Semester

Spring

Date of Graduation

2010

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Wade W. Huebsch

Committee Co-Chair

David C. Lewellen

Abstract

Aircraft contrails are a common phenomenon observed in the sky. They are formed mainly of water, from the ambient atmosphere and as a by-product of the combustion process, in the form of ice crystals. They have been identified as a potential contributor to global warming. Some contrails can be long-lived and create man-made cloud cover, thus possibly altering the radiative balance of the earth. There has been a great deal of research on various aspects of contrail development, but to date, little has been done on the influence of ice crystal shapes on the contrail evolution. In-situ studies have reported that young contrails are mainly quasi-spherical crystals while older contrails can have a much more diverse spectrum of possible shapes. The most common shapes found in contrails are quasi-spherical, hexagonal columns, hexagonal plates, and bullet rosettes. Numerical simulations of contrails to date typically have assumed "spherical" as the default ice shape. This work simulated contrail development with a large eddy simulation (LES) model that implemented both spherical and non-spherical shapes to examine the effects. The included shape effect parameters, such as capacitance coefficient, ventilation factor, Kelvin effect, fall velocity and ice crystal surface area, help to establish the shape difference in the results. This study also investigated initial sensitivities to an additional ice parameter, the ice deposition coefficient. The literature shows conflicting values for this coefficient over a wide range.;In the course of this investigation a comparison of various ice metrics was made for simulations with different assumed crystal shapes (spheres, hexagonal columns, hexagonal plates, bullet rosettes and combination of shapes). The simulations were performed at early and late contrail time, with a range of ice crystal sizes, and with/without coupled radiation. In young and older contrails and without coupled radiation, the difference from the shape effect in ice crystal number, N(t), is not significant compared with the level of uncertainty. In young contrails, the difference between spherical and non-spherical shapes in N(t) is less than 7% for relatively large ice particles and 23% for relatively small ice particles. The ice mass, M(t), is not significantly affected by the crystal shapes, with less than 8% difference. However, the ice surface area, S(t), is the ice metric more sensitive to crystal shape, with a maximum difference of 68%. It increases at late time, though it is mainly governed by geometrical rather than dynamical effects.;The small sensitivity to shape effects in the ice contrail metrics when radiation is not included suggests that the spherical shape will provide a reasonable representation for all shapes found in the in-situ studies. The radiation is included at late time, when the lasting effects of contrails are more critical. The inclusion of coupled radiation increases the level of dispersion in the results and hence increases slightly the differences due to shape effects. The small difference is also observed in the infrared heating rates of contrails.

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