Rime Ice Research Articles

The effect of accretion temperature on microstructure and bending strength of atmospheric ice

Accurate determination of the mechanical response of atmospheric ice is key to understanding the risks associated with ice impact on aircraft during flight. Two types of atmospheric ice which are of particular interest to the aerospace industry are studied. Rime and Glaze ice are each manufactured in an icing wind tunnel facility under controlled conditions. Rime ice is accreted at a temperature of −20°C, and Glaze ice is accreted at −5°C. Quasi-static three-point bend tests are performed on both types of ice to understand the effect of accretion temperature, and therefore microstructure, on strength. The results indicate that the ice accretion temperature, and thus microstructure, has a significant influence on the bending strength. On average, the bending strength of Rime ice is 9.0±0.18MPa compared to 4.4±0.093MPa for Glaze. The comparatively lower accretion temperature of Rime results in smaller grain sizes and higher bending strength. In contrast, the effective modulus appears insensitive to ice microstructure, with an average value of 3.5±0.12GPa for Rime compared to 3.6±0.098GPa for Glaze. Furthermore, the results indicate that both the bending strength and effective modulus are insensitive to the ice storage time.

Evaluation of the shear adhesion strength of impact ice by a new method

Aircraft icing has a detrimental effect on flight safety. The design of aircraft anti-icing/de-icing systems requires a thorough comprehension of the adhesion between the ice and the substrate. In this research, the multi-hole rotating cylinder method (MHRC) is developed in an icing wind tunnel. Simulations of interfacial stress show that ice thickness does not change the evenness of stress, whereas reducing the contact area can impair uniformity. Moreover, the ice layer would not experience any cohesive damage during the experiments. The shear ice adhesion strength is evaluated using the methodology outlined in this research. At lower temperatures, glaze ice demonstrates a rise in its shear adhesion strength, while the change trend is the opposite for rime ice. The lowest adhesion strength is observed at the medium volume diameter (MVD) of 45 µm. Furthermore, the growing liquid water content (LWC), rough surfaces, and surface painting will enhance the interface adhesion of ice to the substrate to varying degrees. Stainless steel substrate exhibits higher adhesion strength with ice as compared to aluminum and titanium. In addition, superhydrophobic surfaces have proven to possess a significant reduction in adhesion strength at higher temperature. The proposed experimental method and platform afford accurate measurements of the shear adhesion strength of impact ice in the icing wind tunnel.

A dynamic collection efficiency formulation implemented in the icing code DICEPS

The Technical University of Braunschweig developed the 2D icing code DICEPS, used for the simulation of ice accretion on aircraft components exposed to icing conditions. In its formulations, DICEPS presents an improvement in the traditional formulation which icing codes use for the calculation of the collection efficiency, which works together with a multi-sub-step algorithm used for the surface deformation during an icing-step. This new formulation continuously updates the impingement mass flow based on the change of the wall-normal as the surface deforms during the multi-sub-step routine in an icing-step, resulting in a dynamic collection efficiency. In the current work, DICEPS and its dynamic collection efficiency formulation (DCEF) are presented and validated through several test cases under three categories: prediction of collection efficiency, rime ice shapes and glaze ice shapes. DICEPS managed to successfully replicate distributions of collection efficiency along with an implemented splashing model. A substantial improvement in the prediction of rime ice shapes via single-step simulations was achieved using the DCEF, while the traditional formulation requires several icing-steps to obtain a converged ice shape. This is also seen when rime ice results are compared to those of FENSAP-ICE, which require as well several icing-steps. In the case of glaze ice, the DCEF presents a time limit since prominent ice features will heavily influence the real collection efficiency, making the DCEF inaccurate after this limit. Nevertheless, the effects that geometrical changes have over parameters such as the heat transfer imply that a multi-step approach is eventually required. By analyzing the results, the DCEF shows that it effectively resembles the physical behavior that geometrical changes impose on the collection efficiency for particles with large inertia, while smaller particles present some inaccuracies, suggesting a proper extension of DCEF for these cases.

