# Exa Meeting with EPA July 6, 2011 Doug Hatfield Kevin Horrigan2

# 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 Environment3 

##  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 

# Outline4 

# Exa’s Best Practice VR6: 24 mm

VR5: 48 mm5 

# Exa’s Best Practice VR7: 12 mm 

### VR8: 6 mm6

# 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/h7

# Volvo Study: SAE 2010-01- 2040  Wind - avg. Drag Predicted within 1% of Experiment – Using Exa’s Best Practice Setup 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 Requirement9 

# 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 

# Outline11 

# 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 curve12 

# 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 m13 

# GCM Positive/Negative Yaw Results

Symmetrical model should not yield asymmetric results!14 

# 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. C D (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.15 

# 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.16 

##  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 

# Outline17 

# 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 m 2 Baseline – No Anemometer Tractor Mounted Anemometer Trailer Mounted Anemometer Yaw 0 0 0 % diff N/A +0.8 +0.419 

# Drag Development Tractor Mounted

Slight drag increase along entire vehicle20 

# Drag Development Trailer Mounted

Drag shift only on trailer21 

# Centerline Velocity Baseline Tractor Mounted Trailer Mounted Anemometer Wake Impacts Flow over Tractor Nothing Directly Downstream22 

# Surface X Force – Baseline23 

# Surface X Force – Tractor Mounted Flow structure altered24 

# Surface X Force – Baseline25 

# Surface X Force – Trailer Mounted Drag Penalty from Additional Area26 

##  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 

# Outline27 

# 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 0,1,3,6,928 

# Trailer Comparison29 

# Drag Summary – Trailer Comparison 28ft Trailer 53ft Trailer30 

# 28ft Trailer vs. 53ft Trailer Drag Open Road

+10% +20%
* Note: Drag coefficients are normalized to value using 53 foot trailer at 0 deg. yaw31 

# 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 tr ailer case with finest resolution of 6 mm.32 

# Surface X- Force 28 foot Trailer 

### 53 foot Trailer33 

# Isosurface of Total Pressure Coeff.34 

# Velocity Magnitude 28 foot Trailer 53 foot Trailer35 

# Transient Animation Velocity Magnitude @ 6deg yaw36 

# 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. yaw37 

# 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