AUTO RESEARCH CENTER COMPUTATIONAL FLUID DYNAMICS STUDY OF CLASS 8 HEAVY

- DUTY TRACTOR S AND 53’ BOX/DRY VAN TRAILER

Simulations conducted :
July & August , 201 1

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

D ate of Report Release:

______________________________________________
Revision

– 5 on 10/10 /11 08/31/2011Date: 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 Computat ional 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

................................ ................................ ................................ ................... 16Date: 08/31/2011 P a g e | 3 ARC CFD Report 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 tr actor 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 tunnel 6 Figure 5: Showing scale

- model of tractor / trailer in ‘digital’ representation of ARC wind tunnel 7 Figure 6: Summary of CFD results, compared to ARC wind tunnel data for a comparable model 9 Figure 7: CFD wind averaged results, compared to ARC wind tunnel data for a comparable model

................................ ................................ ................................ ................................ ............. 10 Figu re 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 su rface static pressure levels 12 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 metho ds of CFD are typically better at predicting these ‘off

- surface’ losses.

.............. 15Date: 08/31/2011 P a g e | 4 ARC CFD Report

1 OBJECTIVE S

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 full

scale: open cooling and moving road and wheels/t i res; closed cooling and moving road and wheels/t i res; and, open cooling with stationary roa d and wheels/t i res. 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 a ngles 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 the ir 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 t he wind tunnel itself.

The scanned tractor model supplied to ARC by the EPA had no representation of the engine bay or underfloor . P arts 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 fo r 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.Date: 08/31/2011 P a g e | 5 ARC CFD Report

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 t est item; general lack of detail on underfloor of scanned da ta 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’, formDate: 08/31/2011 P a g e | 6 ARC CFD Report 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 constr ucted.

Figure 4 : Showing scale

- model of tractor / trailer in ‘digital’ representation of ARC wind tunnelDate: 08/31/2011 P a g e | 7 ARC CFD Report

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 t i re co ntact patches, approx. 0.6mm on the body.

 Full

- Scale Model

– Smallest cell sizes of approx. 1mm for the t i re contact patches,

approx. 2 mm on the body.

The overall mesh size was approximately 40 million cells.

2.2.2 Solving

An initial , ‘steady state’, Reynolds Av eraged Navier

- Stokes (RANS) solution (using the k

epsilon 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 eff

ectively 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.Date: 08/31/2011 P a g e | 8 ARC CFD Report

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 cluster s (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 Rel ated 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 s uch

as the NASA Ames Research CenterDate: 08/31/2011 P a g e | 9 ARC CFD Report Test Scenario 2 (Scale Model ): EPA Rule

(Finest cell size of (1mm on all surfaces & within bou ndary 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 large r than ARC’s, and with a farDate: 08/31/2011 P a g e | 10 ARC CFD Report

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 orde r to visualise the flow:Date: 08/31/2011 P a g e | 11 ARC CFD Report

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 pr essure levelsDate: 08/31/2011 P a g e | 12 ARC CFD Report

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 velocityDate: 08/31/2011 P a g e | 13 ARC CFD Report

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 levelsDate: 08/31/2011 P a g e | 14 ARC CFD Report

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 be en lostDate: 08/31/2011 P a g e | 15 ARC CFD Report

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

- s urface’ 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 technique s 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.

Gi ven 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 t o deliver wind averaged values that better correlate with the physical testing.