Solid & Fluid

spreadeaglerainΜηχανική

24 Οκτ 2013 (πριν από 3 χρόνια και 5 μήνες)

51 εμφανίσεις

Ch1 Introduction

Importance:

生物醫學-
Blood flow


kidney stones


溺水時急救


打氣等等
……

工業

-引擎、汽車(運輸工具)流線造型、如聯


結車之整流罩等等
……
、或
crude oil


across Alaska through an 800
-
mile
-
long



4ft
-
dia. pipe

航空器、飛機氣動力學等等


……

自然界

-大氣、海洋、火山爆發、地底岩漿、海岸


消蝕等等
……



(Show Transparencies relating to Fluid Mechanics)


Solid & Fluid

坨at is a 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.