41
Zaxis Inc. LLC
801.264.1000
2442 South 2570 West
West Valley City, UT 84119
Air Density — If the atmosphere was like water and incompressible, pressure would
decrease uniformly as you went up. In reality, the atmosphere is compressible and den-
sity (mass per unit volume) is proportional to pressure. This relationship, call Boyle’s
Law, implies that density decreases with height in atmosphere. As height increases, less
mass remains above a given point; therefore, less pressure is exerted. At sea level, the
density of air is about 1 kg per cubic meter (8 oz. per cubic foot). Both pressure and
density decrease by about a factor of 10 for every 16 km (10 miles) increase in altitude.
Density does not depend solely on pressure. For a given pressure, density is inversely
proportional to temperature. This relationship, known as Charles’s Law, implies that the
depth of an air column bounded by two constant-pressure surfaces will increase as the
temperature in the column increases.
Density varies mostly with pressure over large vertical distances; at constant height,
pressure variation with temperature becomes important. In the low atmosphere, air is
heavy, with a stable mass of roughly one kilogram per cubic meter (1 oz/cubic foot). A
room of 500 cubic meters (650 cubic yards) thus contains 0.05 metric ton of air. At an
altitude of 3 km (2 miles); however, density is 30% less than at sea level.
This difference in air density can cause variations in ow readings from one location to
another when elevations are quite different and no corrections are made.
Fluids vs. Solids — The distinguishing feature of a uid (gas or liquid), in contrast to a
solid, is how easily the uid can be deformed. If a shearing force — even a very small
force — is applied to a uid, the uid will move and continue to move as long as the
shear acts on it. For example, the force of gravity causes water poured from a cup to
ow. Water continues to ow as long as the cup is tilted. If the cup is turned back up,
the ow stops. The wall of the cup has balanced the forces.
Gas vs. Liquid — Unlike liquids, gases cannot be poured as easily from one open con-
tainer into another, but they deform under shear stress just the same. Because shear
stresses result from relative motion, stresses are equivalent whether the uid ows past
a stationary object or the object moves through the uid.
Although a uid can deform easily under an applied force, the uid’s viscosity creates
resistance to this force. The viscosity of gases, which is much less than that of liquids,
increases slightly as the temperature increases, whereas that of liquids decreases when
the temperature increases. Fluid mechanics is mostly concerned with Newtonian uids,
or those in which stress, viscosity, and rate of strain are linearly related.
To Next Page
Loading...
