AUTO RESEARCH CENTER COMPUTATIONAL FLUID DYNAMICS STUDY OF CLASS 

8 HEAVY-DUTY TRACTORS AND 53’ BOX/DRY VAN TRAILER

Simulations conducted:

July & August, 2011

Conducted by:

Auto Research Center

On Behalf of

The Environmental Protection Agency

CONFIGURATION:

Class 8 Heavy-Duty Tractor and 53’ Box/Dry Van Trailer

Report Release Signature: ______________________________________________

Angus Lock – Head of Aerodynamics, Auto Research Center

Date of Report Release: ______________________________________________

Revision – 5 on 10/10/11

08/31/2011
Date: 08/31/2011 P a g e | 2 ARC CFD Report

Table of Contents

1 OBJECTIVES......................................................................................................................... 4

1.1 Key individuals at ARC: .................................................................................................. 4

1.2 Report Distribution List: .................................................................................................. 4

2 METHOD ............................................................................................................................... 4

2.1 CAD Clean-Up................................................................................................................. 4

2.2 Computational Fluid Dynamics Set-Up ........................................................................... 7

2.2.1 Meshing..................................................................................................................... 7

2.2.2 Solving ...................................................................................................................... 7

2.2.3 Post-Processing......................................................................................................... 8

3 SIMULATION MATRIX....................................................................................................... 8

4 RESULTS ............................................................................................................................... 9

5 CONCLUSIONS................................................................................................................... 16
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Table of Figures

Figure 1: Showing the mis-match between scanned data (blue) and Navistar (grey); the exhaust 

intersecting the chassis rails on the scanned data; the scanned exhaust didn’t match the photo of 

real physical test item; general lack of detail on underfloor of scanned data as supplied. ............. 5

Figure 2: Showing general view of tractor and trailer in final, ‘clean’, form................................. 5

Figure 3: Showing detailed engine bay and underfloor constructed by ARC ................................ 6

Figure 4: Showing scale-model of tractor / trailer in ‘digital’ representation of ARC wind tunnel6

Figure 5: Showing scale-model of tractor / trailer in ‘digital’ representation of ARC wind tunnel7

Figure 6: Summary of CFD results, compared to ARC wind tunnel data for a comparable model9

Figure 7: CFD wind averaged results, compared to ARC wind tunnel data for a comparable 

model............................................................................................................................................. 10

Figure 8: Slice through the flow of the scale-model simulation showing velocity (blue 0m/s, red 

50m/s) ........................................................................................................................................... 11

Figure 9: Quarter view of the scale-model truck showing surface static pressure levels............. 11

Figure 10: Bottom quarter view of the scale-model truck showing surface static pressure levels12

Figure 11: Slice through the flow of the full-scale simulation showing velocity......................... 12

Figure 12: Quarter view of the full-scale truck showing surface static pressure levels ............... 13

Figure 13: Bottom quarter view of the full-scale truck showing surface static pressure levels ... 13

Figure 14: Instantaneous plot of velocity for the baseline 9 degree case showing the highly 

turbulent nature of the flow........................................................................................................... 14

Figure 15: The same image as above, except with time averaged velocity, showing how the detail 

of the flow has been lost ............................................................................................................... 14

Figure 16: Plot showing regions of significant fluctuation in the flow velocity. These type of 

fluctuations would not be captured using traditional steady state CFD approaches..................... 15

Figure 17: Clip showing total pressure, which is a good indicator of regions of high energy loss. 

Transient methods of CFD are typically better at predicting these ‘off-surface’ losses............... 15
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1 OBJECTIVES

The EPA requested a series of Computational Fluid Dynamics (CFD) simulations to be 

conducted on a Class 8 tractor and a 53’ box trailer. Three configurations were simulated at fullscale: open cooling and moving road and wheels/tires; closed cooling and moving road and 

wheels/tires; and, open cooling with stationary road and wheels/tires. Each of these 

configurations was simulated at a range of yaw angles in order to be able to report wind averaged 

drag. Finally a simulation of ARC’s Navistar tractor, and Wabash trailer, in the ARC scale model 

tunnel was run at two yaw angles to provide correlation to the wind tunnel testing.

