Exa Meeting with EPA

July 6, 2011

Doug Hatfield

Kevin Horrigan
2

Outline

 Feedback on Proposed CFD Requirements

– Review Exa’s Best Practice resolution setup

– Impact of proposed EPA requirements

– Exa’s recommended modifications

 Comments on “Wind-average Drag Corrections”

 Review EPA Requested Simulation Results

– Coastdown anemometer position study

– Impact of 28’ Trailer in Open Road Environment
3

 Feedback on Proposed CFD Requirements

– Review Exa’s Best Practice resolution setup

– Impact of proposed EPA requirements

– Exa’s recommended modifications

 Comments on “Wind-average Drag Corrections”

 Review EPA Requested Simulation Results

– Coastdown anemometer position study

– Impact of 28’ Trailer in Open Road Environment

Outline
4

Exa’s Best Practice

VR6: 24 mm

VR5: 48 mm
5

Exa’s Best Practice

VR7: 12 mm

VR8: 6 mm
6

GCM Paper: SAE 2008-01-2612 

6

Experiment Simulation Δ [%]

0.578 0.569 -1.6%

Wind –Averaged Drag Coeff. computed 

using SAE J1252. Vehicle velocity = 88 

km/h, mean wind speed = 11.2 km/h 

CD
7

Volvo Study: SAE 2010-01-2040

 Wind-avg. Drag Predicted within 1% of Experiment

– Using Exa’s Best Practice Setup

(a) 

Wind Tunnel

Note: PowerFLOW simulations of NRC tunnel environment, including walls and mounting fixtures, were compared to 

NRC drag force measurements before corrections are made for blockage. This allows an A-to-B comparison to assess 

simulation accuracy.
8

Case Size Estimates

Volume Element Size in Near Wall Regions

1 mm All

Surfaces

1 mm Applied 

Local

6 mm All

Surfaces

6 mm Applied 

Local 

Est. Total # of Volume 

Elements 11.1 B 8.45 B 132 M 60 M

Est. Fine Equiv. Voxels 5.28 B 2.60 B 97.3 M 23 M

Factor Increase in 

Computational Cost 1374 x 677 x 42 x x

Estimated CPU-hrs 4,120,000 2,030,000 127,000 3,000

Exa’s Current Best Practice Setup:

Ability to achieve very high 

accuracy drag predictions

EPA Proposed CFD Requirement
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Discussion

 1 mm Cells Are Not Necessary for High Accuracy

 Exa Recommended Approach

– A near wall cell size of no greater than 6 mm should be applied 

to local regions on the tractor. These regions should include 

areas of high flow gradients and smaller geometry features 

such as the a-pillar, mirror, visor, grille and accessories.
10

 Feedback on Proposed CFD Requirements

– Review Exa’s Best Practice resolution setup

– Impact of proposed EPA requirements

– Exa’s recommended modifications

 Comments on “Wind-average Drag Corrections”

 Review EPA Requested Simulation Results

– Coastdown anemometer position study

– Impact of 28’ Trailer in Open Road Environment

Outline
11

Wind-Avg. Drag Methodology

 Exa Comments:

– ARC and NRC wind tunnel fail to meet SAE J1252 Spec.

 NRC blockage ratio is greater than 5% for full scale trucks

 ARC maximum yaw angle 7 deg. based on test section size

– Positive and negative yaw angles only req. for WT

 To average out repeatability errors

– NASA GCM model is perfectly symmetrical, yet shows asymmetry 

in drag vs. yaw curve 
12

SAE J1252 Test Section Size Spec.

 SAE J1252 Specification:

– Model frontal area at zero yaw < 5% of test section area

– Model height < 0.3 of test section height

– Projected frontal width at yaw < 0.3 of tunnel width

 Max Yaw at ARC Based on This Requirement:

TSW = 2.3 m

W = 0.324 m

L = 2.77 m 

PW  Lsin W cos  0.3TSW
13

GCM Positive/Negative Yaw Results

Symmetrical model should not yield asymmetric results!
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SAE J1252: Applied to ProStar

Reduced Sweep Results Comparison

Full Sweep:

0°,1°, 3°, 6°, 9°

Three Points:

0°, 3°, 6°

Two Points:

0°, 6°

Wind Avg. CD (Normalized*) 1.000 1.000 1.007

Error Using Reduced Data Set +0.07% +0.7%

* The wind averaged drag coefficient is calculated using SAE J1252, and assumes a wind speed of 7 mph and a truck speed of 55 

mph. The results are normalized to the value calculated from the full sweep of yaw points.
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Summary

 SAE J1252 Applied to Reduced Yaw Data Set

– Still following spec. but with reduced # yaw points

– Applying linear interpolation to data points

 Minimal Error Is Introduced

– Verified on International ProStar

– Verified on other OEM trucks 

 Exa recommmends testing at 0, 2, & 6 Deg. 
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 Feedback on Proposed CFD Requirements

– Review Exa’s Best Practice resolution setup

– Impact of proposed EPA requirements

– Exa’s recommended modifications

 Comments on “Wind-average Drag Corrections”

 Review EPA Requested Simulation Results

– Coastdown anemometer position study

– Impact of 28’ Trailer in Open Road Environment

Outline
17

Anemometer Positions

Tractor 

Mounted 

(EPA Config.)

