# lesson.2.2

```Lesson 3.1 Fluid-FlowPhenomena
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words
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Expressions and Technical Terms
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Words

fluid [`flu:id] n . 流体
boundary[`baundri] n. 边界，疆界，限界

behavior [bi`heivjә:] n. .行为，品行；举止，态度，举动，表现，行动
incompressible [im͵kәm`presbl] a. 不可压缩的
viscosity [vis`kɔstiti] n. 速度，速率
conservation[͵kɔnsә`veiʃәn] n. 【物理学】守恒，不灭
characteristic[͵kærәktә`ristik] n. 有特性的；表示…特性的，…特有的
circulation [͵sә:kju`leiʃәn] n . 循环，环流
Expressions and Technical Terms

fluid flow 流体流动 规格
shear stress 剪切应力
ideal fluid 理想流体
potential flow 势流
conservation of mass 质量守恒
irrotational flow 无旋流
solid boundary 固体边界层
fluid mechanics 流体力学
boundary layer 边界层
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The behavior of a flowing fluid depends strongly on
weather or not the fluid is under the influence of solid
boundaries. In the region where the influence of the wall
is small, the shear stress may be negligible and the fluid
behavior may approach that of an ideal fluid, one which
is incompressible and has zero viscosity.

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The flow of such an ideal fluid is called potential
flow and is completely described by the principles of
Newtonian mechanics and conservation of mass.
Potential flow has two important characteristics : (i)
neither circulations nor eddies can form within the
stream , so that potential flow is also called irrotational
flow , and (ii) friction cannot develop , so that there is no
dissipation of mechanical energy into heat .

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Potential flow can exist at distances not far from a solid
boundary. A fundamental principle of fluid mechanics,
originally stated by Prandtl in 1904, is that, except for
fluids moving at low velocities or possessing high
viscosities, the effect of the solid boundary on the flow is
confined to a layer of the fluid immediately adjacent to
the solid wall.

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This layer is called the boundary layer, and shear forces
are confined to this part of the fluid. Outside the
boundary layer, potential flow survives. Most technical
flow processes are best studied by considering the fluid
stream as two parts, the boundary layer and the
remaining fluid. In some situations such as flow in a
converging nozzle, the boundary layer may be neglected,
and in others such as flow through pipes the boundary
layer fills the entire channel, and there is no potential
flow.

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Within the current of an incompressible fluid under the
influence of solid boundaries, four important effects
appear: (i) the coupling of velocity–gradient and shearstress fields, (ii) the onset of turbulence, (iii) the
formation and growth of boundary layer, and (iv) the
separation of boundary layers from contact with the solid
boundary

⑶边界层的形成和长大,⑷边界层和紧贴便捷层的分离.
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Laminar flow and Turbulence It has long been known
that a fluid can flow through a pipe or conduit in two
different ways. At low flow rates the pressure drop in the
fluid increases directly with the fluid velocity, fluids tend
to flow without lateral mixing and adjacent layers slide
past one another like playing cards

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There are neither cross-currents nor eddies, this
regime is called laminar flow. At high rates it increases
much more rapidly, roughly as the square of the velocity,
turbulence appears and eddies form. The distinction
between the two types of flow was first demonstrated in
a classic experiment by Osborne Reynolds, reported in
1883. It is shown in Fig 8

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Fig 8 Reynolds’ apparatus for tracing patterns
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