First Principles - Sustainable Design: The Science of Sustainability and Green Engineering - page 41

GREEN MECHANICS

Mechanics is the field of physics concerned with the motion and equilibrium
of bodies within particular frames of reference. Green engineering makes use
of the mechanical principles in practically every aspect of pollution: from the
movement of fluids that carry contaminants, to the forces within substances that
affect their properties, to the relationships between matter and energy within
organisms and ecosystems. Engineering mechanics includes statics and dynamics.
Fluid mechanics and soil mechanics are two particularly important branches of
mechanics to the environment.
Statics is the branch of mechanics that is concerned with bodies at rest with
relation to some frame of reference, with the forces between bodies, and with the
equilibrium of the system. It addresses rigid bodies that are at rest or moving with
constant velocity. Hydrostatics is a branch of statics that is essential to environmental
science and engineering in that it is concerned with the equilibrium of fluids
(liquids and gases) and their stationary interactions with solid bodies, such as
pressure. Although many fluids are considered in green engineering, the principal
fluids are water and air.
Dynamics is the branch of mechanics that deals with forces that change or
move bodies. It is concerned with accelerated motion of bodies. It is an espe-
cially important science and engineering discipline because it is fundamental to
an understanding of the movement of contaminants through the environment.
Dynamics is sometimes used synonymously with kinetics . However, we will treat
kinetics as one of the two branches of dynamics, the other being kinematics.
Dynamics combines the properties of the fluid and the means by which it moves.
This means that continuum fluid mechanics varies by whether a fluid is viscous
or inviscid, compressible or incompressible, and by whether flow is laminar or
turbulent. For example, the properties of the two principal environmental fluids,
water in an aquifer and an air mass in the troposphere, * are shown in Table 2.1.

Table 2.1 Contrasts between Plumes in Groundwater and the Atmosphere

Groundwater Plume Air Mass Plume

General flow type Laminar Turbulent

Compressibility Incompressible Compressible

Viscosity Low viscosity Very low viscosity

(1 × 10 − 3 kg m − 1 s − 1 at 288 K) (1.781 × 10 − 5 kg m − 1 s − 1 at 288 K)

* The troposphere is the lowest part of the earth's atmosphere, where living creatures live. Thus, this is the
predominant focus of green engineering. However, spacecraft and other artificial environments have been the
focus of sustainable designs