Trailer 

Mounted 

(Front Position)
18

Drag Summary

 Key Results:

– Small increase in overall drag

 0.8% for tractor mounted configuration

 0.4% for trailer mounted configuration

– Local flow disturbance must also be evaluated

Anemometer Drag

Aref= 9.8 m2 Baseline – No 

Anemometer

Tractor Mounted 

Anemometer

Trailer Mounted 

Anemometer

Yaw 0 0 0

% diff N/A +0.8 +0.4
19

Drag Development

Tractor Mounted

Slight drag increase along entire vehicle 
20

Drag Development

Trailer Mounted

Drag shift only on trailer
21

Centerline Velocity 

Baseline

Tractor Mounted

Trailer Mounted

Anemometer Wake Impacts Flow 

over Tractor

Nothing Directly Downstream
22

Surface X Force – Baseline
23

Surface X Force – Tractor Mounted

Flow structure altered
24

Surface X Force – Baseline
25

Surface X Force – Trailer Mounted

Drag Penalty from Additional Area
26

 Feedback on Proposed CFD Requirements

– Review Exa’s Best Practice resolution setup

– Impact of proposed EPA requirements

– Exa’s recommended modifications

 Comments on “Wind-average Drag Corrections”

 Review EPA Requested Simulation Results

– Coastdown anemometer position study

– Impact of 28’ Trailer in Open Road Environment

Outline
27

Overview of Simulations

Finest 

Resolution 

[mm]

Environmental 

Condition Floor Status

Tires 

Conditio

n

Trailer 

Length

Grille 

Position

Yaw Angles 

Tested

6 Wind Tunnel Static Floor Fixed 28’ Open 0,1,3,6,9

6 Open Road Moving Ground Rotating 53’ Open 0,1,3,6,9

6 Open Road Moving Ground Rotating 53’ Closed 0,1,3,6,9

6 Open Road Moving Ground Rotating 28’ Open 0,3,6

3 Wind Tunnel Static Floor Fixed 28’ Open 0

3 Open Road Moving Ground Rotating 53’ Open 0

3 Open Road Moving Ground Rotating 53’ Closed 0

1.5 Wind Tunnel Static Floor Fixed 28’ Open 0,1,3,6,9

1.5 Open Road Moving Ground Rotating 53’ Open 0,1,3,6,9

1.5 Open Road Moving Ground Rotating 53’ Closed 0,1,3,6,9
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Trailer Comparison
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Drag Summary – Trailer Comparison

28ft Trailer 53ft Trailer
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28ft Trailer vs. 53ft Trailer Drag

Open Road

+10%

+20%

* Note: Drag coefficients are normalized to value using 53 foot trailer at 0 deg. yaw
31

Wind Averaged CD Results

 28’ Trailer results in 15% lower wind-avg. drag

– Note: this is in the open road environment

* The wind averaged drag coefficient is calculated using SAE J1252, and assumes a wind speed of 7 mph and a truck speed of 55 

mph. The results are normalized to the drag coefficient for the open road, moving ground, rotating tire, open grille, 53 foot trailer 

case with finest resolution of 6 mm.
32

Surface X-Force

28 foot Trailer

53 foot Trailer
33

Isosurface of Total Pressure Coeff.
34

Velocity Magnitude

28 foot Trailer

53 foot Trailer
35

Transient Animation

Velocity Magnitude @ 6deg yaw
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Open Road vs. Wind Tunnel Drag

28ft trailer

+56%

+79%

Highlights impact of wind tunnel environment:

Not able to capture correct increase in drag with yaw angle

* Note: Drag coefficients are normalized to value using 53 foot trailer, open road, open grille, at 0 deg. yaw
37

Summary

 28’ Trailer Results in Significant Drag Reduction

– 10% at 0 deg. yaw

– 20% at 6 deg. yaw

– 15% wind-avg.

 Results will Depend on the Tractor Design

– Tractor design influences downstream flow impacting trailer

– Not sufficient to use simple correction method to adjust 28’ 

trailer results to 53’ trailer

 NRC Tunnel Environment Introduces Errors at Yaw

– Drag increase with yaw is overpredicted

 Due to blockage ratio increasing at higher yaw angles

– Influence will be different for Closed vs. Open Test Section