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

Title/Lecturer 2019-12-14 2 Content • Background about chosen Method – Time-Step Size Sensitivity Study * – Turbulence Model Study ** • Chosen Method Case 2 • Simulation Results Case 2 * 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 𝑆𝑓 𝑀 = 3.12 ∙ 10 6 • • 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 −6 𝑡 – 𝐷𝐺𝑀 < 1 • Mesh 𝐃 𝐄 𝐃 𝐌 – 15-20 prisms layers Mesh size 61 million cells 0.268 -0.120 – 61, 102 and 158 million cells 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 𝐃 𝐄 𝐃 𝐌 Method Measurements performed by TU Berlin CFD 0.268 ± 0.002 -0.136 ± 0.001 Wind Tunnel 0.272 ± 0.003 -0.119 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 ( 𝑴/(∆𝒖 ∙ 𝑽 ∞ ) ) CFL Time-step size [s] CFL Time-step size [s] 1.38 ∙ 10 −5 1.4 ∙ 10 −6 1 1 20850 1.4 ∙ 10 −5 1.38 ∙ 10 −4 10 2085 10 2.8 ∙ 10 −5 2.76 ∙ 10 −4 20 1042.5 20 7.0 ∙ 10 −5 6.89 ∙ 10 −4 50 417 50 1.4 ∙ 10 −4 1.38 ∙ 10 −3 100 208.5 100 CFL50 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 Total Pressure and Skin Friction Differences Against CFL1 CFL10 CFL20 CFL100 CFL50 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 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. SBES vs DDES and IDDES Cars, 2014. Fastback Notchback 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 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. SBES vs DDES and IDDES Cars, 2014. Drag difference when increasing yaw angle for 0 ° Fastback Notchback Ekman, P., et al. Assessment of Hybrid RANS-LES Methods for Accurate Automotive Aerodynamic Simulation ., Submitted to Journal of Wind Engineering & Industrial Aerodynamics

Notchback Results – Sensitivity Study 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

Fastback Results – Sensitivity Study 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 p-v SIMPLEC Momentum 2nd order Bounded Central Difference • ANSYS Fluent 2019R1 Turbulence 2nd order Upwind • Stress Blended Eddy Simulation (SBES) Pressure 2nd order Central Difference – Dynamic Smagorinsky SGS model Temporal 2nd order Bounded Implicit Iterative Time-Advancement – k- ω SST RANS model Δ t= 1.375 ∙ 10 −4 s • (corresponding to CFL10) • 5 Inner Iterations SBES is ~25% more expensive than DDES for Τ Simulation Time: 5+20 Convective Flow Units 𝑢 ∙ 𝑉 ∞ 𝑀 • the same mesh and numerical settings • Simulation Cost on 1920 cores • Mesh = Medium Hexapoly • Fastback = 133 658 corehours • Boundary Conditions according to Case 2 description • Estate = 125 429 corehours

Results - Forces • Absolute Forces 𝑫 𝑬 𝑫 𝑴 𝑫 𝑴𝑮 𝑫 𝑴𝑺 Car Body/Method 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 - - - • Force Difference: Estate - Fastback ∆𝑫 𝑬 ∆𝑫 𝑴 Method SBES 0.050 -0.163 WT* 0.049 - Time-Averaging time (20 flow units) * Heft, A., et al. Introduction of a New Generic Realistic Car Model for Aerodynamic Investigations . No. 2012-01-0168. SAE Technical Paper, 2012.

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 Total Pressure and Skin Friction CFL1 CFL20 CFL10 CFL100 CFL50 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 CFL10 CFL20 CFL1 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.