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الكلية كلية الهندسة
القسم الهندسة المدنية
المرحلة 2
أستاذ المادة كاظم نايف كاظم اليساري
6/6/2011 3:32:50 PM
1.1 Preliminary Remarks Fluid mechanics is the study of fluids either in motion (fluid dynamics) or at rest (fluid
statics) and the subsequent effects of the fluid upon the boundaries, which may be either
solid surfaces or interfaces with other fluids. Both gases and liquids are classified
as fluids, and the number of fluids engineering applications is enormous: breathing,
blood flow, swimming, pumps, fans, turbines, airplanes, ships, rivers, windmills, pipes,
missiles, icebergs, engines, filters, jets, and sprinklers, to name a few. When you think
about it, almost everything on this planet either is a fluid or moves within or near a
fluid.
The essence of the subject of fluid flow is a judicious compromise between theory
and experiment. Since fluid flow is a branch of mechanics, it satisfies a set of welldocumented
basic laws, and thus a great deal of theoretical treatment is available. However,
the theory is often frustrating, because it applies mainly to idealized situations
which may be invalid in practical problems. The two chief obstacles to a workable theory
are geometry and viscosity. The basic equations of fluid motion (Chap. 4) are too
difficult to enable the analyst to attack arbitrary geometric configurations. Thus most
textbooks concentrate on flat plates, circular pipes, and other easy geometries. It is possible
to apply numerical computer techniques to complex geometries, and specialized
textbooks are now available to explain the new computational fluid dynamics (CFD)
approximations and methods [1, 2, 29].1 This book will present many theoretical results
while keeping their limitations in mind.
The second obstacle to a workable theory is the action of viscosity, which can be
neglected only in certain idealized flows (Chap. 8). First, viscosity increases the difficulty
of the basic equations, although the boundary-layer approximation found by Ludwig
Prandtl in 1904 (Chap. 7) has greatly simplified viscous-flow analyses. Second,
viscosity has a destabilizing effect on all fluids, giving rise, at frustratingly small velocities,
to a disorderly, random phenomenon called turbulence. The theory of turbulent
flow is crude and heavily backed up by experiment (Chap. 6), yet it can be quite
serviceable as an engineering estimate. Textbooks now present digital-computer techniques
for turbulent-flow analysis [32], but they are based strictly upon empirical assumptions
regarding the time mean of the turbulent stress field.
Chapter 1
Introduction
3
1Numbered references appear at the end of each chapter.
1.2 The Concept of a Fluid
Thus there is theory available for fluid-flow problems, but in all cases it should be
backed up by experiment. Often the experimental data provide the main source of information
about specific flows, such as the drag and lift of immersed bodies (Chap. 7).
Fortunately, fluid mechanics is a highly visual subject, with good instrumentation [4,
5, 35], and the use of dimensional analysis and modeling concepts (Chap. 5) is widespread.
Thus experimentation provides a natural and easy complement to the theory.
You should keep in mind that theory and experiment should go hand in hand in all
studies of fluid mechanics.
From the point of view of fluid mechanics, all matter consists of only two states, fluid
and solid. The difference between the two is perfectly obvious to the layperson, and it
is an interesting exercise to ask a layperson to put this difference into words. The technical
distinction lies with the reaction of the two to an applied shear or tangential stress.
A solid can resist a shear stress by a static deformation; a fluid cannot. Any shear
stress applied to a fluid, no matter how small, will result in motion of that fluid. The
fluid moves and deforms continuously as long as the shear stress is applied. As a corollary,
we can say that a fluid at rest must be in a state of zero shear stress, a state often
called the hydrostatic stress condition in structural analysis. In this condition, Mohr’s
circle for stress reduces to a point, and there is no shear stress on any plane cut through
the element under stress.
Given the definition of a fluid above, every layperson also knows that there are two
classes of fluids, liquids and gases. Again the distinction is a technical one concerning
the effect of cohesive forces. A liquid, being composed of relatively close-packed molecules
with strong cohesive forces, tends to retain its volume and will form a free surface
in a gravitational field if unconfined from above. Free-surface flows are dominated
by gravitational effects and are studied in Chaps. 5 and 10. Since gas molecules
are widely spaced with negligible cohesive forces, a gas is free to expand until it encounters
confining walls. A gas has no definite volume, and when left to itself without
confinement, a gas forms an atmosphere which is essentially hydrostatic. The hydrostatic
behavior of liquids and gases is taken up in Chap. 2. Gases cannot form a
free surface, and thus gas flows are rarely concerned with gravitational effects other
than buoyancy.
Figure 1.1 illustrates a solid block resting on a rigid plane and stressed by its own
weight. The solid sags into a static deflection, shown as a highly exaggerated dashed
line, resisting shear without flow. A free-body diagram of element A on the side of the
block shows that there is shear in the block along a plane cut at an angle through A.
Since the block sides are unsupported, element A has zero stress on the left and right
sides and compression stress p on the top and bottom. Mohr’s circle does not
reduce to a point, and there is nonzero shear stress in the block.
By contrast, the liquid and gas at rest in Fig. 1.1 require the supporting walls in order
to eliminate shear stress. The walls exert a compression stress of p and reduce
Mohr’s circle to a point with zero shear everywhere, i.e., the hydrostatic condition. The
liquid retains its volume and forms a free surface in the container. If the walls are removed,
shear develops in the liquid and a big splash results. If the container is tilted,
shear again develops, waves form, and the free surface seeks a horizontal configura-
4 Chapter 1 Introduction
tion, pouring out over the lip if necessary. Meanwhile, the gas is unrestrained and expands
out of the container, filling all available space. Element A in the gas is also hydrostatic
and exerts a compression stress p on the walls.
In the above discussion, clear decisions could be made about solids, liquids, and
gases. Most engineering fluid-mechanics problems deal with these clear cases, i.e., the
common liquids, such as water, oil, mercury, gasoline, and alcohol, and the common
gases, such as air, helium, hydrogen, and steam, in their common temperature and pressure
ranges. There are many borderline cases, however, of which you should be aware.
Some apparently “solid” substances such as asphalt and lead resist shear stress for short
periods but actually deform slowly and exhibit definite fluid behavior over long periods.
Other substances, notably colloid and slurry mixtures, resist small shear stresses
but “yield” at large stress and begin to flow as fluids do. Specialized textbooks are devoted
to this study of more general deformation and flow, a field called rheology [6].
Also, liquids and gases can coexist in two-phase mixtures, such as steam-water mixtures
or water with entrapped air bubbles. Specialized textbooks present the analysis
1.2 The Concept of a Flu
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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