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- Grabowski, Alexander, 1993
(author)
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VCSEL Equivalent Circuits and Silicon Photonics Integration
- 2022
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Doctoral thesis (other academic/artistic)abstract
- The vertical-cavity surface-emitting laser (VCSEL) is a light source of great importance for numerous industrial and consumer products. The main application areas are datacom and sensing. The datacom industry uses GaAs-based VCSELs for optical interconnects, the short-reach fiber optical communication links used to transfer large amounts of data at high rates between units within data centers and supercomputers. In the area of sensing, VCSELs are largely used in consumer products such as smart phones (e.g. face ID and camera auto focus), computer mice, and automobiles (e.g. gesture recognition and LIDAR for autonomous driving). In this work, an advanced physics-based equivalent circuit model for datacom VCSELs has been developed. The model lends itself to co-design and co-optimization with driver and receiver ICs, thereby enabling higher data rate transceivers with bandwidth limited VCSELs and photodiodes. The model also facilitates an understanding of how each physical process within the VCSEL affects the VCSEL static and dynamic performance. It has been applied to study the impact of carrier transport and capture on VCSEL dynamics. The work also includes micro-transfer-printing of GaAs-based single-mode VCSELs on silicon nitride photonic integrated circuits (PICs). Such PICs are increasingly used for e.g. compact and highly functional bio-photonic sensors. Transfer printing of VCSELs enables the much-needed on-PIC integration of power efficient light sources. The bottom-emitting VCSELs are printed above grating couplers on the PIC and optical feedback is used to control the polarization for efficient coupling to the silicon nitride waveguide. Wavelength tuning, as required by the bio-sensing application, is achieved by direct current modulation.
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| 2. |
- Mathern, Alexandre, 1986, et al.
(author)
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Practical metamodel‑assisted multi‑objective design optimization for improved sustainability and buildability of wind turbine foundations
- 2022
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In: Structural and Multidisciplinary Optimization. - : Springer Science and Business Media LLC. - 1615-1488 .- 1615-147X. ; 65:2
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Journal article (peer-reviewed)abstract
- In this work, we study the potential of using kriging metamodelling to perform multi-objective structural design optimization using finite element analysis software and design standards while keeping the computational efforts low. A method is proposed, which includes sustainability and buildability objectives, and it is applied to a case study of reinforced concrete foundations for wind turbines based on data from a large Swedish wind farm project. Sensitivity analyses are conducted to investigate the influence of the penalty factor applied to unfeasible solutions and the size of the initial sample generated by Latin hypercube sampling. A multi-objective optimization is then performed to obtain the optimum designs for different weight combinations for the four objectives considered. Results show that the kriging-obtained designs from samples of 20 designs outperform the best designs in the samples of 1000 designs. The optimum designs obtained by the proposed method have a sustainability impact 8–15% lower than the designs developed by traditional methods.
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| 3. |
- Abadikhah, Marie, 1992
(author)
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Influence of electrode material and stochastic factors on the performance and microbial community assembly in microbial electrochemical systems
- 2022
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Licentiate thesis (other academic/artistic)abstract
- Microbial electrolysis cells (MECs) are systems with microbial communities in the form of biofilms on electrode surfaces. The electrogenic bacteria in the anode biofilm act as catalysts for the oxidization of organic compounds, leading to release of electrons, generation of electrical current, and production of hydrogen and methane at the cathode. In addition to production of energy carriers, MECs can be used for other applications as well; for example, as biosensors to monitor biochemical oxygen demand or toxicity. The performance of MECs is determined by both deterministic and stochastic factors influencing the microbial communities on the electrode surfaces, most of which as still poorly understood. In this thesis, the effects of electrode materials on microbial community assembly and MEC performance was investigated. Two experiments were carried out. In the first, three cathode materials (carbon nanoparticles, titanium, and steel) were compared. In the second, three anode materials (carbon cloth, graphene, and nickel) were compared. The cathode materials had no significant effect on the performance of the MECs, as opposed to the anode materials where carbon cloth MECs had the highest current density and the shortest lag time during startup. The differences seen in lag time of replicate systems at the start of the experiment indicated a stochastic initial attachment of the electrogenic bacteria on the anode. Different microbial communities develop in the biofilms on the anodes and cathodes. Electrogens from the Desulfobacterota phylum dominated the anode, while various hydrogenotrophic methanogens, e.g., Methanobacterium, were found to dominate on the cathodes. Diversity and null model analysis of the electrode communities highlighted stochasticity and not electrode material as the important factor in the community assembly. Network analysis showed that the cathode communities had fewer negative interactions between taxa in comparison to the anode. Since hydrogen gas generated at the cathode surface can diffuse through the biofilm, all microorganisms on the cathode have access to the substrate, reducing the need for competition between species. In contrast, electrogens require a short distance to the anode to be able to use it as electron acceptor. Limited space on the anode and competition between electrogens shaped the anode communities and explain the higher number of negative interactions observed. Based on the findings in this thesis, it is suggested that stochastic factors have more influence than electrode material on the anode community even though there is a selective pressure for electrogenic bacteria.
