PowerFLOW Results:

Presentation in Ann Arbor

May 26, 2011

Kevin Horrigan

Doug Hatfield
2

Agenda

 Feedback from EPA

 Review Results Delivered on 5/18/11

– Project goals

– Description of geometry process

– Results

– Conclusions

 Comparison of Results to Other Test Methods

– Key considerations: Wind Tunnel and Coastdown

– Highlights from other recent Exa studies

 Proposed Future Work
3

Agenda

 Review Results Delivered on 5/18/11

– Project goals

– Description of geometry process

– Results

– Conclusions

 Comparison of Results to Other Test Methods

– Key considerations: Wind Tunnel and Coastdown

– Highlights from other recent Exa studies

 Proposed Future Work

– Recommended action plan
4

Project Goals

 Evaluate Use of CFD to Predict Drag

 Test Variations to Proposed CFD Requirements

 Compare Results to Other Test Methods

– Coastdown Testing

– Wind Tunnel Testing

 Provide Input for Final GHG Ruling
5

Overview of Simulations

Finest 

Resolution [mm]

Environmental 

Condition Floor Status Tires Condition

Grille 

Position

Yaw Angles 

Tested

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

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

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

3 Wind Tunnel Static Floor Fixed Open 0

3 Open Road Moving Ground Rotating Open 0

3 Open Road Moving Ground Rotating Closed 0

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

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

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

A finest resolution of 1.5 mm was tested instead of 1 mm to allow more 

straightforward comparison to other resolution schemes (i.e. adding 

another resolution level at 1⁄2 the voxel size in the near wall region). 
6

Setup Conditions

 Wind Tunnel Conditions Setup to Match NRC

– Based on previous work on class 8 tractor in NRC tunnel

Wind Tunnel Open Road

Velocity 22.46 m/s 26.82 m/s

Ambient Pressure* 101325 Pa 101325 Pa

Ambient Temperature 30.95 °C 20 °C

Ambient Density 1.161 kg/m31.204 kg/m3

Kinematic Viscosity 1.49e-5 m

2

/s 1.49e-5 m

2

/s

Characteristic Length 2.591 m 2.591 m

Reynolds Number 3.91e6 4.66e6

60 mph

* Measured at outlet of computational domain
7

International ProStar Model

2009 International 

ProStar Premium 6x4

Sleeper Cab w/ High Roof Fairing, and 

Aero Treatment (1660-HD-01)

Engine Family = 9CEXH0912XAK

VIN = 2HSCUAPR6AC168660

Engine = Cummins ISX 450 ST

Rating = 450 hp

Cab Wt. = 20,260 lbs

Frontal Area = 96.91 ft2

Tires = 11R22.5

Front = G395 LHS 

Rear = G362

Wheel Size=22.5x8.25

Axle Ratio = 3.55

Odometer = 55,471

Frontal Area with Box and Tandem 

Trailer = 103.37 sqft, with Flat Bed = 

96.91 sqft

Procurement of Above 2/17/2010

Weekly Rental of Above

Check-In & Test Prep. of Above
8

EPA Scanning Process
9

EPA Scanning Process
10

Geometry Provided to Exa
11

Geometry Provided to Exa
12

PowerFLOW Model

Time to prepare model 

and incorporate more 

detailed underhood 

geometry: 3 man days
13

PowerFLOW Model – Closed Grille

Grille and bumper 

openings were 

sealed for the 

“Closed Grille” 

Simulations
14

NRC Wind Tunnel Setup

NRC 9.1 X 9.1 m Tunnel

– closed-wall atmospheric 

pressure tunnel

Boundary Layer 

Control System

Turntable

Moving Belt
15

NRC Wind Tunnel Setup

28’ Trailer

Fully Detailed 

Tunnel CrossSection and 

Mounting 

Scheme

9.1m
16

NRC Wind Tunnel Setup

Detailed Turntable and 

Mounting Platforms
17

Resolution Setup

Standard Exa

Practice

Refinement 1 Refinement 2

Finest voxel size 6 mm 3 mm 1.5 mm

Total Voxels 60.0 x 106 63.8 x 106 65.8 x 106

Total Surfels 11.7 x 106 12.1 x 106 12.6 x 106

Physical Time / Timestep 28.8 μs 14.4 μs 7.2 μs

Finer resolution 

was added to 

front end regions 

with high 

curvature
18

Lattice-Boltzmann vs. Navier Stokes

 Resolution Requirements Differ for LBM

 PowerFLOW Uses Advanced Boundary Layer Model

– Includes effects for longitudinal pressure gradient

– Skin friction is directly controlled

– Not necessary to calculate velocity gradient near wall

 Requires many cells for N-S finite volume methods

18

Vin Vout

Bounce Back Reflection

Vin Vout

Specular Reflection

Vtin

Vnin

Vtout

Vnout

Slip Condition (frictionless)

