Fluid Mechanics
Fluid Mechanics
Introduction to the topic
Fluids
There are two aspects of fluid mechanics which make it different to solid mechanics:
1. The nature of a fluid is much different to that of a solid
2. In fluids we usually deal with continuous streams of fluid without a beginning or end. In solids
we only consider individual elements.
We normally recognise three states of matter: solid; liquid and gas. However, liquid and gas are both
fluids: in contrast to solids they lack the ability to resist deformation. Because a fluid cannot resist the
deformation force, it moves, it flows under the action of the force. Its shape will change continuously as
long as the force is applied. A solid can resist a deformation force while at rest, this force may cause some
displacement but the solid does not continue to move indefinitely.
The deformation is caused by shearing forces which act tangentially to a surface. Referring to the figure
below, we see the force F acting tangentially on a rectangular (solid lined) element ABDC. This is a
shearing force and produces the (dashed lined) rhombus element A’B’DC.
When a fluid is in motion shear stresses are developed if the particles of the fluid move relative to one
another. When this happens adjacent particles have different velocities. If fluid velocity is the same at
every point then there is no shear stress produced: the particles have zero relative velocity.
Consider the flow in a pipe in which water is flowing. At the pipe wall the velocity of the water will be
zero. The velocity will increase as we move toward the centre of the pipe. This change in velocity across
the direction of flow is known as velocity profile and shown graphically in the figure below:
Velocity profile in a pipe.
Because particles of fluid next to each other are moving with different velocities there are shear forces in
the moving fluid i.e. shear forces are normally present in a moving fluid. On the other hand, if a fluid is a
long way from the boundary and all the particles are travelling with the same
references
http://www.efm.leeds.ac.uk/CIVE/CIVE1400/PDF/Notes/section_all2.pdf