RANS Turbulence Model Sensitivity in Automotive External Aerodynamics: k-? vs. k-? SST Across Sedan and Compact Body Styles with Mesh Independence and y? Verification
Keywords:
ANSYS Fluent 2020 R2, k-ε turbulence model, k-ω SST, automotive aerodynamics, drag coefficient, mesh independence, RANS, external aerodynamics, y⁺ validation, sedan, hatchbackAbstract
This paper presents a rigorous three-dimensional computational fluid dynamics (CFD) investigation of external aerodynamics for two passenger vehicle body styles — a sedan (Geometry A) and a compact/hatchback (Geometry B) — executed in ANSYS Fluent 2020 R2 with the central objective of quantifying RANS turbulence model sensitivity to vehicle body form. Both geometries are simulated at identical inlet velocity V = 27.78 m/s and Reynolds number Re = 3.42×10?, ensuring a Reynolds-matched, single-variable comparison. The standard k-? model is applied to Geometry A (y? ? 65, wall-function regime), while the k-? Shear Stress Transport (SST) model is applied to Geometry B (y? ? 1.2, direct near-wall integration). New simulation results for the compact geometry reveal a maximum domain velocity of 48.94 m/s and a minimum pressure of ?1,487 Pa — a 76% and 94% increase respectively over the sedan values of 35.21 m/s and ?766 Pa — demonstrating substantially more intense adverse pressure gradients and wider separated wake. A three-level mesh independence study (GCI = 0.96%) validates numerical convergence. Predicted drag coefficients are Cd = 0.318 (sedan) and Cd = 0.341 (compact), validated against six published references. The study establishes a quantitative criterion: turbulence model sensitivity scales with rear-body separation intensity — k-? introduces 1.3% Cd error for the sedan but 3.6% for the compact, confirming k-? SST is physically essential for hatchback aerodynamics.
References
[1] S. R. Ahmed, G. Ramm, and G. Faltin, "Some salient features of the time-averaged ground vehicle wake," SAE Technical Paper 840300, 1984. DOI: 10.4271/840300
[2] W. H. Hucho, Aerodynamics of Road Vehicles, 4th ed. SAE International, Warrendale, PA, 1998.
[3] E. Guilmineau, "Computational study of flow around a simplified car body," J. Wind Eng. Ind. Aerodyn., vol. 96, no. 6–7, pp. 1207–1217, 2008. DOI: 10.1016/j.jweia.2007.06.041
[4] A. I. Heft, T. Indinger, and N. A. Adams, "Introduction of a new realistic generic car model for aerodynamic investigations," SAE Technical Paper 2012-01-0168, 2012. DOI: 10.4271/2012-01-0168
[5] H. Lienhart and S. Becker, "Flow and turbulent structure in the wake of a simplified car model," SAE Technical Paper 2003-01-0656, 2003. DOI: 10.4271/2003-01-0656
[6] B. Khalighi et al., "Experimental and computational study of unsteady wake flow behind a bluff body with a drag reduction device," SAE Technical Paper 2001-01-1042, 2001.
[7] M. Z. Siddiqui, M. Agelin-Chaab, and D. Salat, "Numerical investigation of aerodynamic drag on sedan and fastback car bodies," SAE Int. J. Pass. Cars—Mech. Syst., vol. 9, no. 2, pp. 657–668, 2016. DOI: 10.4271/2016-01-1626
[8] W. Mokhtar and C. Britcher, "Computational study of aerodynamics of a sedan-type vehicle using FLUENT," AIAA Paper 2012-0731, 2012.
[9] B. E. Launder and D. B. Spalding, "The numerical computation of turbulent flows," Comput. Methods Appl. Mech. Eng., vol. 3, no. 2, pp. 269–289, 1974. DOI: 10.1016/0045-7825(74)90029-2
[10] F. R. Menter, "Two-equation eddy-viscosity turbulence models for engineering applications," AIAA J., vol. 32, no. 8, pp. 1598–1605, 1994. DOI: 10.2514/3.12149
[11] V. Yakhot and S. A. Orszag, "Renormalization group analysis of turbulence," J. Scientific Computing, vol. 1, no. 1, pp. 3–51, 1986. DOI: 10.1007/BF01061452
[12] T. H. Shih, W. W. Liou, A. Shabbir, Z. Yang, and J. Zhu, "A new k-ε eddy viscosity model for high Reynolds number turbulent flows," Computers & Fluids, vol. 24, no. 3, pp. 227–238, 1995. DOI: 10.1016/0045-7930(94)00032-T
[13] S. Jakirlic, R. Jester-Zurker, and C. Tropea, "9th ERCOFTAC/IAHR Workshop on Refined Turbulence Modelling," ERCOFTAC Bulletin, vol. 55, 2002.
