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Abstract and Figures

Compared to sedans, pickup truck (PU) and sports utility vehicle (SUV) body styles pose additional aerodynamic challenges due to their complex wake structures. The introduction of a realistic generic PU and SUV model as an open access tool is expected to yield benefits to the wider aerodynamics community in characterizing wind tunnel & CFD performance including correlation, development and process control, developing a deeper understanding of the aerodynamic mechanisms unique to PU and SUV body styles, and finally, research and development around a generic realistic PU and SUV model between OEMs, vendors and academia. This paper introduces the Generic Truck Utility (GTU) as a realistic, generic PU truck and interchangeable SUV model. The paper will focus on the design and development of the GTU and will present a summary of preliminary experimental results of the GTU complemented by numerical simulations.
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2020-01-0664
The GTU A New Realistic Generic Pickup Truck and SUV Model
S. Woodiga, P. Norman, K. Howard, N. Lewington, R. Carstairs, B. Hupertz, K. Chalupa
Ford Motor Company
Abstract
Traditionally, ground vehicle aerodynamics has been researched
with highly simplified models such as the Ahmed body and the SAE
model. These models established and advanced the fundamental
understanding of bluff body aerodynamics and have generated a large
body of published data, however, their application to the development
of passenger vehicles is limited by the highly idealized nature of their
geometries. To date, limited data has been openly published on
aerodynamic investigations of production vehicles, most likely due to
the proprietary nature of production vehicle geometry. In 2012, Heft
et al. introduced the realistic generic car model ‘DrivAer’ that better
represents the flow physics associated with a typical production
vehicle. The introduction of the DrivAer model has led to a broad set
of published data for both experimental and computational
investigations and has proven itself invaluable as a correlation and
calibration tool of wind tunnels, the validation of computational fluid
dynamics (CFD) codes and increasing the understanding of the
fundamental flow physics around passenger vehicles.
Automotive sales trends in the United States, published by the
Bureau of Economic Analysis in 2018, indicate that sales of Pickup
Trucks (PUs) and Sports Utility Vehicles (SUVs) have increased over
the past 10 years and are outselling sedans at a rate just over two to
one. Compared to sedans, PU and SUV body styles pose additional
aerodynamic challenges due to their complex wake structures. The
introduction of a realistic generic PU and SUV model as an open
access tool is expected to yield benefits to the wider community,
equivalent to those of the DrivAer passenger vehicle. This paper
introduces the Generic Truck Utility (GTU) as a realistic, generic PU
truck and interchangeable SUV model. The paper will focus on the
design and development of the GTU and will present a summary of
preliminary experimental results of the GTU complemented by
numerical simulations using iconCFD®, an open source based CFD
solver.
Introduction
As we step into a new decade in 2020, the automotive industry is
going through a revolution. Electrification, driving autonomy and
stringent emissions regulations are three major focus areas that will
require intensive yet dynamic development to meet the needs of a
rapidly evolving mobility industry. Aerodynamics has a key role to
play in each of the focus areas mentioned: minimizing aerodynamic
drag is a critical element in delivering competitive fuel economy for
internal combustion (IC) vehicles, extending range while easing
customer range anxiety for electric vehicles including autonomous
vehicles and meeting increasingly stringent regulatory emissions; and
aerodynamic flow control is crucial in ensuring autonomous vehicle
sensors are operable in all ambient conditions.
The aerodynamic performance of an automobile launched within
the past five years is highly optimized for most vehicle lines as the
optimal parameters for vehicle shape and proportions are well known.
A modern-day automotive aerodynamicist focuses on optimizing
details around a vehicle such as wheels, tires, tire defectors, mirrors
and rear-end add on devices such as roof spoilers and D-pillar strakes.
Although these devices additively contribute to a considerable amount
of aerodynamic drag reduction, independently they contribute between
1 to 3 percent drag reduction. The capability to predict and optimize
the aerodynamic performance of these devices requires a numerical or
experimental tool with a high degree of accuracy and precision
accompanied by stringent process control. This is achieved by
benchmarking and developing wind tunnels and CFD codes with
robust reference models.
Le Good and Garry [2] provide a comprehensive review on the
historical perspective of automotive aerodynamics reference models
dating back to the 1970’s. They classified non-production models into
two distinct categories: simple bodies and basic car shapes. An
interesting point raised in their review is that the first fuel crisis of the
1970’s prompted a new focus on aerodynamic drag reduction as a
means of achieving improved fuel consumption. This attracted
significant investment in dedicated automotive wind tunnels and the
birth of numerous reference models such as the Ahmed, SAE and
MIRA models. Not limited to the models mentioned, over the course
of 30 years, the automotive aerodynamics community conducted in-
depth research in fundamental bluff body aerodynamics resulting in
broad collection of published articles reviewed by Le Good and Garry
[2].
