Development of an Analytic Convection Model for a Heated Multi-Hole Probe for Aircraft Applications.

Ice accretion or icing is a well-known phenomenon that entails a risk for the correct functioning of an aircraft. One of the areas more vulnerable to icing is the air data measuring system. This paper studies the icing protection offered by a heating system installed inside a multi-hole probe. The problem is initially solved analytically, creating a tool that can be used in order to predict the heating performance depending on the flying conditions. Later, the performance of the real system is investigated with a heated five-hole probe prototype in a wind tunnel experiment. The measured results are compared with the predictions made by the analytical model. Last, the icing protection provided by the system is estimated with respect to flying altitude and speed. As a result, a prediction tool that can be used in order to make quick icing risk predictions for straight cylindrical probes is delivered. Furthermore, the study provides some understanding about how parameters like altitude and air speed affect the occurrence of ice accretion.

Linking ice accretion and crown structure: towards a model of the effect of freezing rain on tree canopies.

BACKGROUND AND AIMS: Despite a longstanding interest in variation in tree species vulnerability to ice storm damage, quantitative analyses of the influence of crown structure on within-crown variation in ice accretion are rare. In particular, the effect of prior interception by higher branches on lower branch accumulation remains unstudied. The aim of this study was to test the hypothesis that intra-crown ice accretion can be predicted by a measure of the degree of sheltering by neighbouring branches. METHODS: Freezing rain was artificially applied to Acer platanoides L., and in situ branch-ice thickness was measured directly and from LiDAR point clouds. Two models of freezing rain interception were developed: 'IceCube', which uses point clouds to relate ice accretion to a voxel-based index (sheltering factor; SF) of the sheltering effect of branch elements above a measurement point; and 'IceTree', a simulation model for in silico evaluation of the interception pattern of freezing rain in virtual tree crowns. KEY RESULTS: Intra-crown radial ice accretion varied strongly, declining from the tips to the bases of branches and from the top to the base of the crown. SF for branches varied strongly within the crown, and differences among branches were consistent for a range of model parameters. Intra-crown variation in ice accretion on branches was related to SF (R(2) = 0·46), with in silico results from IceTree supporting empirical relationships from IceCube. CONCLUSIONS: Empirical results and simulations confirmed a key role for crown architecture in determining intra-crown patterns of ice accretion. As suspected, the concentration of freezing rain droplets is attenuated by passage through the upper crown, and thus higher branches accumulate more ice than lower branches. This is the first step in developing a model that can provide a quantitative basis for investigating intra-crown and inter-specific variation in freezing rain damage.

Time-Dependent Mechanical Response of Ice Adhesion on Aluminum Substrates.

Ice adhesion on aerospace-relevant materials is both complex and not well understood. Measuring such adhesion and understanding the underlying physics involved require reliable testing techniques that can yield multifaceted data sets. The latter includes the surface morphology, that is, roughness, and its spatial correlation structure, resolving substrate-induced strain, and direct mechanical testing of adhesion. As part of a continuing investigation of ice adhesion on a relevant surface, we performed time-dependent stress ramps on aluminum surfaces. The temperature range explored, from -20 to -7 °C, allowed spontaneous icing and ice morphologies, namely, below or above -15 °C. Additionally, we characterized the spatial correlation surface roughness maps of the specimens. Our novel test protocol yields reproducible and high-precision results when compared with alternative methods reported throughout the literature. The stress-ramp test data using the proposed protocol show that the apparent average critical stress (proportional to the adhesion strength) depends on both stress-ramp rate and temperature. More specifically, the adhesion strength is higher for higher stress rates and increases with decreasing temperature. The stress-ramp test yields the full span of the time-dependent adhesive behavior of ice and particularly the upper bound. Additional stress-concentration analysis is needed to correct for this effect and thereby yield the critical stress rather than the average value produced by our procedure. The results in this work should aid to improve our understanding of ice adhesion mechanisms.

Improved Icephobic Properties on Surfaces with a Hydrophilic Lubricating Liquid.

Slippery liquid-infused porous surfaces were developed recently for icephobic surface applications. Perfluorinated liquids, silicone oil, hydrocarbon, and water were used as lubricating liquids to form a continuous layer on a suitable substrate to prevent icing. However, ice accretion performances of these surfaces have not been reported previously depending on the type of the lubricant. In this work, fluorinated aliphatics, polyalphaolefin, silicone oil, and decamethylcyclopenta siloxane were used as hydrophobic lubricants; water, ethylene glycol, formamide, and water-glycerine mixture were used as hydrophilic lubricants to be impregnated by hydrophobic polypropylene and hydrophilic cellulose-based filter paper surfaces; ice accretion, drop freezing delay time, and ice adhesion strength properties of these surfaces were examined; and the results were compared to those of the reference surfaces such as aluminum, copper, polypropylene, and polytetrafluoroethylene. An ice accretion test method was also developed to investigate the increase of the mass of formed ice gravimetrically by spraying supercooled water onto these surfaces at different subzero temperatures ranging between -1 and -5 °C. It was determined that hydrophilic solvents (especially a water-glycerine mixture) that impregnated hydrophilic porous surfaces would be a promising candidate for anti-icing applications at -2 °C and 56-83% relative humidity because ice accretion and ice adhesion strength properties of these surface decreased simultaneously in these conditions.