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Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model
Linköping University, Department of Management and Engineering, Applied Thermodynamics and Fluid Mechanics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Mathematics, Optimization. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0003-2094-7376
Volvo Car Corporation, Göteborg, Sverige.
Linköping University, Department of Management and Engineering, Applied Thermodynamics and Fluid Mechanics. Linköping University, Faculty of Science & Engineering. Linköping University, Center for Medical Image Science and Visualization (CMIV).ORCID iD: 0000-0001-5526-2399
2019 (English)In: WCX SAE World Congress Experience, SAE International , 2019Conference paper, Published paper (Refereed)
Abstract [en]

In aerodynamic development of ground vehicles, the use of Computational Fluid Dynamics (CFD) is crucial for improving the aerodynamic performance, stability and comfort of the vehicle. Simulation time and accuracy are two key factors of a well working CFD procedure. Using scale-resolving simulations, accurate predictions of the flow field and aerodynamic forces are possible, but often leads to long simulation time. For a given solver, one of the most significant aspects of the simulation time/cost is the temporal resolution. In this study, this aspect is investigated using the realistic vehicle model DrivAer with the notchback geometry as the test case. To ensure a direct and accurate comparison with wind tunnel measurements, performed at TU Berlin, a large section of the wind tunnel is included in the simulation domain. All simulations are performed at a Reynolds number of 3.12 million, based on the vehicle length. Three spatial resolutions were compared, where it could be seen that a hybrid element mesh consisting of 102 million cells only revealed small differences to the finest mesh investigated, well as showing excellent agreement with wind tunnel measurements. An investigation of the temporal resolution is performed, in order to see its effect on the simulation time/cost and accuracy of the results. The finest temporal resolution resulted in a Courant-Friedrichs-Lewy number less than unity, while the coarsest reached a CFL number of around 100. From these results, it is seen that it is possible to reduce the simulation time with more than 90 % (CFL 20) and still keep sufficient accuracy of the forces and important features of the flow field.

Place, publisher, year, edition, pages
SAE International , 2019.
Series
SAE technical paper series, ISSN 0148-7191
National Category
Vehicle Engineering
Identifiers
URN: urn:nbn:se:liu:diva-164924DOI: 10.4271/2019-01-0639Scopus ID: 2-s2.0-85064594517OAI: oai:DiVA.org:liu-164924DiVA, id: diva2:1421129
Conference
WCX SAE World Congress Experience
Available from: 2020-04-02 Created: 2020-04-02 Last updated: 2021-07-15Bibliographically approved
In thesis
1. Important Factors for Accurate Scale-Resolving Simulations of Automotive Aerodynamics
Open this publication in new window or tab >>Important Factors for Accurate Scale-Resolving Simulations of Automotive Aerodynamics
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Road transports are responsible for almost 18 % of the greenhouse gas emission in Europe and are today the leading cause of air pollution in cities. Aerodynamic resistance has a significant effect on fuel consumption and hence the emission of vehicles. For electric vehicles, emissions are not affected by the aerodynamics as such but instead have a significant effect on the effective range of the vehicle.

In 2017, a new measurement procedure was introduced, Worldwide Harmonized Light Vehicles Test Procedure (WLTP), for measuring emissions, fuel consumption, and range. This procedure includes a new test cycle with increased average driving speed compared to the former procedure, which thereby increases the importance of the aerodynamic resistance, as it drastically increases with speed. A second effect is that the exact car configuration sold to the customer needs to be certified in terms of fuel consumption and emissions. The result is that every possible combination of optional extras, which might affect the aerodynamic resistance, needs to be aerodynamically analyzed and possibly improved. From 2021, the European Commission will introduce stricter emission regulations for new passenger cars, with the fleet-wide average lowered to 95 grams CO2=km, which puts an even higher demand on achieving efficient aerodynamics.

Virtual development of the aerodynamics of road vehicles is today used to a great extent, using Computational Fluid Dynamics, as it enables faster and cheaper development. However, achieving high accuracy for the prediction of the flow field and aerodynamic forces is challenging, especially given the complexity of both the vehicle geometry in itself and the surrounding flow field. Even for a simplified generic bluff body, accurately predicting the flow field and aerodynamic forces is a challenge. The main reason for this challenge of achieving results with high accuracy is the prediction of the complex behavior of turbulence. Scale-resolving simulation (SRS) methods, such as Large Eddy Simulation (LES), where most of the turbulent structures are resolved has in many studies shown high accuracy but unfortunately to a very high computational cost. It is primarily the small turbulent structures within the near-wall region that requires a _ne resolution in both space (the mesh) and in time. This fine resolution is the reason for the very high computational cost and makes LES unfeasible for practical use in industrial aerodynamic development at present and in the near future. By modeling the turbulent structures within the near-wall region using a Reynolds-Averaged Navier-Stokes (RANS) model, and resolving the turbulence outside the region with a LES model, a coarser resolution is possible to use, resulting in significantly lower computational cost. Which used RANS model is of high importance, and especially how much turbulent viscosity the model generates, as too high values can result in suppression of the resolved turbulence.

The transitioning between the RANS and LES regions have a significant effect on the results. Faster transition enables more resolved turbulence, favorable for higher accuracy, but needs to be balanced with sufficient shielding of the RANS region. If resolving the turbulence occurs within the near-wall region, and the mesh is not sufficiently fine, it can result in poor accuracy.

By increasing the time-step size and disregarding best-practice guides, the computational cost can be significantly reduced. The accuracy is reasonably insensitive to the larger time step sizes until a certain degree, thereby enabling computationally cheaper SRS to achieve high accuracy of aerodynamic predictions needed to meet present and future emission regulations.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2020. p. 96
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2068
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:liu:diva-164926 (URN)10.3384/diss.diva-164926 (DOI)9789179298630 (ISBN)
Public defence
2020-05-05, C3 C-Building, Campus Valla, Linköping, 13:15 (English)
Opponent
Supervisors
Funder
Linköpings universitetSwedish Energy Agency, 40281-1Swedish National Infrastructure for Computing (SNIC)Swedish Research Council, 2016-07213
Available from: 2020-04-02 Created: 2020-04-02 Last updated: 2025-02-09Bibliographically approved

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Publisher's full textScopushttps://doi.org/10.4271/2019-01-0639

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Ekman, PetterLarsson, TorbjörnKarlsson, Matts

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