In 2012, Heft et al. [3], introduced the DrivAer model as a generic
but realistic automotive reference model. The goal of this model was
to provide a more realistic representation of a production sedan absent
from established aerodynamic reference models, while preserving the
generic nature of the shape. Since then, a number of research topics in
aerodynamics areas of interest such as CFD process development and
characterization [4-12], flow control [13-17], cooling airflow [18,19],
wind tunnel process control [20-23], crosswind and vehicle dynamics
[24-26], real world flow simulation [27], and motorsports [28] have
been published, with the DrivAer model as the test subject. The
DrivAer model has also resulted in a derivative with SUV proportions
and characteristics named the AeroSUV [29]. Although, the DrivAer
model has established its popularity in the domain of automotive
aerodynamics by being a realistic, generic and open access model, a
clear gap still exists in the availability of an established generic,
realistic pickup truck model.
Lokhande et al. [30] introduced a generic PU model that has since
been used for detailed computational and experimental studies [30-39].
The schematic of the Lokhande model is shown in Figure 1. The work
which resulted from this model has provided valuable insight into the
complex flow field around PUs including CFD strategies for
simulating PUs, although key details such as cooling intakes, engine
bay mockup, wheelhouse, wheel & tire assembly, underbody detail and
cab and box variations were not represented. These are critical factors
that affect local flow interactions resulting in global aerodynamic
effects.
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Figure 1. Side View Schematic of the Lokhande Model [30]
Conception and Development of the GTU
As previously discussed, the DrivAer model has established itself
as the reference model for sedans in industry and academia. With
consumer interest shifting towards PUs and SUVs, this is a suitable
time to introduce a PU and SUV aerodynamics reference model. Ford
Motor Company, therefore, would like to introduce a new realistic,
generic pickup truck model, the GTU. The following sections of this
paper will discuss the definition and design process, model overview
and preliminary experimental and numerical results.
At this point, two important questions to answer are What is the
GTU and why develop the GTU? The GTU is a generic, realistic
pickup truck model that can be re-configured to represent an SUV,
similar in philosophy to the DrivAer & AeroSUV. The decision to
develop the GTU can be attributed to four key justifications:
1. A correlation/calibration model to improve experimental
and computational process control for PU and SUV body
styles with larger blockage ratios and complex wake
structures
2. Characterization and investigation of complex aerodynamics
of pickup truck and SUV body styles
3. Investigation of body-on-frame open cooling flows
4. An open access model for industry and academic
collaboration for research on PU and SUV aerodynamics
With the existence of the DrivAer model and its open cooling
variant, the OCDA, the next important question to answer is why not
consider adapting the DrivAer model to create a generic PU/SUV
model. Three key characteristics justify why the DrivAer model is not
suitable for straightforward adaptation. The first is proportional
relevance of a sedan to support realistic pickup trucks and SUV shapes,
especially with the variance of PU cab and box length combinations.
Figure 2 and Figure 3 clearly highlight this incompatibility. The next
important characteristic is relevant ground clearance. PUs and SUVs
have higher ground clearances compared to sedans and a larger ride
height variation to cater for driveline and off-road capability. The
current architecture of the DrivAer model does not support this
requirement. The final point is being able to represent relevant
underbody geometry to reflect body-on-frame architectures of PUs and
certain subset of SUVs. This ability is critical in enabling
representative body-on-frame cooling and underbody flow
representation. Figure 4 and Figure 5 highlight these distinct
differences. With the important justifications discussed above, the
decision was made to pursue an all-new design of a generic, realistic
PU and SUV model that will capture the key requirements for these
vehicle segments.
Figure 2. Overlay Comparison of the GTU and DrivAer Models - Front View
Figure 3. Overlay Comparison of the GTU and DrivAer Models - Side View
Figure 4. DrivAer Underbody
Figure 5. GTU Underbody
Requirements
The requirements for the GTU were defined to (i) parallel the
refinement of the DrivAer specific to pickup trucks and (ii) extend the
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functionality to encompass the unique vehicle configurations set forth
by the industry. To parallel the added refinement and automotive
detail provided by the DrivAer, the GTU was developed by first
capturing the critical design details which define pickups and large
SUV’s by the standards of the industry today: upright grille, defined
front bumper, airdam, tall greenhouse and separate passenger cabin
from cargo area (for PUs). These elements were the initial definition
for the surface design of the GTU and the exact position of the surfaces
were then designated based on a sampling and averaging of the pickup
trucks and large SUV’s in the market place. Care was taken in the A-
surface design to generalize the vehicle and remove any OEM specific
design features while embodying the spirit of the vehicle segment.
