Page 365 - Civil Engineering Formulas

294 CHAPTER TWELVE

θ

w 2
Meniscus

h w 1

l

FIGURE 12.1 Capillary action raises water
in a small-diameter tube. Meniscus, or liquid
surface, is concave upward.

ratio of the tangential shearing stresses between flow layers to the rate of change
of velocity with depth:
!
(12.2)
dV/dy

2
2
where ! shearing stress, lb/ft (N/m )
V velocity, ft/s (m/s)
y depth, ft (m)
Viscosity decreases as temperature increases but may be assumed independent
of changes in pressure for the majority of engineering problems. Water at 70°F
2
2
(21.1°C) has a viscosity of 0.00002050 lb s/ft (0.00098 N s/m ).
Kinematic viscosity is defined as viscosity divided by density . It is
2
2
so named because its units, ft /s (m /s), are a combination of the kinematic
units of length and time. Water at 70°F (21.1°C) has a kinematic viscosity of
2
2
0.00001059 ft /s (0.000001 Nm /s).
In hydraulics, viscosity is most frequently encountered in the calculation of
Reynolds number to determine whether laminar, transitional, or completely tur-
bulent flow exists.