Uson SPRINT LC, Owner'S Manual

Page: 156 / 208

Page 156
Fluids vs. Solids
The distinguishing feature of a fluid (gas or liquid), in contrast to a solid,
is how easily the fluid can be deformed. If a shearing force, even a very
small force, is applied to a fluid, the fluid 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 flow. Water continues to flow as long as the
cup is tilted. If the cup is turned back up, the flow stops because the
gravitational force is then exactly balanced by force of the cup wall.
Gas vs. Liquid
Unlike liquids, gases cannot be poured from one open container into
another, but they deform under shear stress just the same. Because shear
stresses result from relative motion, stresses are equivalent whether the
fluid flows past a stationary object or the object moves through the fluid.
Although a fluid can deform easily under an applied force, the fluid’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 fluids, or
those in which stress, viscosity, and rate of strain are linearly related.
Pressure and Density
Pressure and density are considered mechanical properties of the fluid,
although they are also thermodynamic properties related to the
temperature and entropy of the fluid. For a small change in pressure, the
density of a gas is essentially unaffected. For this reason, gas and all
liquids can be considered incompressible. If density changes are
significant in flow problems, however, then the flow must be considered
compressible. Compressibility affects results when the speed of the flow
approaches the speed of sound.