Figure 6 through Figure 8 illustrate the various views of the long cab,
short box GTU configuration.
After defining the A-surface, focus shifted to development of a
modular concept for the vehicle proportions, to enable the systematic
study of interactions seen by the PU variants. The need to be able to
represent multiple cabin and cargo box lengths prompted market
evaluation to approximate suitable segment averages.
Further requirements included the ability for the vehicle to
simulate (i) SUV’s, (ii) body-on-frame components, (iii) drivetrain
components, (iv) cooling drag, and (v) ride height adjustment. The
SUV model was accomplished by adding surfaces to enclose the cargo
area of the pickup, details of which will be discussed in a later section.
The body-on-frame and drivetrain components were designed to be
very simplified but capture the essence of the components that they
represent. To reduce complexity for cooling drag flow, the GTU
utilizes the cooling pack, underhood components and transmission
defined by the OCDA and adds an optional transfer case to simulate
four-wheel drive vehicles. The ride height and wheelbase modularity
will be highlighted in a later section.
Finally, a vehicle size requirement was defined such that a
physical model could be tested at global wind tunnel facilities with a
reasonable nozzle exit to model frontal area blockage ratio, while
representing an established global PU segment. The proportions of the
model are defined equivalent to the mid-size PU segment, akin to the
Ford Ranger.
Figure 6. GTU Pickup Model – Front Left Corner View
Figure 7. GTU Pickup Model – Front View
Figure 8. GTU Pickup Model – Side View
Model Dimensions, Wheelbase Flexibility, Ride Height
& Wheel/Tire Assembly
As discussed previously, the GTU was sized appropriately to
enable wind tunnel testing in existing global wind tunnel facilities
without the concern of introducing a high blockage ratio. The
dimensions were defined based on sampling the pickup truck segment
and the true size of the GTU falls into the mid-size pickup truck
segment. This means that the GTU’s linear dimensions are
approximately 90% of a full-size pickup truck sold in the North
American market. GTU model dimensions are provided in Table 1.
Table 1. GTU Dimensions
Frontal Area
2.65 m
2
Overall Length
4520 – 5980 mm
Wheelbase
2540 – 4000 mm
Front Overhang
860 mm
Rear Overhang
1120 mm
Box to Cab Gap
20 mm
Overall Width (w/o mirrors)
1830 mm
Track Width
1545 mm
Overall Height
1622 – 1802 mm
Cab Height
1315 mm
The flexibility of the GTU provides for a wide range of vehicle
lengths, as shown in Table 2, of which the bookends for overall length
are 4.52 m and 5.98 m. Several of the longer configurations will not
be viable options for wind tunnels with shorter test sections, however,
the overall length flexibility allows for testing of configurations which
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are compatible with these facilities. Figure 9, displays the section of
the frame which houses the slip mechanism enabling frame length
adjustment. The frame on the model was designed to ensure ease of
adjustment and adequate load carrying capability. As the wheelbase is
adjusted, the driveshaft is appropriately lengthened or shortened to
ensure that geometry accuracy is maintained.
Table 2. GTU Overall Length Combinations
Cab Box Short Medium Long
Short 4520 mm 4885 mm 5250 mm
Medium 4885 mm 5250 mm 5615 mm
Long
5250 mm 5615 mm 5980 mm
Figure 9. GTU Underbody with Tube in Tube Section Highlighted (blue box)
for Wheelbase Adjustments
Ride height at each corner of the vehicle is managed via a slider
block mechanism shown in Figure 10. The slider block is actuated by
rotating a lead screw mechanism. Set screws secure the slider block in
place once the desired ride height is achieved. The GTU has a total
range of ride height adjustment of 180 mm for each wheel, which
corresponds to the front wheel lip to ground (FWLTG) and rear wheel
lip to ground (RWLTG) ranges summarized in Table 3. The ride
height adjustment mechanism takes up the package space that a shock
and spring assembly and wheel hub would typically occupy on a
production PU. Therefore, it is not expected that the mechanism
package would cause significant aerodynamic disruption locally in the
wheelhouse region. The effects of the mechanism can be characterized
by CFD studies and is a potential topic for future research and
publication.