A rotorcraft in-flight ice detection framework using computational aeroacoustics and Bayesian neural networks

This work develops a novel ice detection framework specifically suitable for rotorcraft using computational aeroacoustics and Bayesian neural networks. In an offline phase of the work, the acoustic signature of glaze and rime ice shapes on an oscillating wing are computed. In addition, the aerodynamic performance indicators corresponding to the ice shapes are also monitored. These performance indicators include the lift, drag, and moment coefficients. A Bayesian neural network is subsequently trained using projected Stein variational gradient descent to create a mapping from the acoustic signature generated by the iced wings to predict their performance indicators along with quantified uncertainty that is highly important for time- and safety-critical decision-making scenarios. While the training is carried out fully offline, usage of the Bayesian neural network to make predictions can be conducted rapidly online allowing for an ice detection system that can be used in real time and in-flight.

A bin microphysics parcel model investigation of secondary ice formation in an idealised shallow convective cloud

Abstract. We provide the first systematic study of ice formation in idealised shallow clouds from collisions of supercooled water drops with ice particles (mode 2). Using the University of Manchester bin microphysics parcel model, we investigated the sensitivity of ice formation due to mode 2 for a wide range of parameters, including aerosol particle size distribution, updraft speed, cloud-base temperature, cloud depth, ice-nucleating particle concentration, and freezing fraction of mode 2. We provide context to our results with other secondary ice production mechanisms as single mechanisms and combinations (rime splintering, spherical freezing fragmentation of drops (mode 1), and ice–ice collisions). There was a significant sensitivity to aerosol particle size distribution when updraft speeds were low (0.5 m s−1); secondary ice formation did not occur when the aerosol particle size distribution mimicked polluted environments. Where secondary ice formation did occur in simulated clouds, significant ice formation in the shallower clouds (1.3 km deep) was due to mode 2 or a combination which included mode 2. The deeper clouds (2.4 km deep) also had significant contributions from rime splintering or ice–ice collisional breakup secondary ice production (SIP) mechanisms. While simulations with cloud-base temperatures of 7 ∘C were relatively insensitive to ice-nucleating particle concentrations, there was a sensitivity in simulations with cloud-base temperatures of 0 ∘C. Increasing the ice-nucleating particle concentration delayed ice formation. Our results suggest that collisions of supercooled water drops with ice particles may be a significant ice formation mechanism within shallow convective clouds where rime splintering is not active.

Research on normal ice adhesion strength in icing wind tunnel

Aircraft icing seriously jeopardizes flight safety. The design of aircraft anti-icing/de-icing systems requires a thorough understanding of the adhesion between the ice and the substrate. In this research, an experimental device that can be housed within a wing is designed and constructed. Simulation analysis of the interfacial stresses is performed, which reveals that increasing the load and the interface size led to a deterioration in the uniformity of stresses at the interface. In addition, the ice layer does not undergo cohesive damage during the tests. The normal ice adhesion strength is evaluated in an icing wind tunnel using the methodology outlined in this paper. Glaze ice exhibits an increase in normal adhesion strength at lower temperatures, whereas the trend is reversed for rime ice. The minimum adhesion strength occurs near the medium volume diameter (MVD) of 30 µm. Furthermore, the normal strength is significantly enhanced by increase in wind speed and surface roughness, as well as by surface painting. The adhesion strength of aluminum substrates to ice is greater compared to titanium and stainless steel. Compared to shear adhesion strength, normal adhesion strength is less sensitive to various influencing factors. The proposed experimental framework provides precise measurement of normal adhesion strength of impact ice in the icing wind tunnel.