Normal component inverted

Tangential component unchanged

Momentum balance  normal force only

No Slip Condition

Normal component inverted

Tangential component inverted

Momentum balance 

normal & tangential force
19
20

Transient Animation

Velocity Magnitude @ 6deg yaw
21

Simulation Results

 Comparison of Resolution Tests

 Open Grille vs. Closed Grille

 Impact of Updating Engine Geometry
22

Results Summary

 Average ΔCd of 0.004 between 6mm and 1.5 mm resolution
23

Wind Averaged CD Results

 Resolution Has Very Small Impact on Wind Avg. Cd

– Open road setup: < 1% change

– Wind tunnel setup: < 1% change

* 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 case with finest 

resolution of 6 mm.
24

Resolution Study: Closed Grille Runs

* Note: Drag coefficients are normalized to open road environment with 

open grille at 0 degrees yaw using 6 mm finest resolution.
25

Resolution Study: Open Grille Runs

* Note: Drag coefficients are normalized to open road environment with 

open grille at 0 degrees yaw using 6 mm finest resolution.
26

Resolution Study: WT Runs

* Note: Drag coefficients are normalized to open road environment with 

open grille at 0 degrees yaw using 6 mm finest resolution.
27

Analysis of Resolution Differences

6 mm Setup 1.5 mm Setup

Isosurface of constant total pressure
28

Analysis of Resolution Differences

6 mm Setup

1.5 mm Setup
29

Summary

 Small Results Influence of Increased Resolution

– Average deviation of ~1% across all yaw angles

 Large Computational Cost Increase

– Cost increases for 1.5 mm runs: > 2x factor

 Recommend Standard Exa Best Practice (6 mm)
30

Simulation Results

 Comparison of Resolution Tests

 Open Grille vs. Closed Grille

 Impact of Updating Engine Geometry

 Comparison to Other Test Methods
31

Drag Development – Open vs Closed

• Similar Overall Drag

• However, significant differences in drag 

distribution across truck
32

Open Grille vs. Closed Grille 

Static Pressure

Open

Closed

Different pressure distribution 

between open and closed grille 

models
33

Open Grille vs. Closed Grille 

Centerline Velocity Magnitude

Open Closed

Grille flow redistributed around underhood region
34

Case Study: 

Design change with Open / Closed Grille

 Exa Performed Additional Study

 Goal of Evaluating Drag Delta from Design Change

– Open grille vs. closed grille

Baseline No Side Fairing
35

Case Study: 

Design change with Open / Closed Grille

 Closed Grille Predicts the 

Incorrect Delta Drag

Environmental 

Condition Floor Status Tire Condition

Grille 

Position

Finest

Cell 

Size

Char. 

Area 

[m2]

Δ

(No Fairing - Fairing) 

[%]

Open Road Moving Ground Rotating Open 6.0 9.8 +0.6%

Open Road Moving Ground Rotating Closed 6.0 9.8 +2.3%
36

Case Study: 

Design change with Open / Closed Grille

Baseline

Design Update
37

Case Study: 

Design change with Open / Closed Grille

Baseline

Design Update
38

Summary: Open Grille vs. Closed Grille

 Significant Differences in Pressure Distribution

– Closed grille causes increase in flow around hood

– Leads to different drag contributions of downstream parts 

 Open Grille Necessary for Assessing Designs

– Correct “delta drag” can only be predicted with open grille

– Demonstrated by removal of side fairing study
39

Simulation Results

 Comparison of Resolution Tests

 Open Grille vs. Closed Grille

 Impact of Updating Engine Geometry

 Comparison to Other Test Methods
40

Drag Summary – Engine Comparison

Environmental 

Condition Floor Status Tire Condition

Grille 

Position Yaw Char. Area [m2]

Δ

(Engine 2 – Engine 1) 

[%]

