Fluid Mechanics (Spring 2020) – Chapter 1 - U. Lei (
李雨
)
Chapter 1: Introduction
Definition of fluids
The subject of fluid mechanics
Applications of fluid mechanics
Microscopic (molecular) versus macroscopic
(continuum) approach
Continuum approach
Eulerian and Lagrangian description
Reynolds transport theorem

Fluid Mechanics (Spring 2020) – Chapter 1 - U. Lei (
李雨
)
Fluid: gas + liquid
Three phases of substances:
Solids, liquids and gases
,
depending on different values of pressure and temperature.
The
density
of the solid and liquid are of the same
order, and are in general of three order greater than
that of the gas. The liquid and solid are group as
condensed matters
in Physics.
The
response
of the liquid under the action of an
applied
shear stress
is similar to that of the gas, and is
fundamentally different from that of the solid.
Therefore, the liquid and the gas are grouped
together as a new category, the
fluid
.

Fluid Mechanics (Spring 2020) – Chapter 1 - U. Lei (
李雨
)
Substance subject to a shear stress
γ
is fixed for a given
τ
.
γ
increases continuously as
time
increases for a given
τ
. Propose
G
τ
γ
=
d
dt
γ
τ
μ
=
Elasticity :
Some materials in between: viscoelastic materials / non-Newtonain fluids

Fluid Mechanics (Spring 2020) – Chapter 1 - U. Lei (
李雨
)
Definition of fluid
A fluid is defined as
a substance that
deforms
continuously
when subjected to a
shear stress
, no
matter how small that shear stress may be.

Fluid Mechanics (Spring 2020) – Chapter 1 - U. Lei (
李雨
)
Fluid Mechanics
A discipline for the study of
the
response of the fluid
under
the
action of applied forces

Fluid Mechanics (Spring 2020) – Chapter 1 - U. Lei (
李雨
)
The
response of the fluid
includes its
translation,
rotation
and
deformation.
The
applied forces
are generalized forces associated
with all kinds of the driving mechanisms, including
all kinds of forces such as that associated with the
above
shear stress
, the
pressure
, the
gravitational
force
, the
surface tension
, the
buoyancy
resulting from
an applied temperature contrast, the
electrical force
if
the fluid is a conductor, the
magnetic force
, and others.

Fluid Mechanics (Spring 2020) – Chapter 1 - U. Lei (
李雨
)
Applications of fluid mechanics
We live in
air
and cannot survive without
water
–
importance of fluid mechanics
Two important topics in the last century :
aeronautical and aerospace engineering
geophysical fluid dynamics
Recently, some interesting fluid mechanics problems in
micro-scale appear in the
microelectromechanical
systems (MEMS).

Fluid Mechanics (Spring 2020) – Chapter 1 - U. Lei (
李雨
)
Aerodynamics - examples (1)
(McGraw Hill,
1982
)

Fluid Mechanics (Spring 2020) – Chapter 1 - U. Lei (
李雨
)
Aerodynamics - examples (2)
____

Fluid Mechanics (Spring 2020) – Chapter 1 - U. Lei (
李雨
)
Aerodynamics - examples (3)