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| 4. |
- Konstantinidis, Konstantinos, 1991
(author)
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Numerical analysis of coalescence-induced jumping droplets on superhydrophobic surfaces
- 2022
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Licentiate thesis (other academic/artistic)abstract
- Bio-inspired superhydrophobic surfaces are used in numerous technological applications due to their self-cleaning ability. One of the several mechanisms reported in literature and responsible for self-cleaning is the phenomenon of coalescence-induced jumping of droplets from such surfaces. The phenomenon is observed for scales below the capillary length and when gravity is negligible. Primary applications of this technology are on heat-exchangers or any other that involve surfaces for which anti-icing and water-repellency properties are desired. This thesis comprises two publications that involve high-fidelity numerical investigations on fundamental features of the jumping droplets phenomenon and focuses on two important aspects. The first one is a study on coalescing and jumping of microdroplets (R < 10 µm). The differences in the jumping process (for example, reduction of the merged droplet jumping velocity) are pointed out as a function of the initial size of the droplets. Through an analysis of the energy budget, several degrees of dissipation are found, which is attributed to a competition between viscosity and the strong capillarity on the interface. The second publication focuses on the interaction of the merged droplet with a superhydrophobic surface with hysteresis. It is found that such a case has a reduced jumping velocity as compared to a no-hysteresis one. Using a dynamic contact angle model is beneficial to capture the receding contact angle and provide a more accurate estimation of the overall process. In this work, a combined Immersed Boundary -- Volume-of-fluid method with different contact angle models and a Navier-slip boundary condition is used. The numerical framework has been extensively validated.
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| 5. |
- Steinhagen, Sophie, et al.
(author)
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Harvest time can affect the optimal yield and quality of sea lettuce (Ulva fenestrata) in a sustainable sea-based cultivation : Seasonal Cultivation of Ulva fenestrata
- 2022
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In: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 9
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Journal article (peer-reviewed)abstract
- Seaweed biomass is a renewable resource with multiple applications. Sea-based cultivation of seaweeds can provide high biomass yields, low construction, operation, and maintenance costs and could offer an environmentally and economically sustainable alternative to land-based cultivations. The biochemical profile of sea-grown biomass depends on seasonal variation in environmental factors, and the optimization of harvest time is important for the quality of the produced biomass. To identify optimal harvest times of Swedish sea-based cultivated sea lettuce (Ulva fenestrata), this study monitored biomass yield, morphology, chemical composition, fertility, and biofouling at five different harvesting times in April - June 2020. The highest biomass yields (approx. 1.2 kg fw [m rope]-1) were observed in late spring (May). The number and size of holes in the thalli and the amount of fertile and fouled tissue increased with prolonged growth season, which together led to a significant decline in both biomass yield and quality during summer (June). Early spring (April) conditions were optimal for obtaining high fatty acid, protein, biochar, phenolic, and pigment contents in the biomass, whereas carbohydrate and ash content, as well as essential and non-essential elements, increased later in the growth season. Our study results show that the optimal harvest time of sea-based cultivated U. fenestrata depends on the downstream application of the biomass and must be carefully selected to balance yield, quality, and desired biochemical contents to maximize the output of future sea-based algal cultivations in the European Northern Hemisphere.
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| 8. |
- Chen, Dongliang, et al.
(author)
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The Scanner of Heterogeneous Traffic Flow in Smart Cities by an Updating Model of Connected and Automated Vehicles
- 2022
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In: IEEE transactions on intelligent transportation systems (Print). - : IEEE. - 1524-9050 .- 1558-0016. ; 23:12, s. 25361-25370
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Journal article (peer-reviewed)abstract
- The problems of traditional traffic flow detection and calculation methods include limited traffic scenes, high system costs, and lower efficiency over detecting and calculating. Therefore, in this paper, we presented the updating Connected and Automated Vehicles (CAVs) model as the scanner of heterogeneous traffic flow, which uses various sensors to detect the characteristics of traffic flow in several traffic scenes on the roads. The model contains the hardware platform, software algorithm of CAV, and the analysis of traffic flow detection and simulation by Flow Project, where the driving of vehicles is mainly controlled by Reinforcement Learning (RL). Finally, the effectiveness of the proposed model and the corresponding swarm intelligence strategy is evaluated through simulation experiments. The results showed that the traffic flow scanning, tracking, and data recording performed continuously by CAVs are effective. The increase in the penetration rate of CAVs in the overall traffic flow has a significant effect on vehicle detection and identification. In addition, the vehicle occlusion rate is independent of the CAV lane position in all cases. The complete street scanner is a new technology that realizes the perception of the human settlement environment with the help of the Internet of Vehicles based on 5G communications and sensors. Although there are some shortcomings in the experiment, it still provides an experimental reference for the development of smart vehicles.
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| 9. |
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| 10. |
- Hammarström, Thomas, 1974
(author)
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Evaluation of different approaches to measure partial discharge characteristics within electric motor insulation
- 2022
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In: 2022 International Conference on Electrical Machines and Systems, ICEMS 2022. - 9781665493024 ; , s. 1-6
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Conference paper (peer-reviewed)abstract
- The demand to develop reliable electric motor design solutions stressed by resent development of semiconductor technologies and the need for higher voltage levels, become increasingly important. This paper compares different approaches of measuring and quantifying PD exposure in motor insulation material. In this context, both development and selection of suitable materials and test specimens employing diagnostics suitable for on- and off-line is of importance. To investigate this, electrical methods with a high-pass filter connection, the usage of current transformer as well as an antenna solution are compared for different rise times investigating two types of test objects. The different results are compared, the strength, disadvantages and limitations of each method are presented and implications of this discussed. The results presented constitute a base for development and selection of methods for on or off-line testing.
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