1.1 Key individuals at ARC:

Angus Lock – Head of Aerodynamics created the digital models, ran the simulations, created the 

excel spreadsheet for the raw data and wrote the report

Fritz Marinko - Director of Business Development coordinated the project and prepared the test 

matrix

1.2 Report Distribution List:

ARC – Angus Lock, Mike Camosy, Fritz Marinko

EPA – Arvon Mitcham, Sam Waltzer

URS – Tariq Hasan, Bob Specht

2 METHOD

2.1 CAD Clean-Up

The EPA provided scan data of a full-scale tractor and trailer used for their physical testing.

Navistar provided limited CAD information for the engine bay and cooling pack performance. 

ARC provided detailed CAD of their 1/8 scale physical model, as well as CAD data for the wind 

tunnel itself.

The scanned tractor model supplied to ARC by the EPA had no representation of the engine bay 

or underfloor. Parts were missing or incorrect compared to the photos supplied (and were 

intersecting with other geometry) and did not match the data from Navistar, so a decision was 

made to construct a ‘generic’ tractor model using a combination of the scanned data, the Navistar 

data and a full-scale version of ARC’s International 1/8 scale model. The scanned trailer, as 

supplied, was used for the full-scale runs, with ARC’s own 1/8 scale Wabash trailer used for the 

scale-model runs.

Figure 1, below, illustrates a small selection of examples of the flaws with the scanned data.
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Figure 1: Showing the mis-match between scanned data (blue) and Navistar (grey); the exhaust 

intersecting the chassis rails on the scanned data; the scanned exhaust didn’t match the photo of 

real physical test item; general lack of detail on underfloor of scanned data as supplied.

Following extensive morphing and clean-up a final full-scale model was completed, shown in 

Figure 2, below:

Figure 2: Showing general view of tractor and trailer in final, ‘clean’, form
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Figure 3: Showing detailed engine bay and underfloor constructed by ARC

In order to simulate the scale-model, not only was a detailed CAD model of the tractor and trailer 

required, but in addition a full representation of the ARC wind tunnel was also constructed.

Figure 4: Showing scale-model of tractor / trailer in ‘digital’ representation of ARC wind tunnel
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Figure 5: Showing scale-model of tractor / trailer in ‘digital’ representation of ARC wind tunnel

2.2 Computational Fluid Dynamics Set-Up

Once a ‘clean’, watertight geometry had been created simulations were set up to run in the CFD 

code, OpenFOAM. Essentially the process can be broken down into the following key stages:

2.2.1 Meshing

A ‘domain’ was created at a coarse level of mesh, the geometry was imported and suitable sizes 

were chosen for all the various parts. This resulted in the following sizes being applied to the 

model:

 Scale Model – Smallest cell sizes of approx. 0.3mm for the cooling pack and tire contact 

patches, approx. 0.6mm on the body.

 Full-Scale Model – Smallest cell sizes of approx. 1mm for the tire contact patches, 

approx. 2mm on the body.

The overall mesh size was approximately 40 million cells.

2.2.2 Solving

An initial, ‘steady state’, Reynolds Averaged Navier-Stokes (RANS) solution (using the kepsilon Realizable turbulence model) was obtained to initialise the transient solution. For the 

transient solution a hybrid technique known as Delayed Detached Eddy Simulations (DDES) was 

employed. This effectively blends RANS simulation techniques near the walls, with Large Eddy 

Simulation (LES) techniques away from the walls, leading to a significant reduction in mesh 

requirements, and hence computational effort, over traditional LES techniques.
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As well as setting the boundary conditions to match the simulation matrix, additional data was 

required from Navistar on the air-side performance of their heat exchangers, in order to model 

the losses that occur through these devices. Pressure versus velocity curves for each of the heat 

exchangers were converted into Darcy’s Law coefficients, and entered into the relevant 

dictionary file in OpenFOAM.

In order to capture enough data to deliver a meaningful average value, 1.5 to 2.0 seconds of real 

time were simulated. This resulted in computational times of the order of 1 – 2.5 days on 108

cores of ARC’s compute clusters (2 simulations were run in parallel).