Table 3. GTU Ride Height Range
Position
FWLTG (mm)
RWLTG (mm)
Abs Minimum 780 830
Design Position 870 920
Abs Maximum 960 1010
Figure 10. GTU Ride Height Adjustment Mechanism
Off-the-shelf parts were selected for the wheel assemblies. The
wheels are from a 2019 production Ford Ranger, are 16 inches in
diameter and available globally as a Ford Service Part (Part No: EB3C-
1007-F2A). The selected tires are the light truck & SUV tire, the Pirelli
Scorpion Verde All Season with a section width of 235mm, aspect
ratio of 70% and diameter of 16 inches. On-going research is underway
to determine whether a rigid wheel assembly can be defined for the
GTU, similar to the original specifications for the DrivAer model. This
may assist with potential users investigating scale model concepts.
Pickup Truck Proportional Modularity
To permit assessment of the aerodynamic interactions between
cab and box dimensions, the GTU has three cab configurations
designated as short, medium (+365 mm) and long (+730 mm from
short) roughly corresponding to a regular cab (two door), extended cab
and full four door configuration, respectively. Figure 11, Figure 14
and Figure 17, show various cab lengths with the short box.
The box length is adjustable with three configurations of short,
medium (+365 mm) and long (+730 mm from short). Box length
variation is found on both mid-size and full-size pickup trucks,
however, the full-size pickup trucks usually envelope the three options
selected here, hence their inclusion in the GTU design. Therefore, the
GTU has nine possible configurations for PU evaluation. The full
range of cab and box combinations are summarized in Table 4 and
pictured in Figure 11 through Figure 19, from the shortest to longest
variant.
Table 4. Summary Table of GTU Pickup Truck Configurations
Configuration
Model Type
Cab
Box
1 Pickup Short Short
2
Pickup
Short
Medium
3 Pickup Short Long
4 Pickup Medium Short
5 Pickup Medium Medium
6 Pickup Medium Long
7
Pickup
Long
Short
8 Pickup Long Medium
9 Pickup Long Long
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Figure 11. GTU Pickup Config 1 - Short Cab, Short Box
Figure 12. GTU Pickup Config 2 - Short Cab, Medium Box
Figure 13. GTU Pickup Config 3 - Short Cab, Long Box
Figure 14. GTU Pickup Config 4 - Medium Cab, Short Box
Figure 15. GTU Pickup Config 5 - Medium Cab, Medium Box
Figure 16. GTU Pickup Config 6 - Medium Cab, Long Box
Figure 17. GTU Pickup Config 7 - Long Cab, Short Box
Figure 18. GTU Pickup Config 8 - Long Cab, Medium Box
Figure 19. GTU Pickup Config 9 - Long Cab, Long Box
SUV Proportional Modularity
In addition to pickup truck modularity, the GTU can be
configured to represent an SUV body style. The design of the SUV A-
surface was an extension of the PU A-surface which closed out the box
area with a greenhouse section as depicted in Figure 20.
Figure 20. GTU SUV Config 12Front Left Corner View
The SUV greenhouse has 8º of roof taper starting at the trailing
edge of the side glass panel. A generic roof spoiler follows the roof
taper angle and extends 77 mm beyond the rear glass as illustrated in
Figure 21. The SUV module was designed with flexibility in mind to
not only provide aerodynamicists with changes in overall length, but
also allowing the backlite angle to be varied. Overall length changes
are accommodated by cab length extensions as the SUV module was
designed to attach only to the short box configuration to minimize
engineering design and build complexity. Figure 22 through Figure 24
show images of the backlite angle variations of 50°, 60° and 70°,
respectively. In this case, the backlite angle is defined as the angle
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from the horizontal axis to the rear glass. The roof spoiler for each
backlite angle maintains the same 8º roof taper and 77 mm length from
the rear glass. The three backlite angles can be combined with any of
the three cab lengths, resulting in nine available SUV variants,
summarized in Table 5.