An experimental study of rime ice accretion on bundled conductors

The dynamic ice accretion process on bundled electrical power conductors and icing-induced effects on the aerodynamic forces acting on the conductors were examined experimentally. An Icing Research Tunnel was used to generate a typical atmospheric rime icing condition experienced by power transmission cables. Two pieces of Aluminum-Conductor-Steel-Reinforced (ACSR) cables were mounted in the icing wind tunnel with different spacing and angular displacements with respect to the incoming airflow. While ice accretion was found to take place primarily on the frontal surfaces of the bundled conductors, the accreted ice layers resembled well the twisted outer strands of the ACSR conductors. Averaged outer profiles of the iced conductors were elliptical with the semi-minor axis being the radius of the conductors and the semi-major axis passing through the foremost frontal points of the accreted ice layers. The icing process on the windward conductor was almost unaffected by the existence of the leeward conductor. The ice accretion and resultant aerodynamic force acting on the leeward conductor were found to vary significantly, depending on the spacing and angular displacements between the bundled conductors. The windward conductor was found to induce a “shadowing effect” by intercepting airborne supercooled water droplets to prevent them from impinging onto the leeward conductor, resulting in much less ice accretion and smaller aerodynamic drag acting on the leeward conductor. The aerodynamic force measurements were correlated with the acquired ice accretion images and wake flow field around the bundled conductors to elucidate the underlying physics.

Low-Dimensional Models for Aerofoil Icing Predictions

Determining the aero-icing characteristics is key for safety assurance in aviation, but it may be a computationally expensive task. This work presents a framework for the development of low-dimensional models for application to aerofoil icing. The framework builds on: an adaptive sampling strategy to identify the local, nonlinear features across the icing envelope for continuous intermittent icing; a classic technique based on Proper Orthogonal Decomposition, and a modern Neural Network architecture. The extreme diversity in simulated ice shapes, from smooth and streamlined to rugged and irregular shapes, motivated the use of an unsupervised classification of the ice shapes. This allowed deploying the Proper Orthogonal Decomposition locally within each sub-region, sensibly improving the prediction accuracy over the global model. On the other hand, the Neural Network architecture and the convolutional auto-encoder were found insensitive to the complexity in ice shapes. A strong correlation was found to exist between the ice shape, resulting ice mass and aerodynamic performance of the iced aerofoil, both in terms of the average and variance. On average, rime ice causes a loss of maximum lift coefficient of 21.5% compared to a clean aerofoil, and the average ice thickness is 0.9% of the aerofoil chord. For glaze ice, the average loss of maximum lift coefficient is 46.5% and the average ice thickness is 2.1%. Glaze ice was also found to have three times more surface coverage than rime ice.

Future projections of atmospheric icing in Norway

Atmospheric icing is a major weather hazard, known to cause problems for many sectors of society, in particular for power transmission lines. Considering that such structures are designed with a typical expected lifetime of 70 years, climate change impacts on extreme ice loads should be considered. Atmospheric ice accumulation can be modelled with meteorological input from numerical weather prediction models and extreme loads can thereby be estimated. Here we apply dynamical downscaling of the two global climate models, CESM2 and MPI-ESM1‐2-HR, and three future scenarios, using a regional model, WRF, to obtain high resolution data for future atmospheric ice load projections. Results based on both climate models indicate that future 10-year return values of wet snow loads are generally decreasing in the lowlands and increasing at higher elevation for all three scenarios, linked to a general increase in temperature and extreme precipitation. Results for future 10-year rime ice loads differ between the two downscaled models due to differences in regional warming levels and model biases. More research, involving downscaling of more climate models or the use of other methodologies, is needed to obtain more robust projections and uncertainty estimates before design engineers can make any considerations about future ice loads. However, large uncertainties in climate model projections of regional climate challenge the constraint.

Numerical simulations of ice accretion on wind turbine blades: are performance losses due to ice shape or surface roughness?

Abstract. Ice accretion on wind turbine blades causes both a change in the shape of its sections and an increase in surface roughness. These lead to degraded aerodynamic performances and lower power output. Here, a high-fidelity multi-step method is presented and applied to simulate a 3 h rime icing event on the National Renewable Energy Laboratory 5 MW wind turbine blade. Five sections belonging to the outer half of the blade were considered. Independent time steps were applied to each blade section to obtain detailed ice shapes. The roughness effect on airfoil performance was included in computational fluid dynamics simulations using an equivalent sand-grain approach. The aerodynamic coefficients of the iced sections were computed considering two different roughness heights and extensions along the blade surface. The power curve before and after the icing event was computed according to the Design Load Case 1.1 of the International Electrotechnical Commission. In the icing event under analysis, the decrease in power output strongly depended on wind speed and, in fact, tip speed ratio. Regarding the different roughness heights and extensions along the blade, power losses were qualitatively similar but significantly different in magnitude despite the well-developed ice shapes. It was found that extended roughness regions in the chordwise direction of the blade can become as detrimental as the ice shape itself.

