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Aerodynamic s
Aerodynamics of the VW Caddy –
Best cD Value at 0.30
The city delivery van VW Caddy in fifth generation from Volkswagen Commercial Vehicles achieves
a cD value of less than 0.30, making it the best in its class. The value was achieved primarily
through the consistent use of numerical flow simulation. The air resistance has been improved by
9 % compared with the predecessor model – and this without restricting functionality and usability.
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A U T H ORS
g Aerodynamics is concerned with
Dr.-Ing. Rouven Petzold
is Aerodynamics Engineer in the
Virtual Development of Volkswagen
Commercial Vehicles in Wolfsburg
(Germany).
Dr.-Ing. René Wolf
is Aerodynamics Engineer in the
Development of Volkswagen
Commercial Vehicles in Wolfsburg
(Germany).
the air forces acting on the moving vehicle, in particular with the air resistance,
which is an important control variable for
low fuel consumption and CO2 emissions.
In automotive development, the aerodynamic quality is described by the dimensionless drag coefficient cD. According to
Eq. 1, this cD value is the drag force FD
normalized with the product of dynamic
pressure q∞ and frontal area A:
FD
Eq. 1 ​
​c​D​ = _
​q​∞
​ ​⋅ A​
© Volkswagen Commercial Vehicles
Here, a small cD value and a small frontal area A leads to a low drag force FD.
In commercial vehicles, in addition to
design (styling), freight load capacity
and suitability for everyday use are
more important than in passenger cars.
ATZ worldwide
04|2021 17
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Aerodynamic s
FIGURE 1 Roof spoiler and taillights with defined separation edge form an attic on the Caddy 5 (left), in the detail, the shape of the taillight can
be seen in cross-section; the attic at the 1:2.5 wind tunnel model in clay design (right) (© Volkswagen Commercial Vehicles)
In particular, this is expressed in requirements for ground clearance as well as
loading dimensions. Therefore, the reduction of the frontal area is only suitable to
a very small extent for reducing air resistance; aerodynamicists deal more with
the styling and shape optimization of the
vehicle for a given frontal area.
At the beginning of the Caddy 5 development, the goal “best-in-class cD value”
was set to make the necessary contribution to the reduction of CO2 emissions.
CFD simulation was chosen as the main
development tool because it offers the
possibility to make quick decisions in
the very time-intensive, highly dynamic
styling and concept process and thus to
set the right course for a low cD value
early on. During the development process, two tests on 1:2.5 models were
planned to validate the results; 1:1 mod-
els for aerodynamic development were
deliberately avoided.
OPTIMIZATION OF THE
REAR OF THE VEHICLE
One of the most challenging tasks in
aerodynamics of commercial vehicles
is the optimal design of the rear of the
ve­­hicle, since the possibilities of tapering roof and side (“boat-tailing” [1])
are very limited due to high demands
on loading height and width. An additional complicating factor was the need
to achieve a greater loading width compared with the predecessor Caddy 4.
Under these conditions, the tapering of
roof and side were increased as far as
the project premises allowed.
At the same time, further potential
for optimization at the rear was sought.
Model test 1:2.5
Simulation model
Pre-production
vehicle 1:1
Front-wheel spoiler
-0.006
-0.008
-0.007
Rear-wheel spoiler
-0.002
-0.002
-0.001
Spare tire
-0.002
-0.002
-0.001
Attic (taillights)
-0.005
-0.005
-
Lowering
-0.006
-0.007
-0.005
Without roof rails
0.000
-0.001
-0.002
Closed brake cooling duct
-0.002
-0.001
-0.002
∆cD value [-]
TABLE 1 Comparison of the obtained ∆cD potentials for model test, simulation model
and pre-production vehicle (© Volkswagen Commercial Vehicles)
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In addition to the well-known shaping
of a roof spoiler as a clearly defined
separation edge, a fixation of the separation on the sides is decisive for a low
cD value in city delivery vans. This is
due to the proportions of the Caddy,
which has a high rear end in relation
to its width, when comparing to a car
with a blunt rear (for example the VW
Golf). Here, the large cD potential could
already be shown in the concept phase
with the help of CFD simulation. However, a defined separation edge with
small radius made of sheet steel proved
to be a great challenge in terms of production technology, so that the idea of
a high tail light for forming this sepa­
ration edge was proposed, FIGURE 1
(left). This suggestion enabled an even
more streamlined styling that corresponds in cross-section to the shape
of a roof spoiler. The combination of
taillights and roof spoiler at the rear
of the Caddy 5 now aerodynamically
corresponds to an aerodynamic attic
[1]. The shape of this attic increases
pressure recovery across the sides of
the vehicle and reduces the negative
pressure on the rear surface. Since
Volks­wagen has not yet produced an
aerodynamic attic on any vehicle in
series production, a test was carried
out on a 1:2.5 model in a wind tunnel,
FIGURE 1 (right), to confirm the magnitude of the cD potential of ∆cD = -0.005
for the lateral parts (taillights).
