# Solid & Fluid

Mechanics

Oct 24, 2013 (4 years and 6 months ago)

83 views

Ch1 Introduction

Importance:

Blood flow

kidney stones

……

－引擎、汽車（運輸工具）流線造型、如聯

……
、或
crude oil

across Alaska through an 800
-
mile
-
long

4ft
-
dia. pipe

……

－大氣、海洋、火山爆發、地底岩漿、海岸

……

(Show Transparencies relating to Fluid Mechanics)

Solid & Fluid

From Observation

Solid (hard; not easily deformed);

Fluid (Soft; easily deformed; readily move through air)

From Scientific or Engineering Point View

Molecular
structure of Materials (Solid vs. Fluid; intermolecular
cohesive forces)

Thus liquids can be easily deformed (but not easily

compressed).

Gases

have even greater mlc spacing and freedom of

motion with negligible cohesive intermlc

forces; very easily deformed (and

compressed) and will completely fill the

volume of any container in which they are

placed.

A more specific distinction

How they (gas, liquids, solids) deform under

The action of an external load.

Solid

(within the limit of elasticity)

Shear stress shear modules shear strain

of elasticity

Fluid

a fluid is defined as a substance that deforms

continuously when acted on by a shearing

stress of any magnitude.

plate

no slip condition

H bubble (velocity)

Dimensions, Dimensional Homogeneity, and Units

(See the
text books; Table 1.1; SI/BG)

Characteristics of A Fluid:

Density: (Greek symbol)

Specific volume:

Specific weight:

Specific gravity: S.G.=

Stress at point

tangential stress

normal stress

Define

tension stress

+

compression stress

-

Fluid in static state

unit vectors)

Viscosity: (Solid) shear stress ~ shear strain

(Fluid) shear stress ~ time rate of shear

strain

absolute viscosity

kinematic viscosity

Time rate of strain

(angular deformation)

1.

2.

Velocity gradient change of velocity in x
-
direction in

Y plane.

(
SI)

(
BG)

An important parameter

for momentum transport

(Contact area per second)

Ideal gas law

Non
-
ideal gas

Compressibility of Fluids

Bulk Modulus

Isothermal bulk modulus of elasticity

Coefficient of compressibility

Compression and Expansion of Gases

Isothermal process

Isentropic process

no heat exchanged with the surroundings

Speed of sound

Isentropic process (small disturbances

more rapidly; negligible heat transfer)

Vapor Pressure

Some liquid mlc at the surface

have sufficient momentum to

overcome the intermlc cohesive

forces and escape into the

atmosphere.

pressure is called vapor pressure

Appendix B

Tables B.1 & B.2

Boiling: The formation of vapor bubbles within a fluid

mass, which is initiated when the absolute pressure in the

fluid reaches the vapor pressure.

Sea Level:

T P

water boiling 100

C (212

F) 1atm (14.7psi)

10,000ft:

water boiling 89.4

C (193

F) 1atm (10.1psi)

Cavitation

1. Underwater explosion

2. Ship propeller (Water tunnel at B04)

3. Irregular, narrowed passages of a valve

or pump cause structure damage

Surface Tension

a liquid/ a gas or between two immiscible liquids,

forces develop in the liquid surface

skin.

Ex: a steel needle will float on water if it is carefully

placed on the surface.

other fluids

The difference between the

pressure inside the drop and

the external pressure

The pressure inside the drop

the pressure

surrounding the drop

Continued Surface tension

Q: Would the pressure on the inside of a bubble of

water be the same as that on the inside of a drop of

water of the same diameter and the same temp?

Fig.1.7 Effect of capillary action in small tubes.

Liquid
-
gas
-
solid interface

liquid wet solid surface

q

-

angle of

contact

wetting

Mercury

Non
-
wetting

give some example from

NCFM

Inertial, gravitational & viscous processes
are much more

important in most real flows.