Chapter 1 Notes Fluid Mechanics, 1 Edition Tom Barwell
o Fluid Mechanics
Study of motion of fluids that are practically incompressible (liquids in water, gases at low speed) is called
o Subcategory of Hydrodynamics is hydraulics, which is liquids that flow in pipes or open channels
Gas dynamics deals with the flow of fluids that undergo significant density changes
Aerodynamics is flow of gases (predominantly air) over bodies (aircraft, cars etc)
A fluid is a gas or a liquid, the distinction between a solid and a fluid is made on the basis of the substance’s ability to
resist an applied shear stress. Fluids are continuously deforming under stress, no matter how small or large.
Fluid stress is proportional to strain rate, never stops deforming and approaches a certain rate of strain
Solid stress is proportional to strain, eventually stops deforming under force
Stress is defined as force per unit area (found by dividing the force by the area upon which it acts)
Normal stress is the normal component of the force and is considered as pressure when dealing with fluids
Tangential stress is the force acting on a surface and is called shear stress
A liquid takes the shape of the container it is in, and it forms a free surface in a larger container in a gravitational
field. A gas expands until it encounters the walls of the container and fills all available space. Intermolecular bonds
are strongest in solids and weakest in gases this is because gas molecules are separated by relatively large distances.
The distance between molecules (and therefore intermolecular bonds) generally increases slightly as a solid turns
liquid, with water being the notable exception. Molecules in the gas phase are at a considerably higher energy level
than they are in the liquid or solid phase.
Vapour usually implies a gas that is not far from a state of condensation.
Fluid flow is often confined by solid surfaces. When water is travelling downstream it is confined by rocks, the water
velocity normal to the rock surface must be zero. Also the water approaching the rock at any angle also comes to a
complete stop, and therefore the tangential velocity of water at the surface is also zero.
Nonporous means impermeable to fluid.
A fluid in direct contact with a solid “sticks” to the surface due to viscous effects, and there is no slip. This is known
as the no-slip condition. The flow region adjacent to the wall in which the viscous effects (and velocity gradients) are
significant is called the boundary layer. Both the boundary layer and no-slip condition are due to viscosity.
A consequence of the no-slip condition is that all velocity profiles must have zero values. Another consequence is
surface drag which is the force a fluid exerts on a surface in the flow direction. When a fluid is forced to flow over a
curved surface at sufficiently high velocity, the boundary layer can no longer remain attached the surface and at
some point it separates from the surface, this is called flow separation. When two bodies of different temperatures
are brought into contact, heat transfer occurs until their temperatures are the same, this is called no-termperature-
P a g e | 1 Chapter 1 Notes Fluid Mechanics, 1 Edition Tom Barwell
Classification of fluid flows
Viscous versus Inviscid regions of flow When two fluid layers of variable velocity are relative to each other, the
slower layer tries to slow down the faster layer. This internal resistance
to flow is quantified by viscosity (internal stickiness of the fluid). Zero
viscosity fluids do not exist and thus all fluids involve viscous effects to a
degree. Flows with high frictional effects are called viscous flows. There
are regions where viscous forces are negligibly small compared to
inertial or pressure forces. These are called inviscid flow regions.
Internal versus External flow The flow of an unbounded fluid over a surface such as a plate, a wire, or
a pipe is an external flow. The flow in a pipe or duct is Internal flow if
the fluid is completely bounded by solid surfaces. The flow of liquids in a
duct is called open-channel flow
Compressible versus incompressible Incompressibility is an approximation, and a flow is said to be
flow incompressible if the density remains nearly constant throughout.
Therefore, the volume of every portion of fluid remains unchanged over
the course of its motion when the flow or fluid is incompressible. Liquids
are usually always referred to as incompressible.
Laminar versus Turbulent flow The highly ordered fluid motion characterized by smooth layers of fluid
is called laminar. The flow of high-viscosity fluids such as oils at low
velocity is typically laminar. The highly disordered fluid motion that
typically occurs at high velocities and is characterized by velocity
fluctuations is called turbulent. A flow that alternates between laminar
and turbulent is called transitional. Reynolds Number, Re, is the key
parameter for the determination of the flow regime in pipes.
Natural (unforced) versus forced flow In forced flow, a fluid is forced to flow over a surface or in a pipe by
external means such as a pump or a fan. In natural flows, any fluid
motion is due to natural means such as the buoyancy effect, which
manifests itself as the rise of the warmer fluid and fall of cooler fluid.
Steady versus unsteady flow Steady implies no change at a point with time, unsteady is the opposite.
The term Uniform implies no change with location over a specified
region. Transient is typically used for developing flows as opposed to
unsteady (e.g. firing a rocket, increasing water pressure fr