energy and momentum.

.
To understand such systems, we must start with the basic laws of mechanics
as they apply to fluids, notably the laws of conservation of mass, energy
and momentum. The term fluid includes both liquids and gases, which,
unlike solids, do not remain in equilibrium when subjected to shearing
forces. The hydrodynamic distinction between liquids and gases is that
gases are easily compressed, whereas liquids have volumes varying only
slightly with temperature and pressure. Gaseous volumes vary directly with
temperature and inversely with pressure, approximately as the perfect-gas
law _pV = nRT_. Nevertheless, for air, flowing at speeds <100ms?1 and
not subject to large imposed variations in pressure or temperature, density
change is negligible; this is the situation for the renewable energy systems
analysed quantitatively in this book. It does not apply to the analysis
of gas turbines, for which specialist texts should be consulted. Therefore,
throughout this text, moving air is considered to have the fluid dynamics
of an incompressible fluid. This considerably simplifies the analysis of most
renewable energy systems.
Many important fluid flows are also steady, i.e. the particular type of flow
pattern at a location does not vary with time. So it is useful to picture a set
of lines, called streamlines, parallel with the velocity vectors at each point.
A further distinction is between laminar and turbulent flow
For example, watch the smoke rising from a smouldering taper in still air.
Near the taper, the smoke rises in an orderly, laminar, stream, with the
paths of neighbouring smoke particles parallel. Further from the taper, theflow becomes chaotic, turbulent, with individual smoke particles intermingling
in three dimensions. Turbulent flow approximates to a steady mean
flow, subject to internal friction caused by the velocity fluctuations. However,
even in turbulence, the airflow remains within well-defined (though
imaginary) streamtubes, as bounded by streamlines.