2.2.3 Post-Processing

Automated scripts were used to monitor the forces (and hence coefficients) during the run, and 

this was used to judge if convergence had been achieved. Post-processed images of the flow, as 

well as the forces due to the porous media in the full-scale case, were generated using the open 

source tool ParaView.

Finally all the results were summarised in a spreadsheet, and compared with the relevant wind 

tunnel data.

3 SIMULATION MATRIX

Test Scenario 1: Best Practices

(Finest cell size of (some parts=1mm, some=@ 2mm~4mm)

Environmental 

Condition

Floor 

Status

Tires 

Condition

Grille 

Position

Important Test Related Instructions

Complete Finest Cell Size CFD

1.1 Wind Tunnel *

Fixed 

Ground Fixed Open

 Obtain data points at each of the following 

YAW angles: 

 0°, 6°, -6°, 3°, -3, 9°, -9°

1.2 Open Road Moving 

Ground Rotating Open

 Obtain data points at each of the following 

YAW angles: 

 0°, 6°, -6°, 3°, -3, 9°, -9°

1.3 Open Road Moving 

Ground Rotating Closed

 Obtain data points at each of the following 

YAW angles: 

 0°, 6°, -6°, 3°, -3, 9°, -9°

* The ‘wind tunnel’ configuration is comparable to a large, low blockage facility with a significant boundary layer thickness such 

as the NASA Ames Research Center
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Test Scenario 2 (Scale Model): EPA Rule

(Finest cell size of (1mm on all surfaces & within boundary conditions)

Environmental 

Condition

Floor 

Status

Tires 

Condition

Grille 

Position

Important Test Related Instructions

Complete Finest Cell Size CFD

2.1 Wind Tunnel Moving 

Belt Rotating Open

 Obtain data points at each of the following 

YAW angles:

 6°, 0°

4 RESULTS

A summary of the results (normalized) is presented in the table below:

Figure 6: Summary of CFD results, compared to ARC wind tunnel data for a comparable model

It can be seen that excellent correlation has been achieved for both the scale-model and the 

closed cooling configurations, as well as for the baseline full-scale run excluding zero degree 

yaw. The fixed floor condition represents a test facility far larger than ARC’s, and with a far 

CFD ARC Wind Tunnel Error

C D C D C D

17 100% 0 0.717 0.822 13%

18 100% 6 0.859 0.952 10%

19 100% 3 0.770 0.860 10%

20 100% 9 1.030 1.068 4%

21 100% - 6 0.896 0.943 5%

22 100% - 3 0.783 0.846 7%

23 100% - 9 1.017 1.067 5%

3 100% 0 0.766 0.843 9%

4 100% 6 0.938 0.959 2%

5 100% 3 0.821 0.875 6%

6 100% 9 1.065 1.076 1%

7 100% - 6 0.953 0.951 0%

8 100% - 3 0.818 0.863 5%

9 100% - 9 1.075 1.078 0%

10 100% 0 0.795 0.811 2%

11 100% 6 0.967 0.941 -3%

12 100% 3 0.851 0.849 0%

13 100% 9 1.089 1.062 -3%

14 100% - 6 0.946 0.943 0%

15 100% - 3 0.834 0.838 0%

16 100% - 9 1.042 1.075 3%

1 12.5% 0 0.822 0.843 2%

2 12.5% 6 0.916 0.959 5%

1.3 Open Road, Moving Ground, Rotating 

Tires, Closed Grille

2.1 Wind Tunnel, Moving Ground, Rotating 

Tires, Open Grille

Run No. Scale % Yaw Angle Description

1.2 Open Road, Moving Ground, Rotating 

Tires, Open Grille

1.1 Wind Tunnel, Fixed Ground, Fixed Tires, 

Open Grille
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larger boundary layer thickness, therefore the drag reduction over the moving ground case is 

more significant than in the ARC tunnel.

The discrepancy with the zero yaw for open cooling could have two possible explanations. The 

first is that the engine bay on the full-scale model is entirely generic, and therefore may be 

significantly different to the test vehicle. This would be most prevalent at low yaw angles where 

the engine bay is most exposed. The second possible explanation is that at zero degrees yaw 

there may be a significant low frequency aerodynamic behaviour that requires longer run times.

