| 1. |
- Ekman, Petter, 1988-, et al.
(författare)
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Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model
- 2019
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Ingår i: WCX SAE World Congress Experience. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 2. |
- Ekman, Petter, 1988-, et al.
(författare)
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Aerodynamic Drag Reduction - from Conceptual Design on a Simplified Generic Model to Full-Scale Road Tests
- 2015
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Ingår i: SAE 2015 World Congress & Exhibition. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Konferensbidrag (refereegranskat)abstract
- 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%.
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| 3. |
- Ekman, Petter, 1988-, et al.
(författare)
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Aerodynamic Drag Reduction of a Light Truck - from Conceptual Design to Full Scale Road Tests
- 2016
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Ingår i: SAE 2016 World Congress and Exhibition. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 4. |
- Ekman, Petter, 1988-, et al.
(författare)
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Assessment of hybrid RANS-LES methods for accurate automotive aerodynamic simulations
- 2020
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Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105 .- 1872-8197. ; 206
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Tidskriftsartikel (refereegranskat)abstract
- The introduction of the Harmonized Light Vehicles Test Procedure causes a significant challenge for the automotive industry, as it increases the importance of efficient aerodynamics and demands how variations of optional extras affect the car’s fuel consumption and emissions. This may lead to a huge number of combinations of optional extras that may need to be aerodynamically analyzed and possibly optimized, being to resource-consuming to be done with wind tunnel testing merely. Reynolds Average Navier-Stoles (RANS) coupled with Large Eddy Simulations (LES) have shown potential for accurate simulation for automotive applications for reasonable computational cost. In this paper, three hybrid RANS-LES models are investigated on the DrivAer notchback and fastback car bodies and compared to wind tunnel measurements. Several yaw angles are investigated to see the model’s ability to capture small and large changes of the flow field. It is seen that the models generally are in good agreement with the measurement, but only one model is able to capture the behavior seen in the measurements consistently. This is connected to the complex flow over the rear window, which is important to capture for accurate force predictions.
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| 5. |
- Ekman, Petter, 1988-, et al.
(författare)
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Importance of Sub-Grid Scale Modeling for Accurate Aerodynamic Simulations
- 2021
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Ingår i: Journal of Fluids Engineering. - : ASME. - 0098-2202 .- 1528-901X. ; 143:1
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Tidskriftsartikel (refereegranskat)abstract
- The Ahmed body is one of the most well-investigated vehicle bodies for aerodynamic purposes. Despite its simple geometry, the flow around the body, especially at the rear, is very complex as it is dominated by a large wake with strong interaction between vortical structures. In this study, the flow around the 25 deg Ahmed body has been investigated using large eddy simulations and compared to high-resolution particle image velocimetry (PIV) measurements. Special emphasis was put on studying three commonly used sub-grid scale (SGS) models and their ability to capture vortical structures around the Ahmed body. The ability of the SGS models to capture the near-wall behavior and small-scale dissipation is crucial for capturing the correct flow field. Very good agreement between simulations and PIV measurements were seen when using the dynamic Smagorinsky-Lilly and the wall-adopting local eddy-viscosity SGS models, respectively. However, the standard Smagorinsky-Lilly model was not able to capture the flow patterns when compared to the PIV measurements due to shortcomings in the near-wall modeling in the standard Smagorinsky-Lilly model, resulting in overpredicted separation.
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| 6. |
- Ekman, Petter, 1988-
(författare)
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Important Factors for Accurate Scale-Resolving Simulations of Automotive Aerodynamics
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 7. |
- Fattahi, Sadegh, et al.
(författare)
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Influence of the Timber Shape on the Aerodynamics of a Timber Truck
- 2021
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Ingår i: SAE technical paper series. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191.
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Tidskriftsartikel (refereegranskat)abstract
- The aerodynamic improvement and efficiency of regular goods transportation trucks have been a topic of current interest; however, the timber transport industry has not been receiving as much attention. This is due to the small portion of timber transportation vehicles, compared to regular trucks, not justifying the cost of investigating these vehicles experimentally. Since these vehicles travel large parts of their journey at around 80 km/h, their fuel consumption is heavily affected by the aerodynamic resistance. In Sweden in 2015, there were around 2000 vehicles in operation transporting 6 billion ton-km with an average of 0.025 liter Diesel per ton-km. To understand these vehicles’ aerodynamics, and improve on these in the future, the modelling of the timber stacks is of utmost importance. Computational Fluid Dynamics (CFD) simulations have been utilized to conduct this investigation due to recent advancements and the relatively low cost of these simulations compared to an experimental approach. By investigating the influence of geometrical modifications of the stacks on the flow features and accumulated drag, a generic timber stack was created representative of a real stack for a loaded baseline vehicle. It was found that the shorter log length and a shuffling of the logs in the stack exhibit important flow features contributing to drag not present in the other cases. Based on this, a new baseline loaded truck configuration was created with all stacks being identical to each other. This generic stack was built with logs that were 4.25 m long and 0.35 m in diameter, had a smooth surface, and were stacked with a certain displacement in the lengthwise direction.
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