Normal stress measurements
Whorlow [1992] notes that, of the many methods which have been proposed
for the measurement of various combinations of the first and second normal
stress differences, N1 and N2 respectively, few can give reliable estimates of
N2. Combined pressure gradient and total force measurements in the cone-andplate
geometry, or combined cone-and-plate and plate-plate force
measurements, appear to give reliable values [Walters, 1975]; and satisfactory
results may also be obtained from techniques based on the measurement of the
elevation of the surface of a liquid as it flows down an inclined open duct [Kuo
. [and Tanner, 1974
During rotational flow of liquids which display normal stresses, the tension
along the circular streamlines is always greater than that in other directions so
that streamlines tend to contract unless prevented from doing so by an
appropriate pressure distribution. Determination of the pressure distribution
(say, over the area of the plate in a cone-and-plate system) therefore provides a
means of determining the total force exerted on the plate. Alternatively, and in
practice more generally, the total force is measured directly. As the pressure
distribution and total force measured in a (parallel) plate-plate system depend
in a different manner on the normal stress differences, both cone-and-plate
systems and plate-plate systems have been used to obtain values for both N1
and N2. Numerous modifications to instruments (such as the Weissenberg
Rheogoniometer) to permit more accurate normal force measurements have
been described, some involving the use of piezoelectric crystals as very stiff
. [load cells [Higman, 1973
In the work reported by Jackson and Kaye [1966] the spacing, h, between a
cone and a plate was varied and the normal force measured as a function of gap
size. The same method was used by Marsh and Pearson [1968] who showed
,that
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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