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Important Factors for Accurate Scale-Resolving Simulations of Automotive Aerodynamics
Linköping University, Department of Management and Engineering, Applied Thermodynamics and Fluid Mechanics. Linköping University, Faculty of Science & Engineering.
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 and Acoustics
Identifiers
URN: urn:nbn:se:liu:diva-164926DOI: 10.3384/diss.diva-164926ISBN: 9789179298630 (print)OAI: oai:DiVA.org:liu-164926DiVA, id: diva2:1421275
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, 2016-07213Available from: 2020-04-02 Created: 2020-04-02 Last updated: 2020-05-11Bibliographically approved
List of papers
1. Aerodynamic Drag Reduction - from Conceptual Design on a Simplified Generic Model to Full-Scale Road Tests
Open this publication in new window or tab >>Aerodynamic Drag Reduction - from Conceptual Design on a Simplified Generic Model to Full-Scale Road Tests
2015 (English)In: SAE 2015 World Congress & Exhibition, SAE International , 2015Conference paper, Published paper (Refereed)
Abstract [en]

Road transportation by trucks is the major part of the goods transportations system in the European Union (EU), and there is a need for increased fuel efficiency. While truck manufacturers already spend significant resources in order to reduce the emissions from their vehicles, most truck manufacturers do not control the shape of the trailer and/or swap bodies. These devices are usually manufactured by different companies that cannot consider the overall aerodynamics around the complete vehicle.By use of Computational Fluid Dynamics (CFD) and previous wind tunnel experiments, the flow around a simplified generic tractor-trailer model has been investigated. With better understanding of the flow features around the tractor with attached trailer or swap bodies, an improved design of the trailer and swap body can be achieved, which is the aim for the project. Special emphasis is put on achieving simple, easy to install or implement drag-reducing geometrical modifications to the trailer or swap bodies that can be mounted on existing trucks.Reynolds-Averaged Navier-Stokes (RANS) simulations were used for the conceptual development phase where trends in drag reduction due to the modified geometries were studied using a parameter study, while more advanced scale resolving simulations (SRS) were used in order to investigate the details of the flow fields.The investigation indicates that aerodynamic drag reduction is possible with quite simple geometrical modifications. Some of the results have also been verified through road tests of vehicles in commercial use, which has shown reduced fuel consumption of up to 5%.

Place, publisher, year, edition, pages
SAE International, 2015
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:liu:diva-164920 (URN)10.4271/2015-01-1543 (DOI)
Conference
SAE 2015 World Congress & Exhibition
Available from: 2020-04-02 Created: 2020-04-02 Last updated: 2020-04-02Bibliographically approved
2. Aerodynamic Drag Reduction of a Light Truck - from Conceptual Design to Full Scale Road Tests
Open this publication in new window or tab >>Aerodynamic Drag Reduction of a Light Truck - from Conceptual Design to Full Scale Road Tests
2016 (English)In: SAE 2016 World Congress and Exhibition, SAE International , 2016Conference paper, Published paper (Refereed)
Abstract [en]

Considerable amounts of the everyday goods transports are done using light trucks. In the last ten years (2005-2015), the number of light trucks has increased by 33 % in Sweden. The majority of these light trucks are fitted with a swap body and encounter the same problem as many other truck configurations, namely that several different manufacturers contribute to the final shape of the vehicle. Due to this, the aerodynamics of the final vehicle is often not fully considered. Hence there appears to be room for improving the aerodynamic performance. In this study the flow around a swap body fitted to a light truck has been investigated using Computational Fluid Dynamics. The focus has been on improving the shape of the swap body in order to reduce both the aerodynamic drag and fuel consumption, while still keeping it usable for daily operations. Reynolds-Averaged Navier-Stokes simulations were used for concept evaluation while more advanced Detached Eddy Simulations were performed on the best concept in order to investigate details of the flow. Various concepts were evaluated from which it could be seen that a more streamlined top of the swap body together with a lowered top trailing edge had a significant positive effect on the aerodynamic drag. A full scale light truck was equipped with a swap body with with these modifications for road tests. During a test period, a mean fuel consumption reduction of 12 % was measured, thus indicating a significantly reduced aerodynamic drag.

Place, publisher, year, edition, pages
SAE International, 2016
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:liu:diva-164923 (URN)10.4271/2016-01-1594 (DOI)
Conference
SAE 2016 World Congress and Exhibition
Available from: 2020-04-02 Created: 2020-04-02 Last updated: 2020-04-02Bibliographically approved
3. Aerodynamics of an Unloaded Timber Truck - A CFD Investigation
Open this publication in new window or tab >>Aerodynamics of an Unloaded Timber Truck - A CFD Investigation
2016 (English)In: SAE International Journal of Commercial Vehicles, ISSN 1946-391X, E-ISSN 1946-3928, Vol. 9, no 2, p. 217-223Article in journal (Refereed) Published
Abstract [en]

Reducing energy consumption and emissions are ongoing challenges for the transport sector. The increased number of goods transports emphasize these challenges even more, as greenhouse gas emissions from these vehicles increased by 20 % between 1990 and 2013, in Sweden. One special case of goods transports is the transport of timber. Today in Sweden, around 2000 timber trucks transport around six billion ton kilometers every year. For every ton kilometer these vehicles use around 0.025 liter diesel, and there should exist large possibilities to reduce the fuel consumption and the emissions for these vehicles. Timber trucks spend most of their operation time travelling in speeds of around 80 km/h. At this speed aerodynamic drag contributes to around 30 % of the total vehicle resistance, which makes the aerodynamic drag a significant part of the energy consumption. One of the big challenges with timber trucks is that they travel unloaded half of the time. This put higher demands on possible drag reduction modifications, as they need to function and be practical for both when the timber truck is loaded and unloaded. In this study an unloaded timber truck has been investigated by use of computational fluid dynamics. The recently released Stress Blended Eddy Simulation model has been used for simulating the flow over a timber truck at a Reynolds number of 1.1 million, based on the square root of its frontal area. From the results it could be seen that 52.8 % of the drag is generated by the cab. By investigating a drag reduction device that covered the gap between the bulkhead and the first stake pair, a drag reduction up to 6.7 % was possible, which shows potential for simple modifications that not influence the daily usage.

Place, publisher, year, edition, pages
SAE INT, 2016
National Category
Transport Systems and Logistics
Identifiers
urn:nbn:se:liu:diva-163998 (URN)10.4271/2016-01-8022 (DOI)000389233800010 ()
Available from: 2020-03-05 Created: 2020-03-05 Last updated: 2020-04-02
4. Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model
Open this publication in new window or tab >>Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model
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
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:liu:diva-164924 (URN)10.4271/2019-01-0639 (DOI)
Conference
WCX SAE World Congress Experience
Available from: 2020-04-02 Created: 2020-04-02 Last updated: 2020-04-02Bibliographically approved

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