Figure 21. Roof Taper on the SUV Modules
Figure 22. GTU SUV Config 10- Short Cab, 50° Backlite Angle
Figure 23. GTU SUV Config 11- Short Cab, 60° Backlite Angle
Figure 24. GTU SUV Config 12 - Short Cab, 70° Backlite Angle
Table 5. Summary Table of GTU SUV Configurations
Configuration Model Type Cab SUV Backlite
10
Short
50°
11 SUV Short 60°
12 SUV Short 70°
13 SUV Medium 50°
14 SUV Medium 60°
15
Medium
70°
16 SUV Long 50°
17 SUV Long 60°
18 SUV Long 70°
Open Cooling, Engine Bay, Engine & Driveline
The GTU utilizes the OCDA engine bay and components as it
approximately represents the longtitudinal drivetrain orientation of a
production PU as well as supplement knowledge already gained from
the building and testing of the OCDA [18]. Unique to the GTU, is the
front end opening, which houses a single grille opening with thin,
horizontal grille bars, as shown in Figure 25. The engine and
transmission components, illustrated in Figure 26, emulate the OCDA,
however, as the GTU is designed to represent PU variants, the engine
bay enclosure is modified by removing the lower closeout panel that is
present on the OCDA, as seen in Figure 27. The engine bay has the
capability to be completely enclosed with a flat, uncountoured shield,
similar to the OCDA, as shown in Figure 28.
Figure 25. Cooling Opening on the GTU Model
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Figure 26. Engine Bay Representation on the GTU Model
Figure 27. GTU Underbody with Generic Engine and Driveline Highlighted
(blue box)
Figure 28. Engine Bay Encapsulation Shield
Pressure Taps
To facilitate accurate correlation studies, the GTU was designed
with 530 strategically placed pressure taps, 450 on the pickup truck
and 80 on the SUV modules. All of the critical regions have pressure
tap locations to provide detailed flow information. Pressure tap
distribution across the full model is shown in Figure 29. A higher
density of pressure taps were placed in regions where strong pressure
gradients or flow separations are expected, such as the leading edge of
the hood, the trailing edge of the cab, the A-pillar, side window, box
floor and tailgate. Detailed views of pressure tap placement in some
of these sensitive regions are available in Figure 30 through Figure 35.
Pressure tap locations will be provided in the open access CAD model.
Figure 29. Pressure Tap Distribution on the GTU Model
Figure 30. Pressure tap distribution (in green) on the front left side window
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Figure 31. Pressure tap distribution (in green) on the front left A-pillar
Figure 32. Pressure tap distribution (in green) on the front fender
Figure 33. Pressure tap distribution (in green) on the front wheel arch liner
Figure 34. Pressure tap distribution (in green) on the cab trailing edge
Once the design phase of the GTU model was completed, Ford
Motor Company proceeded to build a full-scale physical model of the
GTU, complete with modular capability of all 18 PU and SUV
configurations, ride height and wheelbase adjustment capability and
open cooling representation with a heat exchanger, engine and
driveline. The next sections will describe the preliminary wind tunnel
test and CFD simulations carried out on the full scale GTU model.
Results
Experimental
Preliminary wind tunnel tests were conducted at the FKFS
Aeroacoustics Wind Tunnel on the full-scale GTU physical model. It
is a ¾ open-jet wind tunnel with a nozzle exit area of 22.45 m2 and a
9.95 m test section length. Road simulation is achieved through an
MTS 5-belt rolling road system. Further details on the facility can be
found in [40,41].
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Figure 35. GTU PU Model at the FKFS Aeroacoustics Wind Tunnel
The primary goal of this test was as a model shakedown to ensure
that all systems were operating as intended before embarking on an
intensive global test campaign. Since this was a familiarization test,
limited test data is presented in this paper, although component deltas,
ride height sensitivities and yaw sensitivities were investigated,
presenting a good first insight into the aerodynamic characteristics of
the GTU. Additional test findings from FKFS and other test facilities,
will be subject of future published work.
All tests were run with GTU Pickup Configuration 7 (long cab,
short box) at a speed of 140 km/h. Aerodynamic coefficients were
obtained by normalizing with a frontal area of 2.65m2. The model was
tested at two trim heights 4×4 trim height with FWLTG and RWLTG
set to 840 mm and 890 mm, respectively and 4×2 trim height with
FWLTG and RWLTG set to 795 mm and 845 mm, respectively. Table
6 summarizes key tested model configurations and their respective
coefficient of drag deltas. All values except for No. 6 were evaluated
against the 4×2 ride height baseline, which recorded a CD of 0.393.
Model change No. 6, removing the airdam at a 4×4 ride height was
evaluated against No. 5, 4×4 ride height baseline, as it was the most
relevant comparison.
The airdam exhibits the larger aerodynamic drag reduction of
28cts in the 4×2 configuration with a lower ground clearance as
opposed to 18cts at the 4 attitude (model change 1 vs. 6). This
behavior has also been observed by Larson and Woodiga [42] in their
detailed study on pickup truck airdams in both moving and static
ground conditions. Cooling drag (model change 2) measures at 12cts
and is similar to published data for the OCDA model, most likely
related to the identical heat exchanger and engine bay representation.
