Freezing Rain Conditions Research Articles

Ice accretion, shedding, and melting on cable-stayed bridges: A laboratory performance assessment

Ice accretion on cable-stayed bridge surfaces increases loading, affects the structures' aerodynamic behavior, and heightens the risk of ice pieces detaching and falling onto bridge traffic. Here we propose a comprehensive laboratory-based procedure for evaluating materials to reduce ice accretion and prevent ice shedding on cable-stayed bridge. We then assess our laboratory-based procedure by testing four high-density polyethylene sheath configurations for cable-stayed bridges. These sheaths differ in their surface finishes, chemistry, and surface roughness. We present a three-step controlled-environment procedure involving (i) the physical characterization of the surface; (ii) an evaluation of surface behavior under laboratory icing tests; and (iii) an assessment of ice accretion at −10 °C under freezing rain conditions, followed by shedding and melting phases. This testing procedure provides a simple means of evaluating geometric changes and surface modifications, including icephobicity, under repeatable and comparable conditions. Our approach could serve in the design and optimization of materials prior to their deployment under actual operational conditions.

Numerical Modeling of Ice Accumulation on Three-Dimensional Bridge Cables under Freezing Rain and Natural Wind Conditions

In order to accurately predict the ice accumulation on bridge cables under two typical freezing rain conditions, rime and glaze ice, this paper proposes a numerical simulation framework based on the three-dimensional Messinger theory. Two technical challenges of determining the flow direction of unfrozen water and solving three-dimensional Messinger equations are solved in this research. Based on the outflow, mass was calculated according to the three-dimensional Messinger theory, and the flow direction of unfrozen water in each cell was determined by the resultant force of air shear stress and water film gravity. To solve the three-dimensional equations, an iterative method without finding the stagnation line was introduced. The final iced geometries were determined when the inflow mass ratio was satisfied with the converge criteria. Moreover, this modified numerical model was programmed and embedded into computational fluid software. For both two typical freezing rain conditions, the effects of temperature and wind speed on iced geometries were studied. The aerodynamic characteristics and galloping instability of bridge cables with different iced geometries were also investigated. These preliminary aerodynamic simulations can provide the basis for the wind-induced vibration analysis of the whole structure.

Differentiating Freezing Drizzle and Freezing Rain in HRRR Model Forecasts

AbstractSupercooled large drop (SLD) icing poses a unique hazard for aircraft and has resulted in new regulations regarding aircraft certification to fly in regions of known or forecast SLD icing conditions. The new regulations define two SLD icing categories based upon the maximum supercooled liquid water drop diameter (Dmax): freezing drizzle (100–500 μm) and freezing rain (>500 μm). Recent upgrades to U.S. operational numerical weather prediction models lay a foundation to provide more relevant aircraft icing guidance including the potential to predict explicit drop size. The primary focus of this paper is to evaluate a proposed method for estimating the maximum drop size from model forecast data to differentiate freezing drizzle from freezing rain conditions. Using in situ cloud microphysical measurements collected in icing conditions during two field campaigns between January and March 2017, this study shows that the High-Resolution Rapid Refresh model is capable of distinguishing SLD icing categories of freezing drizzle and freezing rain using a Dmax extracted from the rain category of the microphysics output. It is shown that the extracted Dmax from the model correctly predicted the observed SLD icing category as much as 99% of the time when the HRRR accurately forecast SLD conditions; however, performance varied by the method to define Dmax and by the field campaign dataset used for verification.

Predicting ice accretion from freezing rain on bridge stay cables

An morphogenetic model that simulates ice accretion on arbitrarily oriented cables exposed to freezing rain has been developed at the National Research Council Canada (NRC). This predictive numerical model has been validated experimentally for a limited number of conditions in the NRC Climatic Testing Facility. The facility is 6 m wide, 6 m high and 30 m long, and can generate a range of freezing precipitation environments. Inclined stay cable models with a diameter of 0.218 m and a length of 2.4 m were exposed to simulated freezing rain. The final experimental ice accretion shapes were scanned and digitized using a handheld laser scanner. These three-dimensional experimental ice shapes were used to partially validate the NRC morphogenetic model. The successfully validated model was used to examine ice accretion on stay cable geometry in an environment representative of freezing rain conditions in Eastern Canada. Prediction of wet snow accretion on already ice covered stay cables has also been undertaken.

