Edward P. Lozowski

A Computational Investigation of Water Droplet Trajectories

Abstract The work of Langmuir and Blodgett is widely referenced among researchers in the field of ice accretion who are interested in the impingement characteristics of cloud and spray droplets on circular cylinders. In this paper we check the accuracy of those early calculations by integrating the droplet trajectories on a modern digital computer. We then demonstrate the changes which result when an improved formulation for the drag coefficient is used. Our results verify that the original Langmuir and Blodgett calculations are essentially correct for most practical purposes. Except for inertia parameters between 0.125 and 0.25, the relative differences between their results and our new ones do not exceed 10%. At very low collision efficiencies, however, the relative error is larger, with Langmuir and Blodgett tending to overpredict the collision parameters. Tables of our newly calculated data and fitting functions for the collision parameters are presented. We recommend them for future use because they ...

Computer simulation of marine ice accretion

Marine icing can arguably be considered to be the primogenitor of icing problems, having occasioned grief to mariners long before aircraft or electrical transmission lines were invented. Adopting an historical perspective, we elucidate the nature of the phenomenon and the scientific and engineering approaches which have been embraced to mitigate it. With a view to encouraging other scientists to address this problem, we also identify the critical knowledge gaps which stand in the way of a final solution, paying particular attention to recent developments and future trends in numerical modelling and wind–tunnel experimentation.

On the Median Volume Diameter Approximation for Droplet Collision Efficiency

Abstract In this note, we examine a shortcut for calculating the overall collision efficiency of a droplet spectrum, known as the “median volume diameter” (mvd) approximation. By calculating the overall collision efficiency of a circular cylinder for a variety of natural droplet spectra, first precisely using a spectrum weighting approach, and then as approximated using the mvd, as well as several other representative droplet sizes, we show by comparison that the mvd approximation is a good one, with an average absolute error of about 0.02. While trying to give some mathematical justification for why the mvd approximation works, we show that it can be derived from a single-point numerical integration formula, and that extension of this formula to 2, 3 or 4 points should give correspondingly better approximations. Detailed comparisons confirm that use of the 2-point formula reduces the average error by one-half, while the 3- and 4-point formulae can reduce it even more, depending on the type of spectrum.

Weather conditions associated with the passage of precipitation type transition regions over eastern Newfoundland

Abstract The passage of a winter storm is accompanied by changes in many surface and near‐surface parameters including temperature, humidity, wind, pressure, precipitation rate and type, cloud base height, visibility and accretion. These parameters were measured in association with the passage of precipitation‐type transitions over Newfoundland during the Canadian Atlantic Storms Program II field experiment. Three simple summaries of the observed weather events were developed. These summaries depend on the observed large‐scale synoptic conditions, which include warm fronts, a cold front and a trough.

Modelling ice accretion on non-rotating cylinders — The incorporation of time dependence and internal heat conduction

Abstract A new model of ice accretion on a non-rotating cylinder has been formulated. While similar in approach to the model by Lozowski, Stallabrass, and Hearty (1983), it features several important innovations. These include internal heat conduction, time-dependence, and solution using a high-level computer language specifically designed for simulating continuous systems.

A theoretical spongy spray icing model with surficial structure

In an attempt to improve the predictive capability of atmospheric icing models, we have developed a theoretical model of spongy ice formation, including the surficial morphology under a falling supercooled liquid film. A steady-state model of freshwater spongy spray icing for a stationary vertical cylinder is presented. A falling film submodel accounts for the flow of excess liquid on the icing surface. Traditional heat and mass balance equations at the outer surface of the falling film are formulated, along with heat and mass balances for the falling film and for the dendritic freezing zone. The rate of advance of the icing interface is calculated by analogy with the rate of advance of freely-growing ice crystals in bulk supercooled liquid. This allows, for the first time, the prediction of ice accretion flux and accretion sponginess, for a specific icing configuration and environmental conditions. An analysis of the models sensitivity to spray temperature reveals that spray supercooling enhances both the rate of accretion and its sponginess. A comparison of the models performance with experiments shows rather good agreement, and suggests that further research into the nature of the icing surface and its effect on the accreted ice is warranted.

Precipitation-based conductor cooling model for Dynamic Thermal Rating systems

This paper presents a precipitation-based conductor cooling model for use in power line ampacity rating applications. It is aimed at better modelling a conductors temperature by incorporating line cooling resulting from precipitation falling on power lines. The improved calculations provide gains in additional line capacity for power transmission networks incorporating advanced Dynamic Thermal Rating systems. Depending on the precipitation rate and other atmospheric variables, the initial work presented in this paper suggests that line cooling gains between 1°C to over 20°C may be obtained. The precipitation based cooling model shows that the highest gains are observed for largest line loads, thus providing cooling where it is needed the most.

Character and stability of a wind-driven supercooled water film on an icing surface-I. Laminar heat transfer

The kinetics of freezing of a supercooled water film flowing on an icing surface is considered here for the case of laminar heat transfer through the water film. The linear growth rate of crystallization (LRC) in a supercooled, laminar water film, flowing over an accreting ice surface, is found to be characterized by unique behaviours at different length scales. The icing surface is envisaged as a semi-infinite horizontal flat plate with vertically impinging supercooled water droplets. The water film begins to develop at the edge of the plate and thickens while it collects impinging water as the horizontal wind stress drives it downstream. At very short distances from the Couette flow origin, where convection prevails over conduction through the water film, the LRC reacts in a stable fashion to instantaneous changes in the external thermodynamic parameters. The LRC accelerates or decelerates, responding to variations in the water film thickness. At intermediate distances from the flow origin, the LRC is independent of sudden perturbations in the water film thickness, and is determined only by the mean external parameters that characterize the heat transfer and flow dynamics at the water film surface. At a still greater distance from the flow origin, the LRC is unstable in a shear-driven, supercooled water film, flowing over an accreting ice surface. The instability gives rise to either complete disappearance of the water film or a total change of the momentum and heat transfer regime.

Novel two-dimensional modeling approach for aircraft icing

A new modeling approach to tackle the challenging problem of in-flight icing prediction is formulated and verified. With use of this new approach, termed morphogenetic modeling, the shape, structural details, and density of aircraft ice accretions are predicted by emulating the behavior of individual fluid elements. A two-dimensional, morphogenetic model is used to predict the ice accretion forming on a cylinder over a range of in-flight conditions. The model predicts rime, glaze, and simultaneous glaze and rime accretions

On the growth of marine icicles

Abstract A new model of saline icicle growth has been developed. It predicts that saline icicles should be shorter and broader than their freshwater cousins and that the length growth of saline icicles eventually stops when the tip temperature reaches the ambient temperature, rather than when the water supply to the tip ceases as for pure icicles. Both of these predictions are borne out by a series of cold‐room experiments in which saline icicles were grown under controlled ambient conditions. When the model predictions are compared with the experimental results, the model is found to behave not too badly considering the assumptions of the model and the limitations of the experiments. Final predicted icicle masses are generally too high, but within 7 to 32% of the observed masses.