Page 82 - Industrial Ventilation Design Guidebook

4,1 FLUID FLOW 47

FIGURE 4.1 Viscosity

Surface Tension
Surface tension is the property of a fluid that produces capillary action, the
rise and fall in a tube.
Water in a tube wets the glass, and the liquid rises, producing a cup. In
the case of mercury, the glass is not wetted and the liquid falls, producing an
inverted cup.

Viscosity
Viscosity is the shear resistance between adjacent fluid layers. Con-
sider in Fig. 4.1 the shearing action between two parallel planes, each of
area A, separated by a distance Y. The tangential force F for a given area
required to slide one plate over the other at a velocity (v) parallel to each
other is

The proportionality factor /u, is the dynamic viscosity of the fluid, its units be-
2
2
ing force x time/length and is expressed as N s m~ or Pa s.
Examination of the thermodynamic properties of fluid tables shows how
the viscosity varies with temperature. In order to obtain a general impression
of this, consider the data in the thermal properties of fluid tables and the vari-
ous values at different temperatures.
Another viscosity unit is the kinematic viscosity v. This is the ratio of vis-
2
1
cosity to density. Common units used for this are the stoke (1 cm s"" ) and the
1
2
centistoke (1 mm s" ).
Because the velocity change in the y direction is linear, Eq. (4.8) can be
written as

When the shearing stress T — F/A,

With most fluids the shearing stress r is linearly proportional to the change of
velocity; hence viscosity /u, is not a function of dv/dy, A fluid having these
characteristics is called a Newtonian fluid.