Case 2 DrivAer Fastback and Estate 1st Automotive CFD Prediction - - PowerPoint PPT Presentation

Case 2 DrivAer Fastback and Estate

1st Automotive CFD Prediction Workshop 2019-12-11

Petter Ekman

Linköping University

Content

• Background about chosen Method

– Time-Step Size Sensitivity Study * – Turbulence Model Study **

• Chosen Method Case 2
• Simulation Results Case 2

2019-12-14 2 Title/Lecturer

* Ekman, P., et al. Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model. No. 2019-01-0639. SAE Technical Paper, 2019. ** Ekman, P., et al. Assessment of Hybrid RANS-LES Methods for Accurate Automotive Aerodynamic Simulation., Submitted to Journal of Wind Engineering & Industrial Aerodynamics

Method – Sensitivity Study

• DrivAer Reference Model – Notchback

– Smooth Underbody

• 𝑆𝑓𝑀 = 3.12 ∙ 106
• 5° of yaw
• Test section included in the simulations

– GroWiKa WT at TU Berlin

• Stationary ground and wheels

Ekman, P., et al. Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model. No. 2019-01-0639. SAE Technical Paper, 2019.

Method – Sensitivity Study

• ANSYS Fluent
• Stress Blended Eddy Simulation (SBES)

– k-ω SST RANS model – Dynamic Smagorinsky SGS Model

• ∆𝑢 = 1.4 ∙ 10−6𝑡

– 𝐷𝐺𝑀 < 1

• Mesh

– 15-20 prisms layers – 61, 102 and 158 million cells

Mesh size 𝐃𝐄 𝐃𝐌 61 million cells 0.268

• 0.120

102 million cells 0.266

• 0.136

158 million cells 0.269

• 0.137

Ekman, P., et al. Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model. No. 2019-01-0639. SAE Technical Paper, 2019.

Method – Sensitivity Study

Comparison to Wind Tunnel Measurements – Following Best Practice Method 𝐃𝐄 𝐃𝐌 CFD 0.268 ± 0.002

• 0.136 ± 0.001

Wind Tunnel 0.272 ± 0.003

• 0.119

Measurements performed by TU Berlin

Wieser, D., et al. Experimental Comparison of the Aerodynamic Behavior of Fastback and Notchback DrivAer Models. No. 2014-01-0613. SAE Int. J. Passeng. Cars, 2014.

Method – Sensitivity Study

• Time-Step Size Investigation

Corresponding time-step size for Case 2 CFL Time-step size [s] (𝑴/(∆𝒖 ∙ 𝑽∞)) 1 1.4 ∙ 10−6 20850 10 1.4 ∙ 10−5 2085 20 2.8 ∙ 10−5 1042.5 50 7.0 ∙ 10−5 417 100 1.4 ∙ 10−4 208.5

CFL50

CFL Time-step size [s] 1 1.38 ∙ 10−5 10 1.38 ∙ 10−4 20 2.76 ∙ 10−4 50 6.89 ∙ 10−4 100 1.38 ∙ 10−3

Ekman, P., et al. Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model. No. 2019-01-0639. SAE Technical Paper, 2019.

Results – Sensitivity Study

Forces - Difference against CFL1

• Drag forces relative insensitive
• Lift forces more sensitive

Ekman, P., et al. Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model. No. 2019-01-0639. SAE Technical Paper, 2019.

Results – Sensitivity Study

CFL10 CFL20 CFL50 CFL100

Total Pressure and Skin Friction Differences Against CFL1

Ekman, P., et al. Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model. No. 2019-01-0639. SAE Technical Paper, 2019.

Results – Sensitivity Study

Ekman, P., et al. Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model. No. 2019-01-0639. SAE Technical Paper, 2019.

Results – Sensitivity Study

SBES vs DDES and IDDES

Notchback Fastback

Ekman, P., et al. Assessment of Hybrid RANS-LES Methods for Accurate Automotive Aerodynamic Simulation., Submitted to Journal of Wind Engineering & Industrial Aerodynamics

Measurements performed by TU Berlin

Wieser, D., et al. Experimental Comparison of the Aerodynamic Behavior of Fastback and Notchback DrivAer Models. No. 2014-01-0613. SAE Int. J. Passeng. Cars, 2014.

