Introduction to Fluid Mechanics

MEE1018: Fluid Mechanics 1 - Lecture 1

3 branches of fluid mechanics

Fluid statics, fluid kinematics and fluid dynamics.

Fluid statics

Mechanics of fluid at rest.

Fluid kinematics

The motion of fluids without considering forces or energy.

Fluid dynamics

Involves the relation between the motion of fluids and the forces exerted by or on them.

Applications of fluid mechanics

Alaska oil pipeline, aircraft, cars (aerodynamics), ferries, wind turbines, dams.

Importance of fluid mechanics

A knowledge of fluid mechanics is required to properly design vehicles, wind turbines, fuel and water supply systems.

Fluid

A substance which deforms continuously (flows) under the action of shearing forces, no matter how small they may be. Liquid and gas.

Shearing force

A force which acts tangentially to the surface.

Forces of a fluid at rest

There are no shearing forces and therefore all forces in the fluid must be perpendicular to the planes on which they act.

Effects of shearing force on a solid

The solid deforms but does not continuously deform.

Provided the elastic limit is not exceeded the deformation is proportional to the applied shear stress and it will disappear when the force is removed.

stress ∝ strain

Effects of shearing stress on a fluid

In control, the fluid element continuous to deform as long as the shearing force is present, i.e. the force flows

stress ∝ rate of strain

Molecular structure of a solid

Molecules densely and regularly packed

Movement restricted to small vibrations

Molecular structure of a liquid

Molecules closely packed but have greater movement

Relative positions of molecules can change

Flows under action of applied forces

Molecular structureof a gas

Molecular spacing 10 ties greater than liquids

Molecules move freely

Intermolecular forces in a solid

Each molecule restrained by its neighbours; strong intermolecular forces

Intermolecular forces in a liquid

Relatively strong intermolecular forces

A given mass of liquid occupies a fixed volume

Intermolecular forces in a gas

Weak intermolecular forces

A given mass of gas has no fixed volume and expands continuously unless restrained by container

Continuum

When the fluid is treated as a continuous substance in which conditions to a point represents the average effects of many molecules.

Why continuum is used

In typical engineering structures the number of molecules is huge and the spacing between them is usually negligible compared with the distance involved in the problem.

Density

Mass of fluid per unit volume

Relative density or specific gravity of a liquid

Ratio of density of liquid to density of water. No units.

Specific volume

The volume occupied by unit mass, i.e. reciprocal of density

Usually applied to gases

Specific weight of fluid

The weight of fluid per unit volume

= mass of fluid per unit volume x g