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The basic principles of engineering science are applied in CFD simulations. These involve fluid mechanics and usually various other phenomena that are typically considered to fall within the category of thermal sciences. A brief overview is provided, but for complete development, interested readers not yet familiar with the details are referred to various textbooks (e.g. [1–3]). Physical phenomena specific to glass processes must be accounted for within the framework of three fundamental principles and will be reviewed later with respect to a few chosen examples. Readers are also directed to a volume edited by Krause and Loch [4] for a collection of excellent examples of numerical simulations applied to glass processes.

Forming the foundation on which CFD models are constructed, the fundamental principles of classical physics account for conservation of mass, momentum, and energy. Conservation of momentum follows from Newton's three laws of motion, whereas energy conservation is of course the first law of thermodynamics. Contrary to what is done for the very small systems simulated in theoretical studies (Chapters 2.8 and 2.9), it is impractical to account for the motion or energy level of each individual atom or structural entity at the scale relevant to industrial processes. Instead, it is recognized that, for length scales of engineering practicality, substances can be characterized with intensive properties (i.e. per unit volume or mass). Because it describes the mass per unit volume of a particular substance, density is a simple example of such a property that is independent of the size of the system. This abstraction allows substances to be treated as a continuum and allows for powerful mathematical models to be constructed.