ice and ridge ice, and the icing scaling law are investigated systematically by theoretic analysis and numerical simulation methods in this paper, and a series of valuable achievements are obtained.(1) Three fundamental steps of numerical simulation on ice accretion (calculation for flow field, trajectories and impingement characteristics of droplets, and thermodynamic process) are thoroughly investigated. Frame and arithmetic for icing numerical simulation are established, which are the ground-work for studying ice with numerical method.①The SIMPLE method is used to solve incompressible, time averaged N-S equations. Viscous effects are considered in icing calculation, which provides more accurate flow field information for the calculation of droplets movements and phase transition. It is different from other methods which neglect viscosity when calculating ice.②Program and steps for calculating droplets impingement characteristic are offered, then the local catch coefficient of mono/multi_droplets size distribution is computed, which agree well with experimental results, therefore, it can be applied in aircraft icing research and anti/deicing design.③A modified thermodynamic model of icing is proposed following analysis of Messinger model, then the numerical method for solving the model is developed, which employs iterative technology, and can give the type of ice directly.(2) An ice research code (IRC) for leading edge ice calculation is established, then the code is used to simulate the formation of leading edge ice on two dimensional object surfaces. Aerodynamic characteristics of iced objects are investigated. The effects of temperature, droplet diameter and liquid water content (LWC) on ice accretion are discussed in detail. These are significant for the study of icing mechanism and the design of anti/deicing equipments.①Rime, glaze and mixed ice accretion on the leading edge of two dimensional cylinder and NACA0012 airfoil are numerical simulated. Ice shapes from computation agree with experimental results, which indicate the validity of model and arithmetic in this paper. The numerical method for ice simulation of multi-element airfoil is described, and then the ice shape on NLR7301 two-element airfoil is obtained, which show that ice accretion on the flap is more severe.②Flow field about iced objects is numerical calculated, and the results reveal that the periodicity of lift/drag properties for cylinder is changed due to ice, especially glaze and mixed ice. Ice accretion on NACA0012 airfoil results in not only a decrease of maximum lift and stall angle but also an increase of drag. Aerodynamic characteristics of NLR7301 two-element airfoil are destroyed severely after icing. Pressure distribution on the main changes more obvious than that on the flap.③The effects of temperature, droplet diameter and LWC on ice accretion are discussed in detail, from which we can know how they affect icing process, that is, the effects of temperature and LWC on freezing fraction can change ice type, and droplet diameter affects mainly on icing region and thickness.(3) A numerical method for droplet movement and impingement characteristics calculation in three-dimensional ice simulation is presented. The technology for solving thermodynamic model on three-dimensional surfaces is proposed. IRC3D, a three-dimensional ice research code, is developed. Then it is used to simulate leading edge ice accretion on three-dimensional wing. Water impingement characteristics and ice shape on three-dimensional NACA0012 straight wing are in accord with those on two-dimensional airfoil, which illustrates that IRC can be used in three-dimensional ice calculation. Ice accretion on the leading edge of ONERA M6 wing is numerically simulated, which demonstrates that from wing root to tip, the maximum local catch coefficient increases, ice become thinker, and the fraction of rime ice decreases, while that of glaze ice increases. Leading edge ice accretion on wing tip of a transport aircraft is computed, and the effects of ice on aerodynamic characteristics are analyzed. Above achievements are valuable for the study of ice accretion on complex, three-dimensional objects.(4) Ridge ice and its effects on aerodynamic characteristics are studied. A method for ridge ice calculation is presented, which can give the mass of ridge ice under specific icing conditions. Then the aerodynamic effects of ridge ice are investigated, structure of flowfield around ridge iced wing is analyzed, which shows the presence of ridge ice results in earlier separation, change of surface pressure distribution, increase of drag, and decrease of maximum lift. The effects of ridge ice on aileron hinge moment are also calculated. We found that the aileron hinge moment also vary greatly due to the change of pressure distribution on aileron surface. It will degrade the control performance and endanger flying aircraft.(5) Icing scaling law is systematically studied. A modified icing scaling law is proposed in this paper. It can be used in icing wind tunnel test as the theoretic basis of test and its parameters selection.①The principle and method for establishing icing scaling law are described. The definition formulas of scaling parameters are derived. After analyze foreign laws, we proposed a modified icing scaling law, which considers the dynamic pressure as a scaling parameter so that the pressure and velocity are coupled. It gets rid of disadvantages in AEDC or ONERA laws.②Based on the modified laws, a method for test parameters selection is established, and the corresponding numerical code is developed.③We put forward a new idea about, without icing wind tunnel, how to validate the scaling law with numerical method. Then IRC are used to test the modified law, which shows the law is valid.④Applied research of icing scaling law is performed. The law is used in the selection of icing test parameter as well as the evaluation of simulation capacity of an icing wind tunnel, which is beneficial to the design for icing wind tunnel.The present research involves three aspects, i.e., leading edge ice accretion, ridg ice and icing scaling law. It is for the first time that numerical method is used for the calculation of ridge ice and the validation of icing scaling law. Some new idea and method are presented. The achievements of this paper are significant in many ice study fields, such as hazard analysis of aircraft icing, design of anti/deicing equipments, test of icing wind tunnel, etc.