First Principles - Sustainable Design: The Science of Sustainability and Green Engineering - page 66

design is the result of collaboration between architects Herzog & de Meuron,
ArupSport, and the China Architecture Research Group. The structure has
been referred to fondly as the “bird's nest,” its structure also serving as fa¸ade
and morphing into roof. An innovative, inflatable “cushion” system similar to
an inflatable bladder provides infill within the structure to allow the stadium to
regulate the environment within the structure in response to changes in wind,
weather, and solar conditions.

The reciprocal of a substance's density is known as its specific volume ( v ). This

is the volume occupied by a unit mass of a fluid. The units of v are reciprocal

density units (m 3 kg − 1 ). Stated mathematically, this is

v = ρ − 1 (2.14)

The weight of a fluid on the basis of its volume is known as the specific weight

( ). Scientists and engineers sometimes use the term interchangeably with density.

Geoscientists frequently refer to a substance's specific weight. A substance's

γ

is
not an absolute fluid property because it depends on the fluid itself and the local
gravitational force:

γ

γ = gp (2.15)

Specific weight units are the same as those for density (e.g., kg m − 3 ).
The fractional change in a fluid's volume per unit change in pressure at con-
stant temperature is the fluid's coefficient of compressibility. Any fluid can be
compressed in response to the application of pressure ( p ). For example, water's
compressibility at 1 atm is 4.9 10 − 5 atm − 1 . This compares to the lesser com-

×

10 − 6 atm − 1 ) and the greater compressibility of pressibility of mercury (3.9 ×

10 − 3 atm − 1 ). A fluid's bulk modulus E is a function of stress
and strain on the fluid and is a description of its compressibility, and is defined
according to the fluid volume ( V ):

hydrogen (1.6 ×

stress
strain =−

dp
dV

E = (2.16)

/ V 1

10 6 kP at 20 ◦ C.
Surface tension effects occur at liquid surfaces (liquid-liquid,

E is expressed in units of pressure (e.g., kP). Water's E = 2.2 ×

liquid-gas,

liquid-solid interfaces). Surface tension is the force in the liquid surface

normal to a line of unit length drawn in the surface. Surface tension decreases
with temperature and depends on the contact fluid. Surface tension is involved in
capillary rise and drop. Water has a very high

σ

value (approximately 0.07 N m − 2
at 20 ◦ C). Of the environmental fluids, only mercury has a higher

σ

σ value (see