3
INTRODUCTION
Fluid mechanics deals with
liquids and gases in motion
or at rest.
Mechanics: The oldest physical science thatdeals with both stationary and moving bodies under the influence of forces.
Statics: The branch of mechanics that deals with bodies at rest.
Dynamics: The branch that deals with bodies in motion.
Fluid mechanics: The science that deals with the behavior of fluids at rest (fluid statics) or in motion (fluid dynamics), and the interaction of fluids with solids or other fluids at the boundaries.
Fluid dynamics: Fluid mechanics is also
4
Hydrodynamics:
The study of the motion of fluids that can
be approximated as incompressible (such as liquids,
especially water, and gases at low speeds).
Hydraulics:
A subcategory of hydrodynamics, which deals
with liquid flows in pipes and open channels.
Gas dynamics:
Deals with the flow of fluids that undergo
significant density changes, such as the flow of gases
through nozzles at high speeds.
Aerodynamics:
Deals with the flow of gases (especially air)
over bodies such as aircraft, rockets, and automobiles at
high or low speeds.
5
Application Areas of Fluid Mechanics
Course Objectives
•
To study the mechanics of fluid motion.
•
To establish fundamental knowledge of
basic fluid mechanics and address specific
topics relevant to simple applications
involving fluids
Syllabus
•
Fluid Properties,
•
Kinematics of fluid flow,
•
Fluid Statics,
•
Dynamics of fluid flow,
•
Concept of Boundary Layer,
•
Dimensional Analysis and
11
What is a Fluid?
Fluid: A substance in the liquid or gas phase.
A solid can resist an applied shear stress by deforming.
A fluid deforms continuously under the influence of a shear stress, no matter how small.
In solids, stress is proportional to strain, but in fluids, stress is
proportional to strain rate.
When a constant shear force is applied, a solid eventually stops deforming at some fixed strain angle, whereas a fluid never stops deforming and approaches a
constant rate of strain.
12 Stress: Force per unit area.
Normal stress: The normal
component of a force acting on a surface per unit area.
Shear stress: The tangential
component of a force acting on a surface per unit area.
Pressure: The normal stress in a fluid at rest.
Zero shear stress: A fluid at rest is at a state of zero shear stress.
When the walls are removed or a liquid container is tilted, a shear develops as the liquid moves to
13
Unlike a liquid, a gas does not form a
free surface, and it expands to fill the
entire available space.
In a liquid, groups of molecules can move relative to each other, but the volume remains relatively constant because of the strong cohesive forces
14
The arrangement of atoms in different phases: (a) molecules are at relatively fixed positions in a solid, (b) groups of molecules move about each other in the liquid phase, and (c) individual molecules move about at random in the gas phase. Intermolecular bonds are strongest in solids and weakest in gases.
Solid: The molecules in a solid are arranged in a pattern that is repeated throughout.
Liquid: In liquids molecules can rotate and translate freely.
15 Gas and vapor are often used as synonymous words.
Gas: The vapor phase of a substance is customarily called a gas when it is above the critical temperature.
Vapor: Usually implies that the current phase is not far from a state of condensation.
On a microscopic scale, pressure is determined by the interaction of individual gas molecules. However, we can measure the pressure on a macroscopic scale with a pressure gage.
Macroscopic or classical approach: Does not require a knowledge of the behavior of individual molecules and provides a direct and easy way to analyze
engineering problems.
For practical use the Poise is normally too large and the unit is often divided by 100 - into the smaller unit centiPoise (cP) - where 1 p = 100 cP 1 cP = 0.01 poise = 0.001 Pascal second = 0.001 N s/m2
SURFACE TENSION AND
CAPILLARY EFFECT
•
Interfaces
•
When phases exist together, the boundary between two of
them is known as an
interface
.
•
The properties of the molecules forming the interface are
often sufficiently different from those in the bulk of each
phase.
Liquid Interfaces
•
Molecules at the surface
(i.e., at the liquid–air
interface) can only develop
attractive cohesive forces
with other liquid molecules
that are situated below and
adjacent to them. They can
develop adhesive forces of
attraction
with
the
molecules constituting the
other phase involved in the
interface, although, in the
case of the liquid–gas
interface, this adhesive
force of attraction is small.
SURFACE TENSION
•
The net effect is that the molecules at the surface of the liquid
experience an inward force toward the bulk.
•
Such a force pulls the molecules of the interface together and,
as a result, contracts the surface, resulting in a
surface tension
.
•
To keep the equilibrium, an equal force must be applied to
oppose the inward tension in the surface.
Effects of surface tension: Capillarity
•
The capillarity phenomenon is due to the rise or depression of the
meniscus of the liquid due to the action of surface tension forces.
•
Capillary action in small tubes which involve a liquid-gas-solid interface is
caused by surface tension. The fluid is either drawn up the tube or pushed
down.
Effects of surface tension: Capillarity
• The forces of attraction binding molecules to one another give rise to cohesion, the tendency of the liquid to remain as one assemblage of particles rather than to behave as a gas and fill the entire space within which it is confined.
• On the other hand, forces between the molecules of a fluid and the molecules of a solid boundary give rise to adhesion between the fluid and the boundary.
• It is the interplay of these two forces that determine whether the liquid will “wet” the solid surface of the container. If the adhesive forces are greater than the cohesive forces, then the liquid will wet the surface; if vice versa, then the liquid will not.
• It is rare that the attraction between molecules of the liquid exactly equals that between molecules of the liquid and molecules of the solid and so the liquid surface near the boundary becomes curved.
• For R > ¼ in ( 7 mm), capillarity is negligible.
Effects of surface tension: Capillarity
•
The strength of the capillary effect is quantified by the
contact (or
Force balance can describe magnitude of capillary rise.
Weight of fluid column = Surface tension pulling force.
Pressure inside liquid droplet and
bubble:
Vapor Pressure
VAPOURISATION of an element or compound is a phase transition. From liquid state to vapour.
Boiling and evaporation are the types of vapourisation.
Evaporation Vs Boiling
• Ordinary evaporation is a surface phenomenon - since the vapor pressure is low and since the pressure inside the liquid is equal to atmospheric pressure plus the liquid pressure, bubbles of water vapor cannot form.
• But at the boiling point, the saturated vapor pressure is equal to atmospheric pressure, bubbles form, and the vaporization becomes a volume phenomena.
CAVITATION
• When the pressure of a liquid falls below the vapor pressure it evaporates, i.e., changes to a gas. If the pressure drop is due to temperature effects alone, the process is called boiling.
• If the pressure drop is due to fluid velocity, the process is called cavitation. Cavitation is common in regions of high velocity, i.e., low pressure such as on turbine blades and marine propellers.
• Cavitation can cause serious problems, since the flow of liquid can sweep this cloud of bubbles on into an area of higher pressure where the bubbles will collapse suddenly. If this should occur in contact with a solid surface, very serious damage can result due to the very large force with which the liquid hits the surface
.