Level-Set Mass-Conservative Front-Tracking Technique for Multistep Simulations of In-Flight Ice Accretion

This paper presents a novel level-set-based approach to model evolving boundary problems for in-flight ice accretion. No partial differential equations are solved as in the standard level-set formulation, but simple geometrical quantities are employed to provide an implicit discretization of the updated boundary. This method avoids mesh entanglements and grid intersections typical of algebraic and mesh deforming techniques, making it suitable for generating a body-fitted discretization of arbitrarily complex geometries as in-flight ice shapes, including the collision of separate ice fronts. Moreover, this paper presents a local ice thickness correction, which accounts for the body’s curvature, to conserve the prescribed iced mass locally. The verification includes ice accretion over an ellipse and a manufactured example to show the proposed strategy’s advantages and robustness compared to standard algebraic methods. Finally, the method is applied to ice accretion problems. A temporal and grid convergence study is presented for automatic multistep in-flight simulations over a NACA0012 airfoil in rime, glaze, and mixed ice conditions.

The Impact of Ice Formation on Vertical Axis Wind Turbine Performance and Aerodynamics

This study investigated the impact of ice formation on the performance and aerodynamics of a vertical axis wind turbine (VAWT). This is an area that is becoming more prevalent as VAWTs are installed alongside horizontal axis wind turbines (HAWTs) in high altitude areas with cold and wet climates where ice is likely to form. Computational fluid dynamics (CFD) simulations were performed on a VAWT without icing in Ansys to understand its performance before introducing ice shapes obtained through the LewInt ice accretion software and repeating simulations in Ansys. These simulations were verified by performing a wind tunnel experiment on a scale VAWT model with and without 3D printed ice shapes attached to the blades. The clean blade simulations found that wind speed had little impact on the performance, while reducing the blade scale severely reduced performance. The ice formation simulations found that increasing the icing time or liquid water content (LWC) led to increased ice thickness. Additionally, glaze ice and rime ice conditions were investigated, and it was found that rime ice conditions that occur in lower temperatures caused more ice to form. The simulations with the attached ice shapes found a maximum reduction in performance of 40%, and the experiments found that the ice shapes made the VAWT unable to produce power.

Experimental Study of Dynamic Icing Process on a Pitot Probe Model

An experimental study was conducted to characterize the dynamic ice accretion process over the surface of a typical aeronautic Pitot probe model under different icing conditions. The experimental study was conducted in the Icing Research Tunnel available at Iowa State University. While a high-speed imaging system was used to record the dynamic ice accretion process, a three-dimensional (3D) scanning system was also used to measure the 3D shapes of the ice layers accreted on the test model. While opaque and grainy ice structures were found to accrete mainly along the wedge-shaped lip of the front port and over the front surface of the probe holder under a dry rime icing condition, much more complicated ice structures with transparent and glazy appearance were observed to cover almost entire surface of the Pitot probe under a wet glaze icing condition. While a flower-like ice structure was found to grow rapidly along the front port lip, multiple irregular-shaped ice structures accreted over the probe holder under a mixed icing condition. The characteristics of the icing process under different icing conditions were compared in terms of 3D shapes of the ice structures, the profiles of the accreted ice layers, the ice blockage to the front port, and the total ice mass on the Pitot probe model. The acquired ice accretion images were correlated with the 3D ice shape measurements to elucidate the underlying icing physics.

Machine learning-based ice detection approach for power transmission lines by utilizing FBG micro-meteorological sensors.

