Chapter 1Fluid Characteristics

ZAIFAH BT. CHE WILDEPARTMENT OF CIVIL ENGINEERING

POLITEKNIK SULTAN IDRIS SHAH

JUNE 2015

DCN3113 HYDRAULICS

FLUIDS

• Fluids – a substance which deforms continuously, or flows when subjected to shear stress.

• Both liquid and gases are classified as fluid.• Hydraulics - The study of liquid and gases at rest

(statics) and motion (dynamics). • Engineering applications

• Dams and reservoirs• Water supply pipelines• Groundwater movement• Pumps, filters, rivers

DCN3113 HYDRAULICS

Physical characteristic of liquid and gases

LIQUID GASES

Fluids do not resist a change in shape.

Assume the shape of container they occupy

Fixed volume No fixed volume

Almost incompressible Easily compressed

Properties do not get affected due to change in temperature

Properties get affected due to change in temperature

Free surface is formed if the volume of container is greater than that of the liquid

No free surface is formed

DCN3113 HYDRAULICS

Fluid properties

• Fluids properties is defined to be a characteristic of the material structure of the fluid.

• There are five properties of fluids• Density• Specific weight• Specific gravity• Specific volume

DCN3113 HYDRAULICS

Density or mass density, 𝜌Mass of substance per unit volume.

Density, = 𝜌 = (kg/).

Specific Weight or Weight Density,

The force exerted by the earth’s gravity up on a unit volume of substance.

Specific Weight, = 𝜔 = = = (N/m³).

DCN3113 HYDRAULICS

Specific gravity (relative density), s

The ratio of the fluid density to the weight density of a standard fluid.

Specific Gravity, s (for liquid) = (dimensionless).

Specific volume, Vs

The volume of fluid occupied by a unit mass or volume per unit mass of a fluid.

Specific Volume, Vs =

= = (m³/kg).

DCN3113 HYDRAULICS

Example 1. Calculate the specific weight, density and specific gravity of

one litre of a liquid which weight 7 N.

2. A container of volume 3.0 has 25.5 kN of an oil. Calculate specific weight, density and specific gravity.

3. Calculate density, specific weight, and weight of one litre of petrol of specific gravity = 0.7

4. One litre of crude oil weight 15.5 N. calculate its specific weight, density, specific gravity and specific volume

5. Mass of a liquid is 5655 g and 3300 litre respectively. Determine weight, density, and specific weight for the liquid.

DCN3113 HYDRAULICS

Viscosity in fluid flows

Viscosity• Viscosity is a quantitative measure of a fluid’s resistance to

flow.• Determines the amount of resistance to shear force• Decrease as temperature increase• Not affected by pressure changes

DCN3113 HYDRAULICS

Dynamic (or Absolute) Viscosity:

The dynamic viscosity() of a fluid is a measure of the resistance it offers to relative shearing motion.

τ = ; kg/ms or Ns/

where ; τ = Shearing stress(Ns/) = dynamic viscosity (Ns/)

DCN3113 HYDRAULICS

Kinematic Viscosity : It is defined as the ratio of absolute viscosity to the

density of fluid.

ν = ; m²/s = 1 stoke

Where;

= dynamic viscosity (Ns/) ρ = density (kg/)

DCN3113 HYDRAULICS

Applications

• Selection of lubricants for various purpose. - we can choose an optimum range of viscosity for

engine oil. - for high load and also for speed operation high

viscous lubricants is required.• In pumping operation

- for high viscous fluid high power will require. - for low viscous fluid low power will require.• In making of blend fuel - less viscous fuels easy to mix.

DCN3113 HYDRAULICS

EXAMPLE

DCN3113 HYDRAULICS