Glaze Ice Shapes Research Articles

Numerical investigation of dynamic icing of wind turbine blades under wind shear conditions

In the wind shear icing environment, the periodically varying inflow conditions experienced by wind turbine blades can significantly enhance the dynamic icing characteristics, which poses a great challenge to wind turbine icing research. To investigate this phenomenon, the Improved Multi-Shot Icing Computational Model (IMSICM) is utilized. In IMSICM, an efficient dynamic icing computational frame is designed by using the 3D Free Wake Lifting Line method coupled with the 2D Viscous and Inviscid Interaction method to calculate the flow field. Meanwhile, the Lagrangian method is applied to compute the water droplet impingement information and the Messinger Model is used to simulate the ice growth. After validation of IMSICM by the icing wind tunnel experimental results, the wind shear icing processes of the NREL 5 MW wind turbine under different wind shear conditions are analyzed. The results show that different from the linear growth behavior of Rime ice, the nonlinear features of Glaze ice development are significant. The wind shear effect has a greater impact on the Glaze ice shape, mainly manifested as the ice protrusions behind the main ice horn are suppressed. This phenomenon is mainly due to the combined effect of water impingement shadowing and heat transfer reduction.

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.

Computational simulation of ice accretion and shedding trajectory of a rotorcraft in forward flight with strong rotor wakes

To ensure safe flight in icing conditions it is essential to understand ice accretion, its effect on aerodynamic performance and ice shedding in any aircraft certification program. Ice fragments shed from a rotorcraft windshield and other locations can be detrimental if they are ingested into the engine intake or impinge on the tail rotor. Because of the lack of experimental data about the forces and moments acting on ice in such a complex flow field, the computational simulation of ice shedding trajectories becomes essential. This study presents a methodology to predict the location of ice accretion and break-off, ice shape and shedding trajectory in a rotorcraft flow field with strong rotor wakes during forward flight. The methodology includes the creation of an aerodynamic database for different ice shapes (rectangle, disc, ellipse, and glaze ice shape) at various combinations of Euler angles; the analysis of rotorcraft flow field by computational fluid dynamics for different advance ratios; and a six degree-of-freedom trajectory analysis using artificial neural networks and the Monte Carlo method. The actuator surface method was applied to account for the rotor wake effect, which is capable of modeling the tip vortices and inboard sheets emanating from the rotor blades. The main results are a probability map of the ice shedding trajectory footprints on the engine intake and tail rotor planes. Disc-shaped ice fragments with a sharp edge turned out to be most dangerous. The rotation of the main rotors also substantially affected ice accretion and shed trajectory, indicating the importance of integrated simulations of all components when designing ice protection systems for rotorcraft.

Theory and Validation of a 2D Finite-Volume Integral Boundary-Layer Method for Icing Applications

A two-dimensional integral boundary-layer method is developed to enable fast and economical computations of boundary-layer flows. The ultimate goal is to provide some experience for the extension of this method in three dimensions. In this study, the unsteady momentum and kinetic energy integral equations are solved numerically, together with a set of closure relations based on assumed velocity profiles for laminar and turbulent flows. The robustness of the method is ensured by a finite-volume formulation based on an upwind scheme and a semi-implicit time discretization. Moreover a control method has been developed in order to avoid the Goldstein singularity. The accuracy of the numerical method in the vicinity of the stagnation point is strongly improved by introducing a consistent corrective source term in the right-hand side of the equation system. The chosen closure relations are validated with test cases of self-similar flows. Numerical results are also compared with those of a full Prandtl equations code for NACA0012, GLC305, and MS317 airfoils test cases to demonstrate the capabilities of the method. Finally, preliminary results are shown proving the ability of the method to deal with iced airfoils even for complex glaze ice shapes.

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.

Scaled ice accretion experiments on a rotating wind turbine blade

This work presents ice accretion tests of a model wind turbine blade mounted on a rotor test stand. The model test blade was scaled to reproduce local flow and icing conditions at the 95% radial station of the NREL Phase VI Rotor. A modified Ruff scaling method was implemented to scale atmospheric icing conditions that could be generated in a laboratory facility. Scaling laws allowed for reduced testing time as well as a reduced chord length of the blades compared to on-site icing events were implemented. Classical Blade Element Momentum Theory (BEMT) was utilized to correlate inflow conditions at the test facility to actual operating conditions of the NREL Phase VI Rotor. Experimentally obtained rime ice shapes are compared to LEWICE predictions and are used to validate the capability of the facility to reproduce representative icing conditions. The rime ice results matched with predictions to within 1% of both ice thickness and ice extent along the blade surfaces. The effects of angle-of-attack, temperature, liquid water content, and icing time on the final ice shapes were investigated in a parametric study. Experimentally obtained glaze ice shapes are used to demonstrate the severity of the icing events on wind turbine torque. The present work comprises testing procedures and experimental data that can be used for future efforts in ice accretion modeling and testing.

An ALE mesh movement scheme for long‐term in‐flight ice accretion

SUMMARYThe rather irregular shapes that glaze ice may grow into while accreting over the surface of an aircraft represent a major difficulty in the numerical simulation of long periods of in‐flight icing. There is a constant need for remeshing: a wasteful procedure. In the framework of ALE formulations, a mesh movement scheme is presented, in which frame and elasticity analogies are loosely coupled. The resulting deformed mesh preserves the quality of elements, especially in the near‐wall region, where accurate prediction of heat flux and shear stresses is required. The proposed scheme handles mesh deformation in a computationally efficient manner by localizing the mesh deformation. The 2D problem of ice accretion over single and multi‐element airfoils is considered here as a numerical experiment. Experimentally measured glaze ice shapes were used to evaluate the performance of the present approach. Copyright © 2011 John Wiley & Sons, Ltd.

