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Case 2DrivAer Fastback and Estate 1st Automotive CFD Prediction Workshop
2019-12-11
Petter EkmanLinköping 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 2Title/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
• 𝑅𝑒𝐿 = 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
• ∆𝑡 = 1.4 ∙ 10−6𝑠
– 𝐶𝐹𝐿 < 1
• Mesh
– 15-20 prisms layers
– 61, 102 and 158 million cells
Mesh size 𝐂𝐃 𝐂𝐋61 million cells 0.268 -0.120102 million cells 0.266 -0.136158 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 𝐂𝐃 𝐂𝐋CFD 0.268 ± 0.002 -0.136 ± 0.001Wind Tunnel 0.272 ± 0.003 -0.119
Measurements performed by TU BerlinWieser, 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] (𝑳/(∆𝒕 ∙ 𝑼∞))
1 1.4 ∙ 10−6 2085010 1.4 ∙ 10−5 208520 2.8 ∙ 10−5 1042.550 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 BerlinWieser, 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 BerlinWieser, 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 StudyNotchback
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 StudyFastback
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 𝑡 ∙ Τ𝑈∞ 𝐿
• 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 IterativeTime-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 𝑪𝑫 𝑪𝑳 𝑪𝑳𝑭 𝑪𝑳𝑹
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 ∆𝑪𝑫 ∆𝑪𝑳
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.
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