Fluid properties - General definitions

1.st Week:
• General definitions
What is Fluid Mechanics?
Fluid mechanics deals with the study of all fluids under static and dynamic situations.
Fluid mechanics is a branch of continuous mechanics which deals with a relationship between forces, motions, and statically conditions in continuous material.
This study area deals with :
• surface tension,
• fluid statics,
• flow in enclose bodies, or flow round bodies (solid or otherwise),
• flow stability.

Primary Dimensions
In fluid mechanics there are only four primary dimensions:
- mass, {M}
- length, {L},
- time, {T},
- and temperature,{ ? }.
These dimensions and their units in both systems are given in Table 1.1.
Note : the kelvin unit uses no degree symbol.
For example, acceleration has the dimensions {LT-2}. The most crucial of these secondary dimensions is force, which is directly related to mass, length, and time by Newton’s second law
F = ma (1.2)
(dimensionally), {F} = {MLT - 2}.

1 newton of force =1 N = 1 kg . m/s2 (1.3)
1 pound of force = 1 lbf = 1 slug. 1 ft/s2 = 4.4482 N

Table 1.1 Primary Dimensions in SI and BG Systems
Primary dimension SI unit BG unit Conversion factor
Mass {M} Kilogram (kg) Slug 1 slug = 14.5939 kg
Length {L} Meter (m) Foot (ft) 1 ft = 0.3048 m
Time {T} Second (s) Second (s) 1 s = 1 s
Temperature {?} Kelvin (K) Rankine (°R) 1 K = 1.8°R




A list of some important secondary variables in fluid mechanics, with dimensions derived as combinations of the four primary dimensions, is given in Table 1.2.

Table 1.2 Secondary Dimensions in Fluid Mechanics
Secondary dimension SI unit BG unit Conversion factor
Area {L2} m2 ft2 1 m2= 10.764 ft2
Volume {L3} m3 ft3 1 m3 = 35.315 ft3
Velocity {LT-1} m/s ft/s 1 ft/s = 0.3048 m/s
Acceleration {LT-2} m/s2 ft/s2 1 ft/s2= 0.3048 m/s2
Pressure or stress{ML-1T-2} Pa= N/m2 lbf/ft2 1 lbf/ft2 = 47.88 Pa
Angular velocity {T-1} s-1 s-1 1 s-1 = 1 s-1
Energy, heat, work {ML2T-2} J= N .m ft . lbf lbf 1 ft . lbf = 1.3558 J
Power {ML2T-3} W= J/s ft . lbf/s 1 ft . lbf/s = 1.3558 W
Density {ML-3} kg/m3 slugs/ft3 1 slug/ft3 = 515.4 kg/m3
Viscosity {ML-1T-1} kg/(m .s) slugs/(ft . s) 1 slug/(ft . s) =47.88 kg/(m . s)
Specific heat {L2T-2?-1} m2/(s2 . K) ft2/(s2 . °R) 1 m2/(s2 . K) =5.980 ft2/(s2. °R)

Fluid Properties:
The following are some of the important basic properties of fluids:
• Density
• Temperature
• Pressure
• Specific Volume
• Specific Weight
• Specific Gravity
• Viscosity

1. Density:{ ?}
Density is the mass per unit volume of a fluid.
Or, it is the ratio between mass (m) and volume (V) of a fluid.
Density is denoted by the symbol ‘?’. Its unit is kg/m3.

In general:
• density of a fluid decreases with increase in temperature.
• It increases with increase in pressure.
2. Temperature: :{ ?