Fluid Mechanics
(Lecture) / Topic 2
MEFLUMECH-T2
OBJECTIVES
By the end of the lecture, the students will be able to:
• Have a working knowledge of the basic properties of fluids and understand the continuum approximation.
•Have a working knowledge of viscosity and the consequences of the frictional effects it causes in fluid flow.
•Calculate the capillary rises and drops due to the surface tension effect
MEFLUMECH – PROPERTIES OF FLUIDS
SYSTEMS AND CONTROL VOLUMES
CLOSED SYSTEM
(also known as a control mass)
If even energy transfer
is not allowed, it is
called isolated system
SYSTEM – it is defined as a quantity of matter or a region in
space chosen for study.
SURROUNDINGS – it is the mass or region outside the
system.
OPEN SYSTEM
(also known as a control
volume)
BOUNDARY – The real or imaginary surface that separates
the system from its surroundings.
MEFLUMECH – PROPERTIES OF FLUIDS
PROPERTIES OF A SYSTEM
PROPERTY – any characteristic of a system
INTENSIVE PROPERTIES – are
those that are independent of the
mass of a system, such as
temperature, pressure, and
density.
EXTENSIVE PROPERTIES – are
those whose
values depend on the size (or
extent) of the system.
Extensive properties per
unit mass are called specific
properties.
Some examples of specific
properties are specific
volume (v V/m) and
specific total energy (e
E/m).
MEFLUMECH – PROPERTIES OF FLUIDS
CONTINUUM
it is very convenient to disregard the atomic nature
of a substance and view it as a continuous,
homogeneous matter with no holes, that is, a
continuum.
The continuum idealization
is implicit in many
statements we make, such
as “the density of water
in a glass is the same at any
point.”
MEFLUMECH – PROPERTIES OF FLUIDS
DENSITY AND SPECIFIC GRAVITY
Density is defined as mass per unit volume
The weight of a unit volume of a substance is
called specific weight.
The reciprocal of density is the specific volume
v, which is defined as volume per unit mass.
specific gravity, or relative density, is defined
as the ratio of the density of a substance to the
density of some standard substance at a
specified temperature
MEFLUMECH – PROPERTIES OF FLUIDS
DENSITY AND SPECIFIC GRAVITY, problem (1)
1.) You have a rock with a volume of 15cm3 and a mass of 45 g. What is its density?
2.) You have a different rock with a volume of 30cm3 and a mass of 60g. What is its density?
a.) In the above two examples which rock is heavier?
b.) Which is lighter?
3.) A sphere made of granite has a radius of 5 cm and a mass of 2 kg, what is its specific gravity?
4.) You have a sample of granite with density 174.8 lbs/ft3. The density of water is 62.4 lbs/ft3. What is the
specific gravity of the granite now?
5.) You have a sample of spherical rock with a specific gravity of 3, it has a mass of 1.5 kg, what is its radius in
inches?
MEFLUMECH – PROPERTIES OF FLUIDS
DENSITY AND SPECIFIC GRAVITY, problem (2)
1.) What is the difference R and Ru?
2.) A spherical balloon with a diameter of m is filled with helium at 20 deg. C and 200 kPa. Determine the
mole number and the mass of the helium in the balloon.
3.) The pressure in an automobile tire depends on the temperature of the air in the tire. When the air
temperature is 25°C, the pressure gage reads 210 kPa. If the volume of the tire is 0.025 m3, determine the
pressure rise in the tire when the air temperature in the tire rises to 50°C. Also, determine the amount of air
that must be bled off to restore pressure to its original value at this temperature. Assume the atmospheric
pressure to be 100 kPa.
4.) The air in an automobile tire with a volume of 0.53 ft is at 90 deg.F and 20psig. Determine the amount of
air that must be added to raise the pressure to the recommended value of 30psig. Assume the atmospheric
pressure to be 14.6 psia and the temperature and the volume to remain constant.
5.) A rigid tank contains 20 lbm of air at 20 psia and 70 deg.F. Air is added to the tank until the pressure and
temperature rise to 35 psia and 90 F, respectively. Determine the amount of air added to the tank.
MEFLUMECH – PROPERTIES OF FLUIDS
VAPOR PRESSURE AND CAVITATION
Saturation Temperature – At a given pressure, the
temperature at which a pure substance changes phase.
Saturation Pressure – at a given temperature, the
pressure at which a pure substance changes phase.
Vapor Pressure – the pressure exerted by its vapor in
phase equilibrium with its liquid at a given temperature.
Partial Pressure - is defined as the pressure of a gas
or vapor in a mixture with other gases.
MEFLUMECH – PROPERTIES OF FLUIDS
VAPOR PRESSURE AND CAVITATION
When the propeller turns quickly, it
creates trail of bubbles behind it.
As the bubble pops it will
make a lot of noise.
MEFLUMECH – PROPERTIES OF FLUIDS
VAPOR PRESSURE AND CAVITATION
The vapor bubbles (called cavitation bubbles since they form cavities in the
liquid) collapse as they are swept away from the low pressure regions,
generating highly destructive, extremely high-pressure waves.
A.) In a water distribution system, the temperature of water is observed to
be as high as 30°C. Determine the minimum pressure allowed in the system
to avoid cavitation.
Cavitation damage
MEFLUMECH – PROPERTIES OF FLUIDS
PHASE DIAGRAM
MEFLUMECH – PROPERTIES OF FLUIDS
CAVITATION, problem (3)
1. In a piping system, the water temperature remains under 40°C. Determine the minimum pressure allowed in
the system to avoid cavitation.
2.) The analysis of a propeller that operates in water at 20°C shows that the pressure at the tips of the propeller
drops to 2 kPa at high speeds. Determine if there is a danger of cavitation for this propeller.
3.) The analysis of a propeller that operates in water at 70°F shows that the pressure at the tips of the propeller
drops to 0.1 psia at high speeds. Determine if there is a danger of cavitation for this propeller.
4.) A pump is used to transport water to a higher reservoir. If the water temperature is 25°C, determine the
lowest pressure that can exist in the pump without cavitation.
MEFLUMECH – PROPERTIES OF FLUIDS