CIEN 30063
DEPARTMENT
HYDRAULICS 1:
PROPERTIES OF FLUIDS
CIEN 30063
DEPARTMENT
Learning Objectives:
At the end of this lesson, the learner will be able to:
• Discuss the different properties of fluids.
•Explain the importance of the different properties of fluids
in solving applications of fluid mechanics/Hydraulics 1.
• Solve problems related to properties of fluids.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
HYDRAULICS is the branch of mechanics that is concerned with the laws
controlling the behavior of water and other liquids in the states of rest and
motion.
• HYDROSTATICS – the study of liquid at rest.
• HYDROKINETICS – deals with the geometry of the motion of liquids
without considering the forces causing that motion.
• HYDRODYNAMICS – deals with the forces exerted by or upon liquids in
motion, including relations between velocities and acceleration involved in
such fluid motion.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
FLUID MECHANICS is a unified course of general fluid motion. Fluids have
dissimilar behavior due to their differences such as density, viscosity,
compressibility and cohesion.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
DISTINCTION BETWEEN A SOLID AND A FLUID
SOLID – the molecules are spaced closer than that of a fluid, and the
intermolecular attraction in a solid is sufficiently strong that it tends to retain its
form, unlike in a fluid.
LIQUID AND GAS (FLUIDS) – in a liquid, the intermolecular attraction is
relatively weak, while in gas, the intermolecular attraction is very weak.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
PHYSICAL PROPERTIES OF FLUIDS
Properties of fluids which are of fundamental importance in the study.
WEIGHT, W: The earth’s gravitational pull upon a body. Its dimension is
Newton (N) in SI units.
MASS, m: The quantitative measure of the amount of matter in a given body.
The dimension used for the mass in SI units is kilogram (kg) which is a base
unit.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
PHYSICAL PROPERTIES OF FLUIDS
The basic relation between the weight and the mass of a body is
𝑾 = 𝒎𝒈
where: 𝑔 = 9.81 m/s2 (32.2 ft/s2), the acceleration of gravity at sea level
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
PHYSICAL PROPERTIES OF FLUIDS
SPECIFIC WEIGHT, 𝜸 : the weight of fluid in a unit volume
𝑾
𝜸=
𝑽
DENSITY, 𝝆 : the mass of fluid contained in a unit volume
𝒎
𝝆=
𝑽
SPECIFIC VOLUME, 𝒗 : the volume per unit mass of fluid, or the reciprocal
of the density
𝑽
𝒗=
𝒎
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
PHYSICAL PROPERTIES OF FLUIDS
SPECIFIC GRAVITY, s: the dimensionless ratio of the specific weight 𝛾 or
density 𝜌 of a fluid to the specific weight 𝛾𝑠 or density 𝜌𝑠 of a standard
substance,
𝜸
𝝆
𝒔=
=
𝜸𝒔 𝝆𝒔
REMARK: For liquids and solids, the standard substance is pure water at 4°C
at which temp, its specific weight, 𝛾𝑠 = 9.81 kN/m3 and density 𝜌𝑠 = 1000
kg/m3 are at their maximum values.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
SAMPLE PROBLEM 1
If a certain gasoline weighs 7 kN/m3, what are the values of its density, specific
volume, and specific gravity relative to water at 4°C?
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
SAMPLE PROBLEM 2
A certain gas weighs 16 N/m3 at a certain temperature and pressure. What are
the values of its density, specific volume, and specific gravity relative to air
weighing 12 N/m3?
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
SAMPLE PROBLEM 3
If 5.30 m3 of a certain oil weighs 43 860 N, calculate the specific weight,
density and specific gravity of this oil.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
SAMPLE PROBLEM 4
The density of alcohol is 790 kg/m3. Solve for its specific weight, specific
gravity and specific volume.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
SAMPLE PROBLEM 5
A volume of 450 liters of a certain fluid weighs 3.50 kN. Compute the mass
density.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
PROPERTIES OF WATER
•
SPECIFIC WEIGHT OF WATER
The specific weight of water is dependent on its temperature, purity, and the
pressure under which it exists. The maximum density, and thus specific weight,
of water occurs at a temperature of 4°C.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
PROPERTIES OF WATER
•
COMPRESSIBILITY AND ELASTICITY OF WATER
The compressibility of water or any other liquid varies inversely as its volume
modulus of elasticity, also known as bulk modulus.
∆P
𝑬𝒗 = −𝒗
∆𝑽
where: p = unit pressure in Pa
v = specific volume in m3/kg
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
PROPERTIES OF WATER
•
VISCOSITY
Viscosity is defined as that property of a fluid which determines the amount of
its resistance to a shearing stress.
dy
𝝁 = 𝝉( )
𝒅𝑽
where: 𝜇 = dynamic/absolute viscosity
𝜏 = shearing stress
V = velocity
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
PROPERTIES OF WATER
•
KINEMATIC VISCOSITY
Kinematic viscosity is the ratio of the viscosity to the density.
𝝁
𝒗=
𝝆
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
PROPERTIES OF WATER
•
COHESION,
ADHESION:
SURFACE
TENSION
AND
CAPILLARITY
Cohesion – property of liquid which enables it to resist tensile stress.
Adhesion – property of liquid which enables it to adhere to another body.
Capillarity – a phenomenon which arises when surfaces of liquids come in
contact with vertical solid surfaces.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
PROPERTIES OF WATER
•
COHESION,
ADHESION:
SURFACE
TENSION
AND
CAPILLARITY
Surface Tension – the noticeable tension effects depend basically upon the
relative amounts of cohesion and adhesion between liquid molecules. This
manifests itself as an imaginary “skin” over the surface which could support
small loads.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
PROPERTIES OF WATER
•
VAPOR PRESSURE
Vapor pressure is the pressure exerted by the gas molecules (vapor) if confined
in a closed space.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
PROPERTIES OF AIR
State of gas follows this equation:
𝑷
𝝆=
𝑹𝑻
where:
𝜌 = density
P = absolute pressure in Pa
R = gas constant in N-m/kg-K
T = absolute temperature in K
𝑷𝒈
𝜸=
𝑹𝑻
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
SAMPLE PROBLEM 1
A cubic meter of air at 101.3 kPa and 15°C weighs 12 N. What is its specific
volume? Gas constant R of air is equal to 287 N-m/kg-K.
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
SAMPLE PROBLEM 2
If the dynamic viscosity of water at 4°C is 1x10-3 N-s/m2, what is the kinematic
viscosity in the English units?
HYDRAULICS 1:
PROPERTIES OF FLUIDS
DEPARTMENT
SAMPLE PROBLEM 3
A fuel oil having a viscosity of 0.297 Pa-s flows through a circular pipe 15 cm
in diameter. At the center of the pipe, the velocity is 1.20 m/s and decreases to a
minimum value at the pipe wall. The value of the velocity at any point in the
cross section a distance x from the center is
V = 63.4 (r2 – x2)/
r being the radius of the pipe. Compute the shear stress at a point midway
between the center and the wall.