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- Ekman, Petter, 1988-, et al.
(author)
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Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model
- 2019
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In: WCX SAE World Congress Experience. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Conference paper (peer-reviewed)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. |
- Babajan, Tigran, 1986-
(author)
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Oligarchs, State Power and Mass Opinion : A Study of the Role of Oligarchs in Post-Soviet Pseudo-democracies
- 2018
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Doctoral thesis (other academic/artistic)abstract
- This thesis attempts to understand the role of oligarchs in post-Soviet pseudo-democratic regime trajectories. The two dominant, competing streams of prior work have emphasized either the importance of state power or that of mass opinion in these processes in order to explain why some regimes successfully maintain their grip on power, while others make gradual steps towards democratization. However, the role of oligarchs has been largely overlooked; a gap filled by this study. It employs both widely accepted, as well as unique data to approach the research problem. The work presented in this thesis involves large-N surveys, analyses of media reports and an in-depth case study. There are three key findings:First, for an overwhelming majority of citizens in post-Soviet pseudo-democracies, oligarchs’ actual, negative influence on the political system as well as popular perceptions of unfair wealth concentration are conducive to beliefs that a non-democratic regime is what is needed in order to set things right in their country.Second, the findings from Armenia, Georgia, Russia and Ukraine demonstrate that four politically motivated, rival oligarchs focused on enhancing their reputation and organizational capacity in their struggles for power. In all four countries, regimes used their state power to counter the rival oligarchs. Evidence from Armenia and Russia provides a nuanced understanding of these interactions. In particular, incumbents in these countries used their state power to manipulate public opinion in order to damage the reputation and credibility of the rival oligarchs. This thesis suggests that the main regime leaders do so in order to portray themselves as more trustworthy than their oligarch rivals.Third, a survey analysis of Kyiv university students shows that they view most oligarchs as corrupt and harmful to their political system. According to the survey results, oligarchs’ concentration of wealth appears to disqualify them from being legitimate political players. Yet, further evidence from this thesis suggests that people differentiate between oligarchs as a group and as individuals, which allows at least some oligarchs to enhance their reputation by using their vast material wealth. I conclude this thesis by discussing possible policy and societal implication of the results and by setting out some new venues of future research.
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| 3. |
- Ekman, Petter, 1988-, et al.
(author)
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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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In: SAE 2015 World Congress & Exhibition. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Conference paper (peer-reviewed)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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| 4. |
- Trbakovic, Amela, et al.
(author)
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A new synthetic granular calcium phosphate compound induces new bone in a sinus lift rabbit model
- 2018
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In: Journal of Dentistry. - : Elsevier BV. - 0300-5712 .- 1879-176X. ; 70, s. 31-39
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Journal article (peer-reviewed)abstract
- Objectives The aim of this study was to investigate if a synthetic granular calcium phosphate compound (CPC) and a composite bisphosphonate-linked hyaluronic acid–calcium phosphate hydrogel (HABP·CaP) induced similar or more amount of bone as bovine mineral in a modified sinus lift rabbit model. Material and methods Eighteen adult male New Zeeland White rabbits, received randomly one of the two test materials on a random side of the face, and bovine mineral as control on the contralateral side. In a sinus lift, the sinus mucosa was elevated and a titanium mini-implant was placed in the alveolar bone. Augmentation material (CPC, HABP·CaP or bovine bone) was applied in the space around the implant. The rabbits were euthanized three months after surgery and qualitative and histomorphometric evaluation were conducted. Histomorphometric evaluation included three different regions of interest (ROIs) and the bone to implant contact on each installed implant. Results Qualitative assessment (p=<.05), histomorphometric evaluations (p=<.01), and implant incorporation (p=<.05) showed that CPC and bovine mineral induced similar amount of bone and more than the HABP·CaP hydrogel. Conclusion CPC induced similar amount of bone as bovine mineral and both materials induced more bone than HABP·CaP hydrogel. Clinical significance The CPC is suggested as a synthetic alternative for augmentations in the maxillofacial area.
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