Page 97 - Aeronautical Engineer Data Book

Section 5

Basic fluid mechanics

5.1 Basic poperties
5.1.1 Basic relationships
Fluids are divided into liquids, which are virtually
incompressible, and gases, which are compress­
ible. A fluid consists of a collection of molecules
in constant motion; a liquid adopts the shape of a
vessel containing it whilst a gas expands to fill any
container in which it is placed. Some basic fluid
relationships are given in Table 5.1.

Table 5.1 Basic fluid relationships
Density ( ) Mass per unit volume.
3
3
Units kg/m (lb/in )
Specific gravity (s) Ratio of density to that of
water, i.e. s = / water
Specific volume (v) Reciprocal of density, i.e. s =
3
3
1/ . Units m /kg (in /lb)
Dynamic viscosity ( ) A force per unit area or shear
stress of a fluid. Units Ns/m 2
2
(lbf.s/ft )
Kinematic viscosity ( ) A ratio of dynamic viscosity to
2
density, i.e. = µ/ . Units m /s
2
(ft /sec)
5.1.2 Perfect gas
A perfect (or ‘ideal’) gas is one which follows
Boyle’s/Charles’ law pv = RT where:
p = pressure of the gas
v = specific volume
T = absolute temperature
R = the universal gas constant
Although no actual gases follow this law totally,
the behaviour of most gases at temperatures