Effect of Ice Accretion on the Aerodynamic Responses of a Pipeline Suspension Bridge

Abstract Ice accretion is a major concern that may endanger the operation safety of structures and even cause serious casualties in cold regions. Under freezing rain conditions, the size and shape ...

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>

Study on effectiveness of anti-icing and deicing performance of super-hydrophobic asphalt concrete

Super-hydrophobic asphalt concrete specimens were collaboratively designed and prepared by using modified hydrophobic materials and nano-materials. The anti-icing and deicing performance of super-hydrophobic asphalt concrete at different working conditions was compared to the traditional asphalt concrete through the combination of simulation test and theoretical analysis. The ice-repellency of super-hydrophobic asphalt concrete was characterized by means of contact angle and surface energy. It was found that the super-hydrophobic asphalt concrete could effectively promote the water drop rolling from its surface, and the rolling rate could reach 80%. Compared to that of the traditional asphalt concrete, super-hydrophobic asphalt concrete could significantly reduce the adhesion forces between ice and road. The observed effect of super-hydrophobic asphalt concrete was a delay of frost formation and evolution. It was indicated that the residual rate of ice on super-hydrophobic asphalt concrete was about 21.6% and 48.6% of those on traditional asphalt concrete respectively when freezing snow and freezing rain conditions came. It was proved that super-hydrophobic material had better durability by abrasion test. Theoretical results showed that the surface energy on the surface of super-hydrophobic specimens was about 5% of this on traditional asphalt concrete. It was proved that the skid resistance performance of super-hydrophobic asphalt concrete could conform to the requirements of the specification and the economic benefit of super-hydrophobic asphalt pavement material is considerable in the long run. The findings demonstrated the potential of using super-hydrophobic asphalt concrete on highways in adverse weather to provide better anti-icing and deicing performance and contribute to traffic safety in winter.

Study of anti-icing performance of insulator strings bottom-coated with semiconductive silicone rubber coating

Prevention of icing flashover on transmission lines has become an important problem. A new method for mitigating effects of glaze ice on energized insulators has been developed. Semiconductive silicone rubber was painted on the bottom of suspension disc insulators, while the upper ceramic surface was untreated. An electrically conductive path was formed when supercooled water or droplets bridged the uncoated areas. The power loss was limited when there was no precipitation. The heat and partial discharge energy generated by the leakage current melted the original ice accretion on the insulator surface. The electrical performance of the insulator string with this kind of coating under freezing rain conditions was comparable with that of a clean insulator string. The impact of the ambient temperature on the anti-icing performance was also evaluated. In addition, the performance of the insulator with bottom surface semiconductive coating under fog conditions was compared with an uncoated string.

Spatial and Microphysical Characteristics of Low-Ceiling, Temperature-Inverted Clouds in Warm Overrunning and Freezing-Rain Conditions: A Case Study

AbstractIn-flight microphysical measurements in classical freezing-rain conditions were used to study the vertical and horizontal characteristics of the precipitation and associated low-ceiling, stratiform clouds, which are usually present as overcast in freezing-rain conditions. The low overcast is usually based in the surface cold layer but may extend up into the inversion, or transition layer, between the overrunning warm air and the surface cold layer. This gives the cloud an unusual temperature-inverted structure—supercooled in the lower half and warmer than freezing in the upper half. The low cloud is also subject to wind shear and turbulence that is due to the warm overrunning. The apparent effects of this are 1) increased cloud droplet concentrations in clusters up to a few hundred meters wide that occur sporadically in the cloud layer, 2) possible forcing of cloudy air upward from lower levels against the resistance of the temperature inversion and into the transition layer, and 3) highly variable air temperatures during level flight in the inversion layer.

Laboratory Investigations of the Electrical Performance of Ice-covered Insulators and a Metal Oxide Surge Arrester

This review paper summarizes the main results of the laboratory investigations conducted at Tampere University of Technology (Finland) on ice-covered insulators and a metal oxide surge arrester (MOA). The most important factors affecting the dielectric strength of high voltage (HV) insulators in freezing rain conditions are the resistivity of the freezing water, the length and the number of icicles between insulator sheds and electrodes and the state of the ice deposit (i.e. how wet it is). The studies verify that most of the commercially available HV insulators may flashover even with a normal phase-to-ground ac voltage in rather light salty (or otherwise slightly contaminated), but long-duration freezing rain conditions. The ac strength of a clean insulator string under rime ice during melting period was approximately 75% of the ac strength in dry conditions. Accordingly, the ac strength of the contaminated (ESDD ap 0.3 mg/cm2 NaCl) insulator strings during the melting period of rime was on average 65% of the AC strength in dry conditions. The icing of an MOA consisting of two or more units in series may have harmful effects on the electrical performance of the MOA. With ac voltage stress an unevenly ice-coated MOA may be thermally stressed due to the leakage current transition from the ice covering of one unit to the interior of another unit not covered with so much ice. With switching impulse current surges the residual voltage across an ice-covered unit rather easily causes an external flashover. This leads to a sudden increase in the surge current: with the MOA of two units studied the current peak reached as much as twice the peak value in normal operation without ice covering.