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FIGURE 2 Visualization of detached flow areas by an isosurface with a total pressure coefficient equal to zero for Caddy 4 (left) and
Caddy 5 (right) – the front of the Caddy 5 is free of detached flow (© Volkswagen Commercial Vehicles)
Additionally, TABLE 1 shows some ∆cD
values of the model measurement in
comparison to the simulation results and
the values on the later pre-production
vehicle. Especially the measurement of
the attic and the lowering on the model
have contributed to the improvement of
the prediction quality of the simulation
model. The comparison of the results
shows a good agreement of the values,
which underlines the suitability of model
test and simulation as development tools.
FRONT OPTIMIZATION
The front shaping with today’s auto­
motive stylings confronts aerodynamicists with further tasks. On the one
hand, a high sweep of the front is implemented, which makes a straight approach
of the flow in front of the front wheel
arch impossible; on the other hand, in the
area of the fog lights, a styling element is
often chosen which suggests an air inlet
and often leads to detached flow at the
front due to its pronounced edges [2].
One possibility to make this basic
styling aerodynamically favorable is a
so-called Air Curtain. With the Caddy 5,
intensive optimization loops between
aerodynamics and styling departments
have resulted in an optimal solution
without an Air Curtain, which on the
one hand shows a frame-shaped element,
but on the other hand does not cause
detached flow. In FIGURE 2 these areas of
detached flow are visualized by an isosurface with a total pressure coefficient
of zero. It can be seen that the front of
the Caddy 5 does not cause detached flow
in front of the wheel arch. This leads to a
reduction of the cD value by -0.002 compared to the predecessor in this area.
ENGINE UNDERHOOD FLOW
In addition to the aerodynamic development of the upper car body, the engine
underhood flow and ensuring the cooling
air mass flows was also investigated
using the same simulation model. Since
both disciplines were organized from a
single source, an optimal cooling air flow
with low leakage could be implemented,
which contributes to the low cD value.
Here it is important to keep the cooling air opening in the front end as small
as possible and to guide all air through
FIGURE 3 Sealing measures as well as air ducting of the Caddy 5 in blue;
the cooling package is colored green (© Volkswagen Commercial Vehicles)
ATZ worldwide
04|2021 the cooling package. For this purpose,
a cooling air duct was designed which
seals very well between the air inlet and
the cooling package. The quality of such
a seal can be quantified by the so-called
air efficiency. This value is the ratio of
air mass flow through the cooling package to air mass flow through the front
end. In the case of the Caddy 5, this air
efficiency is over 70 %. FIGURE 3 shows
the sealing measures.
UNDERBODY AERODYNAMICS
For a low cD value, an underbody that
is as smooth and closed as possible is
advantageous. This is usually achieved
by attaching cladding components.
From the front area up to the rear axle,
a smoothly closed underbody paneling
can be easily implemented. FIGURE 4
shows the aerodynamically effective
cladding components of the Caddy 5
compared to its predecessor. One can see
the much larger paneling percentage of
the Caddy 5 from the front to the rear
axle. Behind that paneling however, the
work on the commercial vehicle-type
underbody of the rear end presents
aerodynamicists with a major task.
The high variance of the rear end on
the underbody due to all-wheel drive,
long wheelbase and various aggregates
and exhaust gas systems makes the technical and economic implementation of
a flat underbody paneling in the rear
area difficult. The accommodation of
the spare wheel is another challenge. In
commercial vehicles, it is accommodated
on the underbody for reasons of accessibility when the vehicle is loaded. These
conditions lead to the fact that implementing an aerodynamically effective
paneling was impossible. This made it
all the more important to smooth the
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C OVER STORY
Aerodynamic s
spring seats was implemented, which
leads to a further smoothing of the
underbody and results in an advantage
of ∆cD = -0.002, FIGURE 5. The improved
underbody paneling, lowering and
cranking of the axle tube lead to an
overall improvement of ∆cD = -0.017
and thus contribute significantly to
achieving a low cD value.
