fluid_mechanics

5.3 First Law of Thermodynamics—The Energy Equation 235
(b) The power lost due to friction can be obtained from
Eq. 5.85 as
W # loss Qh L 162.4 lb/ft 3 211.96 ft 3 /s2115 ft2
1830 ft#lb/s 11 hp550 ft#lb/s2
3.33 hp
(Ans)
COMMENTS The remaining 10 hp 3.33 hp 6.67 hp
that the pump adds to the water is used to lift the water from the
lower to the upper lake. This energy is not “lost,” but it is stored
as potential energy.
By repeating the calculations for various head losses, h L , the
results shown in Fig. E5.25b are obtained. Note that as the head
loss increases, the flowrate decreases because an increasing portion
of the 10 hp supplied by the pump is lost and, therefore, not
available to lift the fluid to the higher elevation.
3.5
3
Q, ft 3 /s
2.5
2
1.5
(15 ft, 1.96 ft 3 /s)
1
0.5
0
0 5 10
h L , ft
15 20 25
F I G U R E E5.25b
A comparison of the energy equation and the Bernoulli equation has led to the concept of
loss of available energy in incompressible fluid flows with friction. In Chapter 8, we discuss in detail
some methods for estimating loss in incompressible flows with friction. In Section 5.4 and
Chapter 11, we demonstrate that loss of available energy is also an important factor to consider in
compressible flows with friction.
F l u i d s i n t h e N e w s
Smart shocks Vehicle shock absorbers are dampers used to provide
a smooth, controllable ride. When going over a bump, the relative
motion between the tires and the vehicle body displaces a
piston in the shock and forces a viscous fluid through a small orifice
or channel. The viscosity of the fluid produces a head loss that
dissipates energy to dampen the vertical motion. Current shocks
use a fluid with fixed viscosity. However, recent technology has
been developed that uses a synthetic oil with millions of tiny iron
balls suspended in it. These tiny balls react to a magnetic field
generated by an electric coil on the shock piston in a manner that
changes the fluid viscosity, going anywhere from essentially no
damping to a solid almost instantly. A computer adjusts the current
to the coil to select the proper viscosity for the given conditions
(i.e., wheel speed, vehicle speed, steering-wheel angle, lateral acceleration,
brake application, and temperature). The goal of these
adjustments is an optimally tuned shock that keeps the vehicle on
a smooth, even keel while maximizing the contact of the tires with
the pavement for any road conditions. (See Problem 5.107.)
5.3.4 Application of the Energy Equation to Nonuniform Flows
The forms of the energy equation discussed in Sections 5.3.2 and 5.3.3 are applicable to onedimensional
flows, flows that are approximated with uniform velocity distributions where fluid
crosses the control surface.