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الكلية كلية الهندسة
القسم الهندسة المدنية
المرحلة 2
أستاذ المادة كاظم نايف كاظم اليساري
6/6/2011 3:48:56 PM
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 Fluid 5
Static
deflection
Free
surface
Hydrostatic
condition
Solid Liquid
A A A
(a) (c)
(b) (d)
0
0
A A
Gas
(1)
– p – p
p
p
p
= 0
2
1
– = p – = p
1
Fig. 1.1 A solid at rest can resist
shear. (a) Static deflection of the
solid; (b) equilibrium and Mohr’s
circle for solid element A. A fluid
cannot resist shear. (c) Containing
walls are needed; (d) equilibrium
and Mohr’s circle for fluid
elementA.
1.3 The Fluid as a Continuum
of such two-phase flows [7]. Finally, there are situations where the distinction between
a liquid and a gas blurs. This is the case at temperatures and pressures above the socalled
critical point of a substance, where only a single phase exists, primarily resembling
a gas. As pressure increases far above the critical point, the gaslike substance becomes
so dense that there is some resemblance to a liquid and the usual thermodynamic
approximations like the perfect-gas law become inaccurate. The critical temperature
and pressure of water are Tc 647 K and pc 219 atm,2 so that typical problems involving
water and steam are below the critical point. Air, being a mixture of gases, has
no distinct critical point, but its principal component, nitrogen, has Tc 126 K and
pc 34 atm. Thus typical problems involving air are in the range of high temperature
and low pressure where air is distinctly and definitely a gas. This text will be concerned
solely with clearly identifiable liquids and gases, and the borderline cases discussed
above will be beyond our scope.
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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