Ice Shape Characterization Using Self-Organizing Maps

A method for characterizing ice shapes using a self-organizing map (SOM) technique is presented. Self-organizing maps are neural-network techniques for representing noisy, multi-dimensional data aligned along a lower-dimensional and possibly nonlinear manifold. For a large set of noisy data, each element of a finite set of codebook vectors is iteratively moved in the direction of the data closest to the winner codebook vector. Through successive iterations, the codebook vectors begin to align with the trends of the higherdimensional data. In information processing, the intent of SOM methods is to transmit the codebook vector set, which contains far fewer elements and requires much less memory or bandwidth than the original noisy data set. When applied to airfoil ice accretion shapes, the properties of the codebook vectors and the statistical nature of the SOM methods allows for a quantitative comparison of experimentally measured mean or average ice shapes to ice shapes predicted using computer codes such as LEWICE. In the present study, SOM characterizations are applied to a rime ice shape, a glaze ice shape at an angle of attack, a bimodal glaze ice shape, and a multi-horn glaze ice shape. Improvements and future explorations are discussed.

Asymmetrical Flow Simulation of Icing Effects in S-Duct Inlets at Angle of Attack

The effect of flow angularity on an S-duct inlet with icing is computationally investigated. Flow angularity is simulated through angle-of-attack, and sideslip in addition to asymmetrical ice accretion on the inlet lip. A commercial CFD code, STAR-CCM+ is used for the steadystate computations with the shear-stress transport (SST) k! turbulence model. Symmetrical and asymmetrical glaze ice shapes are computationally simulated on the inlet lip. The symmetrical glaze ice uniformly covers the entire cowl lip; whereas the asymmetrical glaze ice is simulated on a 1=4 sector of the inlet lip and is positioned on top, bottom or side of the inlet lip. The results indicate that flow angularity, whether in angle-of-attack or sideslip, aggravates the low performance of inlets with icing. The total pressure recovery suffers an additional 2% loss and the inlet mass flow rate drops by 7% when the inlet is at C20 angle of attack, as compared to zero angle, for flight Mach number of 0.34. The extent of loss in total pressure and a drop in mass flow rate depends on the asymmetrical icing location as well as the inlet angle-of-attack and sideslip. In addition, the ice-induced flow blockage is identified as a critical inlet performance parameter, since the symmetrical (360) glaze ice with its wider flow blockage creates a lower total pressure recovery than the asymmetrical (90) glaze ice at all angles of attack or sideslip.

FENSAP-ICE-Unsteady: Unified In-Flight Icing Simulation Methodology for Aircraft, Rotorcraft, and Jet Engines

In-flight ice accretion, even though driven by a steady flow airstream, is an inherently unsteady phenomenon. It is, however, completely ignored in icing simulation codes (one-shot) or, at best approximated via quasi-steady modeling (multishot). The final ice shapes thus depend on the length of the total accretion time (one-shot), or of the arbitrarily prescribed time intervals (multishot), during which the impact of ice growth on both airflow and water impingement is ignored. Such a longstanding heuristic approximation is removed in this paper by coupling in time the dilute two-phase flow (air and water droplets flow) with ice accretion, and is implemented in a new code, FENSAP-ICE-Unsteady. The two-phase flow is solved using the coupled Navier–Stokes and water concentration equations, and the water film characteristics and ice shapes are obtained from laws of conservation of mass and energy within the thin film layer. To continually update the geometry of the iced surface in time, arbitrary Lagrangian–Eulerian terms are added to all governing equations to account for mesh movement in the case of stationary components. In the case of rotating/stationary interacting components, a dynamically stitched grid is used. The numerical results clearly show that unsteady modeling improves the accuracy of both rime and glaze ice shape prediction, compared with the traditional quasi-steady approach with frozen solutions. The unsteady model is shown to open the door for a unified approach to icing on fixed wings, on helicopters with blades/rotors/fuselage systems. Problems of current concern in the icing community such as the ingestion of ice crystals at high altitude become tractable with the new formulation.

Heat Transfer Coefficient Measurement on Iced Airfoil in Small Icing Wind Tunnel

The local convective heat transfer coefe cient distribution is measured on an iced airfoil in a small closed-loop icing tunnel at the Centre d’ Essais des Propulseurs, the French engine test center. The airfoil surface is heated with a modulated laser source, and the heat e ux variation of the ice is recorded with an infrared camera. The method is validated on a cylinderin dry air and in icing conditions. Themethod isthen applied to a 145-mm-chord airfoil covered with typical rime, glaze, and mixed ice shapes. The effect of air velocity is studied by performing tests at airspeeds from 80 to 123 m/s (Reynolds numbers from 1 :25 ££ 10 6 to 1:9 ££ 10 6). It was found that the heat transfer coefe cient strongly depends on the surface condition of the ice. In relatively smooth zones, values are approximately equal to 400 W/m 2± C, whereas values greater than 1000 W/m 2± C are observed in nonuniform and rough areas. Results and limitations of the method are discussed and compared to numerical results obtained on similar ice shapes.

Modeling of surface roughness effects on glaze ice accretion

The cause and effects of roughness on accreting glaze ice surfaces were studied with microvideo observations. Distinct zones of surface water behavior were observed, including a smooth wet zone in the stagnation region with a uniform water film, a rough zone where surface tension effects caused coalescence of surface water into stationary beads, and a zone where roughness elements grow into horn shapes. In addition, a zone where surface water ran back as rivulets and a dry zone where rime feathers formed were observed. The locations and behaviors of these zones are discussed. A simple multizone modification to the glaze ice accretion model is proposed to include spatial variability in surface roughness. Two test cases using the multizone model showed significant improvements for the prediction of glaze ice shapes.