The next step was to convert the above CD values to the wind averaged drag numbers (also 

normalized) presented in the table below:

Figure 7: CFD wind averaged results, compared to ARC wind tunnel data for a comparable 

model

The wind average values demonstrate that better correlation can be achieved when the CFD 

results are combined in this way, than for individual yaw points. The results for closed cooling 

are especially close, as this case removes one of the areas of major uncertainty with the scanned 

data, namely the engine bay. The fixed floor simulation was really meant to represent a large, 

low blockage wind tunnel with a significant boundary layer thickness, and therefore the 

comparison with the ARC tunnel, which has very low boundary layer thickness, is purely 

indicative.

Finally a selection of images is presented of the scale-model and baseline open cooling cases in 

order to visualise the flow:

Speed CFD Wind Tunnel Error

75 0.785 0.854 8%

70 0.794 0.862 8%

65 0.805 0.873 8%

60 0.819 0.886 8%

55 0.836 0.902 7%

50 0.860 0.924 7%

75 0.837 0.885 5%

70 0.846 0.893 5%

65 0.857 0.903 5%

60 0.871 0.915 5%

55 0.889 0.930 4%

50 0.912 0.950 4%

75 0.868 0.866 0%

70 0.877 0.875 0%

65 0.888 0.885 0%

60 0.902 0.898 0%

55 0.919 0.914 -1%

50 0.942 0.936 -1%

Description

1.2 Open Road, Moving Ground, Rotating Tires, Open 

Grille

1.3 Open Road, Moving Ground, Rotating Tires, Closed 

Grille

1.1 Wind Tunnel, Fixed Ground, Fixed Tires, Open Grille

Wind Average
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Figure 8: Slice through the flow of the scale-model simulation showing velocity (blue 0m/s, red 

50m/s)

Figure 9: Quarter view of the scale-model truck showing surface static pressure levels
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Figure 10: Bottom quarter view of the scale-model truck showing surface static pressure levels

Figure 11: Slice through the flow of the full-scale simulation showing velocity
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Figure 12: Quarter view of the full-scale truck showing surface static pressure levels

Figure 13: Bottom quarter view of the full-scale truck showing surface static pressure levels
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Figure 14: Instantaneous plot of velocity for the baseline 9 degree case showing the highly 

turbulent nature of the flow

Figure 15: The same image as above, except with time averaged velocity, showing how the detail 

of the flow has been lost
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Figure 16: Plot showing regions of significant fluctuation in the flow velocity. These type of 

fluctuations would not be captured using traditional steady state CFD approaches

Figure 17: Clip showing total pressure, which is a good indicator of regions of high energy loss. 

Transient methods of CFD are typically better at predicting these ‘off-surface’ losses.
Date: 08/31/2011 P a g e | 16 ARC CFD Report

5 CONCLUSIONS

Significant errors / compromises were present in the scanned data which led ARC to create a 

‘generic’ tractor for use in the full-scale runs. This has almost certainly compromised the 

accuracy achieved and demonstrates the requirement for good CAD/scan data for CFD 

simulations.

The use of advanced transient CFD techniques has demonstrated significantly better accuracy 

than traditional RANS techniques (which usually have errors in the region 5 to 10%), but does 

come at a computational cost. A minimum level of mesh size and run time are essential to 

achieve good results.

Given the compromises with the geometry, excellent correlation has still been achieved for both 

the scale-model and the closed cooling configurations, as well as for the baseline full-scale run 

excluding zero degree yaw. The fixed floor condition represents a test facility far larger than 

ARC’s, and with a far larger boundary layer thickness, therefore the drag reduction over the 

moving ground case is more significant than in the ARC tunnel.

Wind averaged values show an even better correlation than individual CFD runs, as errors at 

lower yaw angles are compensated for by better results at high yaw angles. Combined with the 

issues at zero yaw, ARC would therefore recommend using the full SAE yaw sweep (including 1 

degree values) for CFD predictions, in order to deliver wind averaged values that better correlate 

with the physical testing.