DETAIL OPTIMIZATION
FIGURE 4 Comparison of the aerodynamic underbody paneling for Caddy 4 (top) and Caddy 5 (bottom) –
the underbody of the Caddy 5 is smoother and more closed (© Volkswagen Commercial Vehicles)
FIGURE 5 The offset of the axle tube (light blue) of the rigid axle smoothes the underbody and thus
improves the underbody flow (© Volkswagen Commercial Vehicles)
underbody by skillfully shaping the
installed components.
In order to achieve a high payload, the
Caddy 5 has a rigid axle. From an aerodynamic point of view, a rigid axle is
unfavorable, as the axle tube is located
in the underbody flow due to the concept
and obstructs the air flow under the
vehicle. In order to compensate for this
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disadvantage, the vehicle was lowered
at the rear as far as the serviceability
permits in a first step. In addition to the
aerodynamically positive effect of lowering the vehicle, this measure also brings
the axle tube closer to the underbody
and thus reduces its negative influence.
In a second step, an additional cranking
of the axle tube in the area between the
In addition to these fields of activity,
intensive detail optimization was pursued. Some examples are explained in
the following. One important detail is
the design of the A-pillar. In addition
to a large radius, the transitions to the
windshield and side windows are of
particular importance. These should be
as smooth, continuous and tangential
as possible. In the Caddy 5, the step
between the windshield and the A-pillar
has been significantly reduced compared
to its predecessor. FIGURE 6 shows an isosurface with a total pressure coefficient
of zero in the area of the A-pillar; a significantly reduced proportion of de­­
tached flow can be seen. The improvement in the cD value here is ∆cD = -0.003
compared to the predecessor.
The exterior mirrors also have a high
relevance for aerodynamics and must be
intensively optimized in an interdisciplinary manner, since it must fulfill a
multitude of technical functions in
ad­d ition to the requirements of styling
and design. The exterior mirror of the
Caddy 5 was optimized in detail in
numerous studies. First of all, it has a
cD advantage over the predecessor vehicle conceptually due to the almost parallel position of the side surface of the
mirror head to the side window. A backward-facing step was implemented on
the underside of the mirror head which
leads to reduced soiling of the mirror
glass in rain. Often, downward ex­­
truded spoiler lips are used here, which
decrease soiling but increase drag. The
measure implemented on the Caddy 5
meets the requirements for low cD value
and for low soiling.
A further detail optimization was carried out for the brake cooling channel
in the area of the underbody. Here, its
shape has been optimized so that a targeted flow of air to the brake is achieved
and only as much air reaches the brake
as is needed for cooling.
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One System —
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FIGURE 6 Visualization of detached flow areas
using an isosurface with a total pressure coef­
ficient of zero for Caddy 4 (top) and Caddy 5
(bottom) – the lower step from windscreen to
A-pillar in the Caddy 5 reduces turbulences
(© Volkswagen Commercial Vehicles)
AMS Screen Imaging System
SUMMARY
Volkswagen Commercial Vehicles has
been able to significantly improve the
aerodynamics of its city delivery van.
The new VW Caddy in fifth generation
achieves a best-in-class cD value of less
than 0.300 for the aerodynamically
best derivatives, which represents an
improvement of approximately 9 %
over its predecessor. The vehicle represents a milestone in terms of commercial vehicle aerodynamics, as this value
was achieved without impeding functionality and usability; for example, the
Caddy 5 has even a greater loading width
than its predecessor. This success was
achieved primarily through the consistent use and the high level of acceptance
of CFD simulation as a basis for decisionmaking in the development process.
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REFERENCES
[1] Schütz, T. (ed.): Hucho – Aerodynamik des
Automobils: Strömungsmechanik, Wärmetechnik,
Fahrdynamik, Komfort. Wiesbaden: Springer
Vieweg, 2013
[2] Blacha, T.; Islam, M.: The Aerodynamic Concept
of the Audi Q5. In: ATZworldwide 3/2017, pp. 42-47
ATZ worldwide
04|2021 Goniophotometer
2 LumiCam
Imaging camera
3 Vehicle lights
4 Baffles
5 Projection screen
6 DSP
Photometer
Instrument Systems GmbH | Optronik Line
info-berlin@instrumentsystems.com
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