1.2.2Non-Newtonian fluid behaviour
A non-Newtonian fluid is one whose flow curve (shear stress versus shear rate)is non-linear or does not pass through the origin, i.e. where the apparentper-mviscosity, shear stress divided by shear rate, is not constant at a given teature and pressure but is dependent on flow conditions such as flow geometry, shear rate, etc. and sometimes even on the kinematic history of the fluid element under consideration. Such materials may be conveniently grouped intothree general classes:
(1) fluids for which the rate of shear at any point is determined only by
the value of the shear stress at that point at that instant; these fluids are
variously known as ‘time independent’, ‘purely viscous’, ‘inelastic’ or
‘generalised Newtonian fluids’, (GNF);
(2) more complex fluids for which the relation between shear stress and shear
rate depends, in addition, upon the duration of shearing and their kinematic
history; they are called ‘time-dependent fluids’, and finally,
(3) substances exhibiting characteristics of both ideal fluids and elastic solids
and showing partial elastic recovery, after deformation; these are categorised
as ‘visco-elastic fluids’.
This classification scheme is arbitrary in that most real materials often exhibit
a combination of two or even all three types of non-Newtonian features.
Generally, it is, however, possible to identify the dominant non-Newtonian
characteristic and to take this as the basis for the subsequent process calculations.
Also, as mentioned earlier, it is convenient to define an apparent
viscosity of these materials as the ratio of shear stress to shear rate, though
the latter ratio is a function of the shear stress or shear rate and/or of time. Each
type of non-Newtonian fluid behaviour will now be dealt with in some detail.
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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