Open Road Moving Ground Rotating Open 0 9.8 +1.3%

Open Road Moving Ground Rotating Open 1 9.8 +0.9%

Open Road Moving Ground Rotating Open 3 9.8 +2.0%

Open Road Moving Ground Rotating Open 6 9.8 +0.6%

Open Road Moving Ground Rotating Open 9 9.8 -0.2%
41

Drag Development

Engine Swap – 0deg

Engine 1

Engine 2
42

Engine Swap 

Centerline Velocity Magnitude

Engine 1 Engine 2
43

Engine Swap 

Underbody Surface Pressure

Engine 1 Engine 2
44

Summary: Engine Update

 Measurable Drag Difference

 Altered mass flow split 

– Different component drag contributions

 Recommend Using Correct Engine Bay Parts
45

 Review Results Delivered on 5/18/11

– Project goals

– Description of geometry process

– Results

– Conclusions

 Comparison of Results to Other Test Methods

– Key considerations: Wind Tunnel and Coastdown

– Highlights from other recent Exa studies

 Proposed Future Work

– Recommended action plan

Agenda
46

Open Road vs. Wind Tunnel Drag

+42%

+53%
47

Open Road versus WT: Static Pressure

Open Road Wind Tunnel Lower pressure 

due to wind 

tunnel blockage 

from walls

Static floor and 

presence of 

wheel mounts 

alters pressure 

distribution in 

underbody
48

Drag Development

Open Road vs. Wind Tunnel
49

Open Road versus WT: Static Pressure

Open Road Wind Tunnel
50

Open Road vs. WT: Velocity Magnitude

Wind Tunnel Walls

Open Road

Wind Tunnel
51

Isosurface of Total Pressure Coeff.
52

 ProStar Results Consistent with Volvo Study

– SAE Paper: 2010-01-2040

Comparison to Other Studies

2010-01-2040

(a) (b)

Wind Tunnel Simulation
53

NRC Wind Tunnel 

(28’ Trailer @ 0° Yaw)

Lower pressure 

due to wind 

tunnel blockage 

from walls

Static floor and 

presence of 

wheel mounts 

alters pressure 

distribution in 

underbody

Wind Tunnel Effects vs. Open Road

Open Road 

(28’ Trailer @ 0° Yaw)
54

Volvo Study: SAE 2010-01-2040

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

(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.
55

Comparison to Open Road Setup

0.6

0.7

0.8

0.9

1.0

1.1

1.2

-10.0 -5.0 0.0 5.0 10.0

Normalized drag

Yaw angle [°]

Experiment (blockage corrected)

Simulation (28' trailer)

Simulation (53' trailer)

-50

-40

-30

-20

-10

0

10

0.0 0.2 0.4 0.6 0.8 1.0

Normalized drag difference [%]

Location

Drag difference = Open road - Wind tunnel 

Wind Tunnel 

Results are 

Corrected Down 

by 25%

Simulation of 

open road 

environment 

shows a 

decrease of 37%

53’ trailer simulation 

in open road 

environment shows 

different yaw 

performance
56

Wind Tunnel Study: KW T2000

Simulation of Wind Tunnel 

Simulation of Open Road

Published Study (SAE COMVEC 2007)
57

Summary: Open Road vs. Wind Tunnel

 Large Difference Between Open Road and WT

– Consistent with existing studies

 Paper with Volvo: SAE 2010-01-2040

 Kenworth study at University of Washington Tunnel

 Many Contributors

– High blockage environment

 Further exaggerated at yaw 

– Different trailer

– Measurement system influencing flow

– Others
58

 Comparison of Resolution Tests

 Open Grille vs. Closed Grille

 Impact of Updating Engine Geometry

 Comparison to Other Test Methods

– Wind Tunnel

– Coastdown Testing

Simulation Results
59

Comparison to Coastdown Testing

 Conducted Study in Early May

 Simulations with OEM To Investigate:

– Impact of uncontrolled test conditions

 Wind conditions

 Tractor/trailer vertical movement 

– Other factors affecting comparison to CFD

 Variable Reynolds # during coastdown

 Impact of truck mounted anemometer
60

Wind Conditions During Coastdown

 Assume Typical Low Wind Conditions

– Wind direction perpendicular to truck

– Average speed of 4 mph

– Oscillating between 2 and 6 mph at 2 Hz
61

Impact on Yaw Angle

Relative Air 

Velocity

Wind Velocity

Truck 

Velocity
62

Sample Raw Anemometer Data

SW Runs

NE Runs
63

Simulation Results

* Simulation CD results are normalized to a 

value of 0.65 at a truck speed of 60 mph
64

Summary: Coastdown Comparisons

 Wind Conditions Can Have Significant Impact

– Even relatively low wind speeds result in high yaw 

– Can increase aerodynamic drag by up to 46% 

 Variability Makes Comparison to CFD Difficult

– Ideally, CFD should match measured conditions

 On-board anemometer measurements from single run

– Exa recommends coastdown testing with low wind speeds

 Other Important Coastdown Considerations

– Exa is performing additional studies

 Influence of anemometer

 Road roughness

 Sensitivity of tractor trailer vertical movement

– Suggest follow-up meeting to review results
65

 Review Results Delivered on 5/18/11

– Project goals

– Description of geometry process

– Results

– Conclusions

 Comparison of Results to Other Test Methods

– Key considerations: Wind Tunnel and Coastdown

– Highlights from other recent Exa studies

 Proposed Future Work

– Recommended action plan

Agenda
66

Proposed Future Work

 Additional Invesigations of Coastdown Effects

– Wind conditions and variable truck speed

– Anemometer influence

– Sensitivity of tractor-trailer movement

 Additional Simulations on the ProStar

– Fill in additional points on original matrix

– To be discussed with EPA