Based on internal benchmarking data, side mirror contributions (model
change 4) are in a reasonable range considering their size and body
style application. Ride height sensitivity to aerodynamic drag is highly
dependent on the presence of the airdam, with the ‘airdam oncase
showing a drag decrease of 6 counts per 10mm of ride height reduction
compared to 4 counts per 10mm of ride height reduction for the
airdam offcase. This behavior is analogous to airdam height/ground
distance sweeps performed by Larson and Woodiga [42].
Table 6. Experimental Component Deltas for GTU PU Configuration 7
No Model Change
CD
1
Remove Airdam +0.028
2 Closed Cooling -0.012
3
Remove Front Solid Axle
-0.001
4 Remove Side Mirrors -0.012
5 4×4 Ride Height +0.029
6
Remove Airdam at 4×4 Ride Height +0.018
Figure 36 illustrates the yaw sweep curve of the GTU for four
cases – 4×4 and 4×2 ride heights both with and without airdams. The
total vehicle drag vs. yaw angle curves are generally symmetric for all
cases, indicating that despite some minor asymmetry in the underbody
layout, it does not appear to significantly influence the total vehicle
drag characteristics across the yaw polar. Clear differences in the drag
polar slopes are exhibited for both 4×2 and 4×4 configurations when
comparing airdam on/off. With the airdams installed, both 4×2 and
4×4 configurations exhibit near linear drag increases with angle either
side of 0°. However, removing the airdam results in a reduced slope
in the drag polar between and ±5° and an inflection in the drag polar
slope above 10° and below -10°. Although insufficient data exists to
determine the root cause of this behavior, the most plausible
explanation is the interaction between the airdam and the flow on the
leeward side of the model and it presents an interesting case for further
investigation. The diminishing aerodynamic contributions of the
airdam at higher yaw angles with both 4×2 and 4×4 configurations
exhibiting a reduced aerodynamic benefit beyond a 7.5° yaw angle is
similar to the behavior observed and documented by Larson and
Woodiga [42].
Initial shakedown test conclusions are that the PU GTU
configuration exhibits a number of aerodynamic characteristics
previously measured on production vehicles. The hope is that the GTU
will prove invaluable in furthering understanding flow mechanisms
associated with these types of vehicles.
Figure 36. GTU Yaw Sweeps in Various Aerodynamic Configurations
Numerical
In this section, we present preliminary CFD simulations of the
GTU. Simulations were carried out using iconCFD®’s unsteady
incompressible solver with a set up similar to the one described in [43].
Further details are given in Table 7.
Table 7. CFD Simulation Details
Simulation Details
Turbulence Model SA-DDES
Mesh Details Unstructured, Size: ~100 Million, y+: ~30
Run Length 6 s (Avg. taken over final 5.5 s)
Time Step 2×10-4 s
Boundary
Conditions
Inflow: Velocity inlet
Outlet: Pressure outlet
Walls/Ceiling: Slip wall
Floor: Slip wall (excepting moving wall at
5-belt locations from wind tunnel and no
slip wall surrounding vehicle and
extending downstream)
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Vehicle: No slip except wheels which are
rotating wall boundary condition
Simulation Size
Length (streamwise): 60 m
Width: 50 m
Height: 30 m
This set up is similar to open road conditions, and it does not
account for wind tunnel specific geometry (nozzle, vehicle supports,
test section, and collector), thus some differences between CFD and
experimental results can be expected. Using the methods discussed in
[44] to calculate the uncertainty in the average of the CD signal, the
95% confidence interval for the
CD values is ~0.002 for all the runs.
A list of the CFD runs and a comparison between CFD and experiment
is shown in Table 8. As shown in Table 8, the CFD results are always
directionally consistent with experiment, although the magnitude of
the changes is over predicted for model changes 1, 2, 3, and 6, and
under predicted for model changes 4 and 6.
Table 8. Comparison between CFD and Experiment. All CD values are
defined using 4×2 Baseline as the comparator
No Model Change CD Exp CD CFD
1 Remove Air Dam +0.028 +0.035
2 Closed Cooling -0.012 -0.017
3 Remove Front Solid Axle -0.001 -0.003
4 Remove Side Mirrors -0.012 -0.010
5 4 Ride Height +0.029 +0.021
6 Remove Airdam at 4×4 RH +0.018 +0.021
Visualizing the flow field around the GTU highlights several flow
features which often occur around pickup truck geometries. Figure 37
shows the y = 0 clip of the velocity flow field for the 4×2 ride height
configuration. A recirculation region is present in the bed of the truck
as well as directly behind the tailgate in the wake. Figure 38 shows
the Cp,total = 0 isosurface from behind and above the vehicle. Two large
counter rotating vortices are visible as protuberances in the isosurface
behind the tailgate.
