Fundamentals A Basic Laws

The fundamental principles that apply to the analysis of fluid flows are few and can be described by the "conservation laws'':

1. Conservation of mass

2. Conservation of energy (first law of thermodynamics)

3. Conservation of momentum (Newton's second law)

To these may also be added:

4. The second law of thermodynamics

5. Conservation of dimensions ("fruit salad'' law)

6. Conservation of dollars (economics)

These conservation laws are basic and, along with appropriate rate or transport models (discussed below), are the starting point for the solution of every problem.

Although the second law of thermodynamics is not a "conservation law,'' it states that a process can occur spontaneously only if it goes from a state of higher energy to one of lower energy. In practical terms, this means that energy is dissipated (i.e., transformed from useful mechanical energy to low-level thermal energy) by any system that is in a dynamic (nonequili-brium) state. In other words, useful (mechanical) energy associated with resistance to motion, or "friction," is always "lost" or transformed to a less useful form of (thermal) energy. In more mundane terms, this law tells us that, for example, water will run downhill spontaneously but cannot run uphill unless it is "pushed" (i.e., unless mechanical energy is supplied to the fluid from an exterior source).

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