Severe icing of transmission lines causes power failures, tower collapses, and other adverse events, which hinders the normal operation of society. Existing icing monitoring methods have problems of irregular monitoring and poor accuracy. In this study, we propose a comprehensive model for predicting hard rime and glaze ice using temperature, humidity, and historical icing data. The results of the experimental verification conducted for nine icing cycles in different years and geographic locations demonstrate that the proposed technique can effectively predict multiple types of icing while ensuring correlation coefficients > 0.99 and mean squared error < 4%.

Experimental study on icing and anti-icing of a rotating conical spinner

Icing and hot-air anti-icing tests of a rotating conical spinner were carried out in an icing wind tunnel. The surface temperature was measured using a calibrated infrared thermal imager. The icing process and the water film flow on the spinner were recorded by a high-speed camera. In the icing test with temperature of −21.7 °C and LWC (liquid water content) of 0.5 g/m3, white rime ice grows and covers the entire surface. From the cone tip the ice thickness decreases from 5.8 mm to a minimum of 3.2 mm, while in the rear part the ice thickness increases. In the icing test with temperature of −9.4 °C and LWC of 1.0 g/m3, long pine needle-shaped ice forms downstream the cone tip area. The anti-icing surface temperature first increases and then decreases along the spinner generatrix, with a maximum value near the tip. Under nearly identical incoming flow conditions the higher the mass flow of hot air is, the greater the decrease in surface temperature is. For the anti-icing tests with LWC of 0.5 g/m3, from t = 40 s, as the mass flow of the hot air is low the spinner surface is completely covered by water film. There exists a complete evaporation area near the cone tip with a high mass flow. For the anti-icing tests with LWC of 1.0 g/m3, from t = 40 s the spinner surface is completely covered by water film in all test runs.

Influence of positive DC electric field on rime icing on the insulator and its experimental verification in the natural environment

AbstractThe electric field imposes a significant effect on ice accretion on the insulator. The impact of an AC electric field on insulator icing in an artificial climate chamber has been addressed. Compared with AC, DC electric field exerts an attraction on supercooled water droplets, which is more detrimental to the power system. Moreover, the icing on energised insulators is rarely studied in the natural environment. In the present research, an electrical, fluid coupled‐field simulation was carried out on the insulator. The force characteristics and motion deviation features of water droplets at different positive DC electric field strengths were studied. On this basis, rime icing on the insulator energised with different positive DC voltages was investigated in the field. Results show that the charged droplet shifts along the electric field line under a positive DC voltage. The deviation degree of the droplet increases with the increment of the voltage and droplet size and decreases with an increase in wind speed. As the voltage rises, the surface roughness, icing amount and icing length of rime ice‐covered insulator increase. Consequently, compared with that without energisation, the local collision coefficient, icing amount and icing length increase by 17.6%, 46.4% and 63.9%, respectively, under a 40 kV DC voltage.

Scaling formulation of multiphase flow and droplet trajectories with rime ice accretion on a rotating wind turbine blade

In this paper, scaling and similitude are investigated for ice accretion on a rotating wind turbine blade. Numerical CFD icing simulations are performed using FENSAP ICE software to test the scaling methods and verify the results. A small blade section is scaled six times, and each case is tested at specific flow conditions. Scaled conditions for velocity (streamwise and rotational), droplet size, and icing time are examined. CFD solutions for the flow field (air and droplet) are obtained in terms of the velocity, droplet trajectories, pressure coefficient distributions, ice thickness, and ice shapes, by quantifying the significant parameters involved in the icing process. Recommendations for parameters to be used for rime ice scaling on a rotating blade are presented, and numerical results are reported to support those recommendations. Numerical results and test conditions are obtained at sea level in wind tunnel facilities for experimental investigation. This paper presents the formulation of non-dimensionlized governing equations for scaling of the flow field, droplet trajectories, and ice accretion on a rotating blade model. It derives a set of non-dimensional groups which govern the flow parameters for a rotating blade, including the Strouhal number, Froude number, drag coefficient of droplets and droplet based Reynolds number, droplet inertia parameter, and two other new parameters. The scaling methodology can also be utilized to determine alternative test conditions to predict rime ice conditions on a rotating blade. The study provides valuable insight to predict ice accretion on large wind turbine blade sections in the field based on scaled smaller blade sections tested in a laboratory setting.

