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The application and comparison of multistep ice accretion analyses

Brouwer, W.R. (2015) The application and comparison of multistep ice accretion analyses.

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Abstract:In order to design an effective anti-icing system on aircraft, it is imperative to numerically simulate the ice accretion with high precision. Ice shapes that accrete on an aerofoil can be classified into two major groups, rime ice shapes and glaze ice shapes. Rime ice shapes follow the contour of the aerofoil, yielding a new shape that is very similar to the original aerofoil. Glaze ice usually consist of horns, which can grow quite big, resulting in rather exuberant ice shapes. Numerically simulating ice accretion can be done with either a single step or a multistep method. Because accreting ice can have a big influence on the flow around an aerofoil, calculating the final shape of the ice in one step can give a very different result from reality. A multistep method divides the total accretion time in a number of intervals, running the clean aerofoil for the first interval, resulting in an ice shape. This ice shape is then used to calculate a new flow around the aerofoil, continuing on untill the last time interval. From the cases that have been run in this study it can be concluded that rime ice cases often can be run with a single step method, but that for glaze ice cases, the use of a multistep method is necessary. A grid refinement study has been done, resulting in a rule of thumb for the simulation of three-dimensional aerofoils. Because of the trade-off between computational speed and accuracy of the solution, it was concluded that a gridsize of 1/600th of the chord length of an aerofoil was needed to produce a sufficiently accurate solution. This gridsize has been used in the subsequent analyses of the cases, using small refinement regions for iced areas. When using multistep methods, there is a possibility that the ice will grow in on itself. Because the ice grows outward of the surface normal, it is possible in sharp corners for ice to cross over on itself. This of course, is not a physical solution, hence a code has been developed to repair this ingrowth. When repairing the ice shape it is assumed that the ice normally would have grown on the position where it grew in on itself, adding the missed out ice at this position. Comparing two different ice shapes can be a very troublesome task, by just comparing them visually it is difficult to say which ice shape is a better match. By developing a method of comparison which yields a single scalar, it is possible to quantitatively compare two ice shapes, making it easy to compare different ice shapes with a set ice shape. It is then possible to compare the different step methods with the experimental ice shapes provided by AeroTex for this study. In this study two comparison methods have been developed. The first method makes use of the cosine similarity method, along with the Euclidian distance vector, to give a value between 0 and 1 for two ice shapes. The second method makes use of 6 different geometrical features of an ice shape, along with the Fourier components of the ice shape, also yielding a value between 0 and 1. In both of these methods a value of 0 is a complete match between two ice shapes, and a value of 1 is a complete mismatch. The cosine similarity method serves as a simple indicator to give a preliminary view on the matching of two shapes, whereas the hybrid comparison model is used for the final analysis. It is possible to change the parameters used for the hybrid
Item Type:Internship Report (Master)
AeroTex, United Kingdom
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
Keywords:ice accretion, multistep numerical methods
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