Figure 37. Normalized Velocity Clip at y = 0 at 4×2 Ride Height, Open
Cooling Geometry. Velocity direction is shown through LIC plots.
Figure 38. Isosurface of Cp,total = 0 at 4×2 Ride Height
Open Access Model & Data Sharing
As discussed previously, the benefits of a generic model are best
realized if industry and academia have access to this information,
facilitating collaboration. The GTU CAD geometry will be made
available publicly as an open-access model, similar to the DrivAer,
OCDA and AeroSUV. Initially, the publicly released configurations
will be:
Wheelbase Option 3 (3270mm) with:
1. Three cab and box combinations
i. Configuration 3 Short Cab, Long Box
ii. Configuration 5 Medium Cab, Medium Box
iii. Configuration 7 Long Cab, Short Box
2. Three SUV backlite angles
i. Configuration 16 Long Cab, 50° Backlite
ii. Configuration 17 Long Cab, 60° Backlite
iii. Configuration 18 Long Cab, 70° Backlite
The model will be hosted on the ECARA website
(http://www.ecara.org) similar to the OCDA and AeroSUV models.
As mentioned earlier, a full-sized physical model has also been
constructed with full modularity capability of all 18 configurations.
Ford Motor Company is open to collaboration for testing at external
wind tunnel facilities. Data exchange and sharing rules will be handled
on a case by case basis, similar to what has been practiced for the
OCDA model built and owned by Ford Motor Company.
Summary
In summary, we have successfully conceptualized, designed and
built a new realistic, generic pickup truck and SUV model The GTU.
The development process included laying out critical requirements for
PUs and SUVs and ensuring they were captured in the final design of
the model. The PU configuration of the GTU represents nine distinct
levels of proportionality with three cab and box combinations,
respectively. Similarly, the SUV model is capable of nine individual
configurations with three backlite angle and cab length combinations.
The model includes a simplified engine bay and open cooling
capabilities similar to the OCDA model including simplified elements
of a grille, cooling intake ducting, fan shroud, engine and driveline.
A full-scale model was built with preliminary experimental
investigations conducted on GTU PU Configuration 7. CFD
simulations of the tested configurations show good directional
Page 11 of 12
11/11/2019
agreement with experimental results. Early indications are the GTU
has met its objective in delivering a generic realistic PU and SUV
model. The proportional flexibility presented by the model will
provide a broad area of experimental and numerical research in the
future.
Similar to the DrivAer, OCDA & AeroSUV models, the open
access release and usage of this model will greatly benefit the
automotive aerodynamics community threefold – characterizing wind
tunnel & CFD performance including correlation, development and
process control, developing a deeper understanding of the
aerodynamic mechanisms unique to PU and SUV body styles, and
finally, research and development around a generic realistic PU and
SUV model between OEMs, vendors and academia.
The Ahmed, SAE and MIRA models have contributed to
extensive research and development in fundamental bluff body
aerodynamics in the 1980s. It is hoped that the GTU along with other
recent generic realistic models will contribute in the same way in
advancing the science of bluff body aerodynamics.
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Contact Information
Sudesh Woodiga
Ford Motor Company
Vehicle Energy Management Engineering – Aerodynamics
17200 Southfield Road
Allen Park
MI 48101
Tel. +1 313 236 2373
swoodiga@ford.com
Acknowledgments
The authors would like to thank Mr. Levon Larson, Mr. Ronald Gin,
Mr. Lou Polik, Ms. Jodi McGrew, Dr. Mehrdad Shademan, Mr. Lothar
Krueger, Ms. Rhiannon Gardiner and Ms. Katie Pusey for their
significant contributions in the conception, design and build of the
GTU model.
Definitions/Abbreviations
GTU
Generic Truck Utility
CFD
Computational Fluid
Dynamics
PU
Pickup Truck
SUV
Sports Utility Vehicle
IC
Internal Combustion
OCDA
Open Cooling DrivAer
FWLTG
Front Wheel Lip to Ground
RWLTG
Rear Wheel Lip to Ground
ResearchGate has not been able to resolve any citations for this publication.
