Fundamental concepts

Question 1

WHAT IS A FLUID?

Answer 1

fluid is any substance that deforms continuously when
subjected to a shear stress (tangential force per unit area), no
matter how small the stress is.

1. A fluid at rest shows inability to resist shear stress
2. Deformations in a fluid are permanent: they do not disappear when the
   applied shear stress tends to zero

Question 2

Sheer stress on a liquid

Answer 2

Liquids show resistance to change in volume (a liquid in a tank “sits” on the
bottom part of the tank and has a free surface!). Little compressible

Question 3

Sheer stress on a gas

Answer 3

Gases tend to easily change their volume under small forces (a gas uses
the whole volume of a container). High compressible

Question 4

Define stress

Answer 4

STRESS is defined as force per
unit area [N/m2]

Question 5

Define the normal stress

Answer 5

the normal stress is called PRESSURE
The supporting walls of a fluid eliminate
shear stress, and thus a fluid at rest is at
a state of zero shear stress

Question 6

Body forces can be defined as:

Answer 6

all external forces acting without physical contact on all particles
of the system and proportional to their mass.
Usually expressed as force per unit mass
Gravitational and magnetic forces are body forces

Question 7

Surface forces can be defined as:

Answer 7

all forces acting on a portion of the continuum system through
its boundary surface
Pressure and viscous forces are examples of surface forces
Surface forces on a fluid particle lead to stress

Question 8

Specific weight

Answer 8

The specific weight, γ, of a substance is its weight
per unit volume, specific weight of a fluid depend on temperature
and pressure

Question 9

Specific gravity

Answer 9

The specific gravity, SG, of a substance is the ratio
of its weight to that of an equal volume of water at a
specified temperature, usually 4 ◦C

Question 10

Viscosity

Answer 10

THE RELATION BETWEEN THE RATE OF
DEFORMATION AND THE APPLIED SHEAR STRESS

Question 11

Newtonian fluid

Answer 11

the rate of deformation is
proportional to the shear stress

Question 12

non - newtonian fluids

Answer 12

the rate of deformation is not directly
proportional to the shear stress

Question 13

No slip condition

Answer 13

a fluid in motion comes to a complete stop at contact with the surface and thus it has a zero velocity, relative to the surface, a fluid in direct contact with a solid “sticks” to the surface and there is no slip