A Study of Ice Accretion Shape on Cables Under Freezing Rain Conditions

The influence of atmospheric conditions (specifically precipitation rate and external heat flux) on the freezing rain ice accretion forming on a non-rotating, horizontal cylinder is studied, using an analytical model based on a simple form of the equations for conservation of mass and heat balance. In keeping with the freezing rain application, but in order to simplify this first step, we have assumed vertical incidence of precipitation (no wind) and no dripping from the accretion (hence light to moderate precipitation rates with relatively low air temperatures). The initial ice accretion shape and the location of its center of mass are examined as a function of the ratio of the precipitation mass flux to the total heat flux lost from the ice surface. An increase in the flux ratio leads to a quantifiable downward displacement of the accretion center of mass. We complement this analysis with numerical simulations, using an improved, two-dimensional version of the Szilder-Lozowski morphogenetic model that predicts the evolution of the accretion shape. For the first time, the freezing probability, which is the critical model parameter, is expressed as a function of location and atmospheric conditions for an accretion shape evolving with time. Using the morphogenetic model, we examine the influence of atmospheric conditions on the accretion shape and ice load. In particular, we address the question of what gives rise to extreme ice loads by identifying the range of atmospheric conditions that tends to maximize (or minimize) the ice load for a given amount of precipitation. The results of this research are applicable to predicting ice formation on overhead transmission lines.

PREDICTING AND REDUCING GLAZE ICE ACCRETION ON ELECTRIC POWER LINES WITH JOULE HEATING: THEORY AND EXPERIMENTS

An analytical model is developed for the prediction of glaze ice accretion with runback water for electric power lines including the Joule heating effect. In this model, the external air flow is coupled with the liquid film flow by slip (non-zero velocity) boundary conditions a the liquid-air interface. In this way, corrections to previous rime ice models are given in order to account for runback water and its effect on wet ice growth in freezing rain conditions with ambient temperatures slightly below 0[°C]. Also, the process of Joule heating in icing conditions is examined from a thermodynamic optimization viewpoint in a manner which permits efficient power transmission while dissipating heat in order to reduce ice accumulation. Good agreement is achieved between theoretical predictions of the glaze ice accretion and experimental results from the freezing rain simulator at the University of Manitoba.

Flashover test of Artificially iced Insualtors

Test apparatus and procedures were developed to test energized, non-contaminated, 120 kV system insulators under simulated freezing rain conditions. The flashover performances were compared between non-ceramic composite transmission line insulators and standard porcelain 53/4 x 10 inch suspension insulators. Standard and long leak Multicone porcelain insulators were also compared in the icing tests to standard porcelain station post insulators. Icing tests at several voltage stresses provided the full flashover-range of the insulators from the minimum or zero flashovers in ten trials up to the maximum or 10 flashovers in 10 trials. Using the desired insulator voltage stress flashover curve for specific insulator types and the voltage stress applied to the insulator, the comparative probability of flashover due to ice storms on the system can be evaluated for 120 kV, 230 kV and 345 kV systems.

Flashover Tests of Artificially Iced Insulators

Test apparatus and procedures were developed to test energized, non-contaminated, 120 kV system insulators under simulated freezing rain conditions. The flashover performances were compared between non-ceramic composite transmission line insulators and standard porcelain 5 3/4 x 10 inch suspension insulators. Standard and long leak Multicone porcelain insulators were also compared in the icing tests to standard porcelain station post insulators. Icing tests at several voltage stresses provided the full flashover range of the insulators from the minimum or zero flashovers in ten trials up to the maximum or 10 flashovers in 10 trials. Using the desired insulator voltage stress flashover curve for specific insulator types and the voltage stress applied to the insulator, the comparative probability of flashover due to ice storms on the system can be evaluated for 120 kV, 230 kV and 345 kV systems.