Conference Paper
The airflow that enters the front grille of a ground vehicle for the purpose of component cooling has a significant effect on aerodynamic drag. This drag component is commonly referred to as cooling drag, which denotes the difference in drag measured between open grille and closed grille conditions. When the front grille is closed, the airflow that would have entered the front grille is redirected around the body. This airflow is commonly referred to as cooling interference airflow. Consequently, cooling interference airflow can lead to differences in vehicle component drag; this component of cooling drag is known as cooling interference drag. One mechanism that has been commonly utilized to directly influence the cooling drag, by reducing the engine airflow, is active grille shutters (AGS). For certain driving conditions, the AGS system can restrict airflow from passing through the heat exchangers, which significantly reduces cooling drag. The difference in drag between the AGS vanes being open and closed is referred to as AGS drag. Another vehicle component that influences the cooling drag is chin spoilers. Chin spoilers are components that lie within cooling interference airflow paths for many vehicles and can be used/designed to affect cooling drag. This study focuses on the influence of the chin spoiler on cooling and AGS drag of a production-level F-150 in a wind tunnel test environment. The chin spoiler variables tested were height and curvature (sweep). All experiments were conducted in both stationary and moving ground wind tunnel conditions at 80 MPH between yaw angles of ±7°. In addition to overall vehicle drag coefficients, surface pressures at discrete locations and cooling pack airflow rates were measured to provide better insight into the internal and external airflow behavior. Ground and yaw conditions were shown to heavily influence chin spoiler design. Cooling and AGS drag were also strongly influenced by chin spoiler face height at 0° yaw; at higher angles of yaw this influence was lessened but was still present. Chin spoiler sweep was shown to have a significantly lesser (though non-negligible) impact than chin spoiler face height on all metrics in all conditions.
Conference Paper
Wind tunnel testing is conducted to determine the aerodynamic characteristics of a vehicle under controlled and well-defined boundary conditions. Differences in wind tunnel facility layout, design, and subsequent onset flow conditions may result in differing aerodynamic conditions being attained for the same test property in different test facilities. Several OEMs develop vehicles in different regions and utilize local test facilities during the vehicle design process. Understanding the flow characteristics and correlations between test facilities is therefore essential to ensure that global processes can utilize data obtained in any region. Typically, automotive facility correlations are derived by evaluating a fleet of production level test properties in each facility. Adopting a test fleet approach for facility correlation yields three key issues; firstly, there are significant logistics and timing constraints. Secondly, over time the test fleet will deteriorate and potentially introduce random errors in the test data. Thirdly, test facility modifications may require repeat fleet assessments. This article aims to detail the development of a full-scale generic test property with the ability to better represent complex flow phenomena associated with road vehicles. Alternate rear-end geometries, permitting assessment of key flow phenomena associated with differing body styles, will be assessed using a single rolling chassis in six automotive wind tunnel facilities. Initial uncorrected results will be presented along with comparisons to the equivalent computational assessments for specific configurations. These initial results will then be summarized to show how Ford Motor Company plans to move forward with the experimental data. Finally, planned future work to ensure continual suitability of the test property as a reliable correlation and calibration tool in the automotive industry will be outlined.
Conference Paper
In this article the unsteady aerodynamic properties of a 25% scale DrivAer notchback model as well as the influence of the wind tunnel environment on the resulting unsteady aerodynamic forces and moments under crosswind excitation are investigated using experimental and corresponding numerical methods. Research Institute of Automotive Engineering and Vehicle Engines Stuttgart (FKFS) swing® (side wind generator) is used to reproduce the essential properties of natural stochastic crosswind in the open jet test section of the Institute for Internal Combustion Engines and Automotive Engineering (IVK) model scale wind tunnel (MWK). The results show that the test environment of an open jet wind tunnel alters the amplitudes of side force and yaw moment under crosswind excitation when compared to an ideal environment neglecting wind tunnel interference effects. The presented approach for the quantification of the unsteady behavior of the flow field provides a basic understanding of the phenomena occurring under the dynamic deflection of a wind tunnel jet. It is shown that the wind tunnel jet has a dynamic behavior that is superimposed on the aerodynamic response of the vehicle. The investigation of different model scales gives information about the interaction of the dynamic jet behavior with the vehicle response as well as the magnitude of the expected influence. For the first time, a validated simulation model of the unsteady aerodynamic vehicle properties of the realistic DrivAer model as well as of the model scale wind tunnel is introduced. The presented simulation environment without interference effects allows the quantification of the unsteady aerodynamic response of a vehicle without external influences.