Results – Sensitivity Study

SBES vs DDES and IDDES Drag difference when increasing yaw angle for 0°

Notchback Fastback

Ekman, P., et al. Assessment of Hybrid RANS-LES Methods for Accurate Automotive Aerodynamic Simulation., Submitted to Journal of Wind Engineering & Industrial Aerodynamics

Measurements performed by TU Berlin

Wieser, D., et al. Experimental Comparison of the Aerodynamic Behavior of Fastback and Notchback DrivAer Models. No. 2014-01-0613. SAE Int. J. Passeng. Cars, 2014.

Results – Sensitivity Study

Notchback

SBES vs DDES and IDDES

Ekman, P., et al. Assessment of Hybrid RANS-LES Methods for Accurate Automotive Aerodynamic Simulation., Submitted to Journal of Wind Engineering & Industrial Aerodynamics

Results – Sensitivity Study

Fastback

SBES vs DDES and IDDES

Ekman, P., et al. Assessment of Hybrid RANS-LES Methods for Accurate Automotive Aerodynamic Simulation., Submitted to Journal of Wind Engineering & Industrial Aerodynamics

Chosen Method – Case 2

• ANSYS Fluent 2019R1
• Stress Blended Eddy Simulation (SBES)

– Dynamic Smagorinsky SGS model – k-ω SST RANS model

• Δt= 1.375 ∙ 10−4s

(corresponding to CFL10)

• 5 Inner Iterations
• Simulation Time: 5+20 Convective Flow Units 𝑢 ∙

Τ 𝑉∞ 𝑀

• Mesh = Medium Hexapoly
• Boundary Conditions according to Case 2 description

p-v SIMPLEC Momentum 2nd order Bounded Central Difference Turbulence 2nd order Upwind Pressure 2nd order Central Difference Temporal 2nd order Bounded Implicit Iterative Time-Advancement

SBES is ~25% more expensive than DDES for the same mesh and numerical settings

• Simulation Cost on 1920 cores
• Fastback = 133 658 corehours
• Estate = 125 429 corehours

Results - Forces

• Absolute Forces
• Force Difference: Estate - Fastback

Car Body/Method 𝑫𝑬 𝑫𝑴 𝑫𝑴𝑮 𝑫𝑴𝑺 Fastback – SBES 0.229

• 0.035
• 0.120

0.086 Fastback – WT* 0.243

• Estate - SBES

0.279

• 0.198
• 0.154
• 0.044

Estate – WT* 0.292

• Method

∆𝑫𝑬 ∆𝑫𝑴 SBES 0.050

• 0.163

WT* 0.049

• * Heft, A., et al. Introduction of a New Generic Realistic Car Model for Aerodynamic Investigations.
• No. 2012-01-0168. SAE Technical Paper, 2012.

Time-Averaging time (20 flow units)

Results - WSS

Results - Pressure

• Comparison to Heft, A., et al. * and

* Heft, A., et al. Introduction of a New Generic Realistic Car Model for Aerodynamic

• Investigations. No. 2012-01-0168. SAE Technical Paper, 2012.

Results - Pressure

• Comparison to Avadiar, T., et al. *
• Offset of Cp = 0.05

* Avadiar, T., et al. Characterisation of the wake of the DrivAer estate vehicle. Journal of Wind Engineering & Industrial Aerodynamics, 2018.

Conclusions

• Possible to be aggressive with time-step size

– Drag relative insensitive – Lift more sensitive

• High accuracy achieved with SBES

– Able to capture the complex flow over the rear window – Base pressure correlate well with measurements – Good drag prediction for different yaw and car configurations – Excellent trend prediction – ~25% more expensive than DDES k-ω SST

Acknowledgements

Thanks to TU Berlin and especially Dirk Wieser for sharing measurement data Thanks to National Supercomputer Centre at Linköping University for providing computational resources

Thank you!

Petter.ekman@liu.se

Extra Material

CFL10 CFL20 CFL50 CFL100

Total Pressure and Skin Friction

CFL1

Ekman, P., et al. Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model. No. 2019-01-0639. SAE Technical Paper, 2019.

Extra Material

Surface Pressure CFL1

CFL10 CFL20 CFL50 CFL100

Ekman, P., et al. Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model. No. 2019-01-0639. SAE Technical Paper, 2019.