Environmental Engineering Reference
In-Depth Information
coefficient of the SUV plus trailer is higher than anticipated. The ratio of the trailer
tongue length (~vehicle-vehicle spacing) to SUV length is
L
tongue
L
SUV
¼
1
:
22
4
:
256
¼
0
:
286
This is clearly within the vehicle-to-vehicle spacing zone where the aero-
dynamic drag coefficient of the trailing vehicle (i.e. the covered trailer) is a higher
fraction of the combined drag than the lead vehicle. The other factor for the high
drag coefficient when pulling a covered trailer is that utility trailers have poor
shape factors for aerodynamic purposes, being basically a vertical wall with only
moderately rounded edges.
A more thorough evaluation of vehicle drag would be made by the scale model
testing in a wind tunnel. We conclude this discussion of vehicle with trailer in tow
by noting that drag coefficient
C
d
has four components: wheel drag, skin friction
drag, roof equipment drag and pressure wave drag. Each of these will be briefly
explained with regard to their influence on the overall drag coefficient:
Wheel drag
is due to turbulent flow beneath the vehicle and the churning due to
the wheels. It can be a large fraction of the total drag coefficient. In passenger
trains this component is typically 38-66% of the total drag.
●
Skin friction drag
is a retarding force that results from shearing stresses of the
airstreams over the sides of the vehicle. This also includes floor pan and roof,
but these are grouped into wheel drag and roof equipment in this discussion.
Skin friction drag in a passenger train is 27-30% of the total.
●
Roof equipment drag
is the drag due to skin friction and roof-mounted luggage
racks, antenna, sun/moon roofs and the like. The most common type of roof
equipment in an SUV is the roof cargo rack (used for skis, cargo/luggage
containers etc.). In a train, this component may be 8-20% of the total.
●
Pressure wave drag
is a characteristic of the vehicle nose and tail shape and
generally independent of the overall length of the vehicle. In a passenger train
and high speed train, this component is only 8-13% of the total.
●
In vehicle design the aerodynamic drag coefficient would be decomposed into
its pressure wave drag and skin friction drag components as shown in (11.12):
F
aero
¼
0
:
5
r
AC
do
V
2
þ
0
:
5
r
LC
fo
V
2
ð
N
Þ
ð
11
:
12
Þ
where
A
is the vehicle frontal area,
L
is the overall length,
r
is the corrected air density
(11.1), and drag coefficient
C
do
is the pressure drag and
C
fo
is the skin friction drag.
11.4 Class-8 tractor test
The reason for including heavy duty trucks such as the over-the-road class-8 is to
further clarify the trailer towing case. Semi-tractor-trailers continue to be optimized
for aerodynamic drag and more efficient drivelines. Aerodynamics are improved by
trends to a more tear-drop shape, use of side skirts to cover fuel tanks and other
attachments, plus fairings on the roof to streamline the airflow over the cab and up