[14] M. Lanfrit, "Best practice guidelines for handling automotive external aerodynamics with FLUENT," ANSYS/Fluent Internal Report, v1.2, 2005.
[15] T. Indinger, A. I. Heft, and N. A. Adams, "Investigation of unsteady flow structures in the wake of a realistic generic car model," AIAA Paper 2012-0448, 2012. DOI: 10.2514/6.2012-448
[16] F. R. Menter, M. Kuntz, and R. Langtry, "Ten years of industrial experience with the SST turbulence model," Turbulence, Heat and Mass Transfer 4, pp. 625–632, 2003.
[17] I. B. Celik, U. Ghia, P. J. Roache, and C. J. Freitas, "Procedure for estimation and reporting of uncertainty due to discretisation in CFD applications," J. Fluids Eng., vol. 130, no. 7, 2008. DOI: 10.1115/1.2960953
[18] H. K. Versteeg and W. Malalasekera, An Introduction to Computational Fluid Dynamics, 2nd ed. Pearson Education, 2007.
[19] S. Krajnović and L. Davidson, "Flow around a simplified car, part 1: large eddy simulation," J. Fluids Eng., vol. 127, no. 5, pp. 907–918, 2005. DOI: 10.1115/1.1989371
[20] M. Huminic and A. Huminic, "Aerodynamic study of a generic car model with wheels and underbody diffuser," Int. J. Automotive Technology, vol. 18, no. 3, pp. 397–404, 2017. DOI: 10.1007/s12239-017-0040-6
[21] Y. Xu and R. Zhu, "Aerodynamic drag reduction of electric vehicles by optimizing underbody panels," Energies, vol. 13, no. 11, p. 2897, 2020. DOI: 10.3390/en13112897
[22] P. Barua and W. T. Tian, "Electric powertrain of two-wheeled electric vehicles: a time-varying global sensitivity analysis," Control Syst. Optim. Lett., vol. 3, no. 2, 2025.
[23] P. Roache, Verification and Validation in Computational Science and Engineering. Hermosa Publishers, 1998.
[24] J. D. Herman et al., "Method of Morris effectively reduces the computational demands of global sensitivity analysis," Environ. Model. Softw., vol. 47, pp. 146–157, 2013. DOI: 10.1016/j.envsoft.2013.03.004
[25] E. A. Mercker and J. Wiedemann, "On the correction of interference effects in open jet wind tunnels," SAE Technical Paper 960671, 1996. DOI: 10.4271/960671
[26] A. Altaf, A. A. Omar, and W. Asrar, "Passive drag reduction of square back road vehicles," J. Wind Eng. Ind. Aerodyn., vol. 134, pp. 30–43, 2014.
[27] G. Franck, N. Nigro, M. Storti, and J. D'Elia, "Numerical simulation of the flow around the Ahmed vehicle model," Latin Am. Appl. Research, vol. 39, no. 4, pp. 295–306, 2009.
[28] ANSYS Inc., ANSYS Fluent Theory Guide, Release 2020 R2, Canonsburg, PA, 2020.
[29] ANSYS Inc., ANSYS Fluent User's Guide, Release 2020 R2, Canonsburg, PA, 2020.
[30] M. Mirzaei, S. Krajnović, and B. Basara, "PANS simulations of flows around two different Ahmed body configurations," Computers & Fluids, vol. 102, pp. 273–292, 2014.
[31] K. Krajnović, "Shape optimization of bluff bodies by passive flow control: a review," J. Fluids Struct., vol. 84, pp. 418–446, 2019.
[32] K. Chen, Z. Hu, and H. Wang, "Multi-objective optimization of electric two-wheeler powertrain for fuel economy and performance," Applied Energy, vol. 261, p. 114393, 2020.
[33] C. Hinterberger, M. Garcia-Villalba, and W. Rodi, "Large eddy simulation of flow around the Ahmed body," Lecture Notes in Applied and Computational Mechanics, vol. 19, Springer, 2004.
[34] B. Sarlioglu et al., "Driving toward accessibility: improvements for EVs," IEEE Ind. Appl. Mag., vol. 23, no. 1, pp. 14–25, 2017.
[35] H. Lim, T. G. Thomas, and I. P. Castro, "Flow around a cube in a turbulent boundary layer: LES and experiment," J. Wind Eng. Ind. Aerodyn., vol. 97, pp. 96–109, 2009.
Downloads
Published
Issue
Section
License
Copyright (c) 2026 American Scientific Research Journal for Engineering, Technology, and Sciences
This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
Authors who submit papers with this journal agree to the following terms.
