CIVIL ENGINEERING FORMULAS

294 CHAPTER TWELVE

ratio of the tangential shearing stresses between flow layers to the rate of change
of velocity with depth:

(12.2)

where!shearing stress, lb/ft^2 (N/m^2 )
Vvelocity, ft /s (m/s)
ydepth, 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
(21.1°C) has a viscosity of 0.00002050 lbs/ft^2 (0.00098 Ns/m^2 ).
Kinematic viscosityis defined as viscosity divided by density . It is
so named because its units, ft^2 /s (m^2 /s), are a combination of the kinematic
units of length and time. Water at 70°F (21.1°C) has a kinematic viscosity of
0.00001059 ft^2 /s (0.000001 Nm^2 /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.



!

dV/dy

θ

h

Meniscus

w 1

l

w 2

FIGURE 12.1 Capillary action raises water

in a small-diameter tube. Meniscus, or liquid

surface, is concave upward.