An experimental study of dynamic icing process on an Aluminum-Conductor-Steel-Reinforced power cable with twisted outer strands

An experimental campaign was conducted to investigate the dynamic icing process on an Aluminum-Conductor-Steel-Reinforced (ACSR) power cable with twisted outer strands and characterize the resultant wind loads experienced by the power cable under both dry rime and wet glaze icing conditions. In addition to recording the dynamic icing process over the cable surface with a high-resolution imaging system, the three-dimensional (3D) shapes of the ice layers accreted on the cable model were also quantified by using a novel 3D profile scanning system. While the evolution of the wake flow behind the iced ACSR cable was quantified with a Particle Image Velocimetry (PIV) system, the variation of the wind loads experienced by the cable model was also examined based on the measurements of force/moment transducers. Under the dry rime icing condition, opaque, grainy ice was found to accrete only within the impinging zone of airborne water droplets over the cable frontal surface without any noticeable water runback flow over the cable surface. In comparison, substantial unfrozen water was observed to run back over the cable surface under the wet glaze icing condition, causing the formation of transparent, glazy ice layer over a much wider area over the cable surface. Correlating with the formation of more complicated glazy ice humps/horns over the ACSR cable surface, the accumulated ice mass was found to grow much faster during glaze icing process, in comparison to that of rime icing case. While the resultant wind loads acting on the iced ACSR cable was found to decrease gradually with the increasing icing time under the rime icing condition, the corresponding values were found to increase continuously under the glaze icing condition.

Three-dimensional numerical simulation of rime ice accumulation on silicone rubber insulator and its experimental verification in the natural environment

Icing on transmission lines is a challenging problem in the world. Rime icing on silicone rubber insulator was taken as research object in this paper. The position coordinates of supercooled water droplets collided with the surface of the insulator were extracted, the local collision coefficient (E) of the droplet was calculated by a triangular area ratio method. The “point-line-face-solid” ice shape reconstruction algorithm was proposed to realize three-dimensional numerical simulation of rime ice accretion on the insulator. The results shown that the effect of median volume diameter (MVD) of the droplet on the E is greater than that of wind speed (v). The most serious icing occurs on the rod and the edge of the shed. Due to the shielding effect of the shed, the icing on the rod presents the shape of the nose. The length, thickness and amount of rime icing on the insulator calculated by the model increases with time, whose average errors with that obtained by the tests are 7.68%, 7.96%, 10.34%, and 9.45%, respectively. Moreover, the simulated icing distribution is well consistent with the experimental results. Therefore, the effectiveness of the numerical simulation method was proved in the present research.

Three‐Dimensional Numerical Simulation of Rime Ice Accretion on Polymeric Insulator Based on Computational Fluid Dynamics and its Experimental Verification

AbstractIce accretion on transmission lines seriously threaten the stable operation of power grid. The current research focuses on the discharge development and flashover characteristics of ice‐covered insulator. Due to the irregular structure of the insulator, very few studies concentrate on the mechanism of ice accumulation. In the present study, the local collision coefficient was determined, and then a three‐dimensional numerical model for rime ice deposit accumulation on polymeric insulator was established based on computational fluid dynamics and verified by experiments. Results show that the local collision efficiency decreases along the rod and the edge of the shed from the front stagnation point to both sides, whose maximum values are 0.73 and 0.74 in the research scope. Rime ice amount on the insulator increases with the increment of environmental parameters including wind velocity, liquid water content, and median volume diameter of water droplet. It was proved by numerical simulation and artificial tests that the position with a high collision efficiency is heavily covered with rime ice. Through comparison and analysis, the numerical model can well simulate the process of rime ice accretion on polymeric insulator, and the relative error of ice amount between calculated by the model and that obtained by the test is less than 18.8%. The research in this paper realizes the visualization of rime ice‐coating process of polymeric insulator. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.