Christian-Albrechts-Universität zu Kiel
Recent publications
In this paper, a three-phase voltage-source converter (VSC) based isolated matrix-type AC/DC converter is proposed for wind energy conversion system (WECS). The line-frequency transformer (LFT) is replaced by the high-frequency transformer (HFT), and there is no need for the intermediate electrolytic capacitors used in conventional two-stage power conversion system. Moreover, the bidirectional switches used for conventional matrix converter (MC) are eliminated in the proposed topology. Consequently, the complicated four-step commutation scheme is avoided and the additional conduction loss caused by serial connection of power switches can be eliminated. The proposed isolated matrix-type AC/DC can be regarded as the dual VSC circuit, where two VSCs alternately work during each switching cycle. Hence, the voltage vectors based space vector modulation (SVM) can be employed and the control schemes for non-isolated VSCs can be used for the proposed converter. For safe operation, the commutation strategy is proposed for the leakage-inductor current of HFT. Both simulations and experiments are conducted to verify the performance of the proposed converter.
Active neutral point-clamped (ANPC) multilevel converters with integrated flying-capacitor (FC) technique are widely used in many industrial applications owing to their low losses, reduced stress on devices, bidirectional operation, and leakage current attenuation capability. Their major setback related to the half dc-link voltage utilization at the ac output has also recently been addressed using the concept of active boost neutral point-clamped (ABNPC) converters with a switched-capacitor (SC) technique. Nonetheless, such a voltage-boosting capability in ABNPCs causes an excessive voltage stress on devices, when the provided dc-link voltage is larger than double of the grid peak voltage. To address this critical challenge, the concept of dual-mode (DM) converter that can retain the quality of ac output with a reduced voltage stress on devices over a wide range of the input dc voltage is introduced. Following this concept, a family of three DM-ANPC five-level (5 L) converters is presented in this article, where they can be operated in either buck or boost mode. This allows to operate the converters with a wide range of input dc voltage utilization and an improved performance. Each of the proposed DM-ANPC-5 L converters requires 10 power switches, while they offer a bidirectional power flow feature with an integrated FC and/or SC technique. Extensive analysis, comparative study, and several simulation and experimental results obtained from a laboratory-built prototype operating in the grid-connected condition are presented to validate the performance of the proposed converters.
Simultaneous power and data transmission is possible by data injection into control signals of power converters without additional hardware. In this paper, a modified analog hysteretic current control is discussed which combines data transmission with efficient quasi-square-wave zero-voltage switching. A half-bridge GaN converter with 99.1 investigated with respect to power conversion efficiency and data transfer. Measurements show that, compared to digital PWM, a wider range of current ripple modulation is available for data transfer before the power efficiency drops. The feasibility of talkative power conversion in the presence of frequency jitter is demonstrated.
Studying tropical hydroclimate and productivity change in the past is critical for understanding global climate dynamics. Northwest Australia is an ideal location for investigating Australian monsoon dynamics, the variability of the Indonesian Throughflow (ITF), and their impact on past productivity and Pacific warm pool evolution, which remain poorly understood during the 40 kyr world in the mid‐early Pleistocene. In this study, we present multi‐proxy records from International Ocean Discovery Program (IODP) Site U1483 in the Timor Sea spanning the last 2000 ka, including orbitally‐resolved records from the 40 kyr world between 2000 and 1300 ka. Our results suggest that northwest Australia underwent a step of increased aridification and that productivity in the Timor Sea declined during the transition from ∼ 1700 to ∼ 1400 ka. We attribute this aridification to the reduced moisture supply to this region caused by the ITF restriction and warm pool contraction. We ascribe the declined productivity to a decrease in the nutrient supply of the Pacific source water associated with global nutrient redistribution. At orbital timescale, multiple mechanisms, including sea level changes, monsoon, and the Intertropical Convergence Zone (ITCZ) dynamics, and variations in the ITF and Walker circulation could have controlled variations of productivity and terrigenous input in the Timor Sea during the 40 kyr world. Our bulk nitrogen and benthic carbon isotope records suggest a strong coupling to biogeochemical changes in the Pacific during this period. This research contributes to a better understanding of tropical hydroclimate and productivity changes during the 40 kyr world.
Purpose of Review Since the introduction of Archaea as a new domain of life more than 45 years ago, progress in their phylogenetic classification and knowledge of their exclusive biological characteristics has identified archaea as unique microorganisms which are widespread in extreme but also in various moderate ecosystems, including eukaryotic hosts. However, archaea are still neglected players within microbiomes, and research on archaea-bacteria interactions is still in its infancy due to methodological challenges. Recent Findings This review summarizes the current knowledge of archaea as components within microbiomes and focuses on their interactions with their bacterial neighbors and the principles of archaeal interactions. Summary Archaea are common constituents of animal and human microbiomes, which are dominated by Euryarchaeota. The gastrointestinal tract is the most studied body site, where archaea account for up to 4% of all microorganisms, primarily represented by methanogens. No archaeal pathogen has yet been identified, although methanogens are hypothesized to be indirectly involved in pathogenicity. Archaeal interactions comprise symbiotic relationships, and the cell membrane and wall might be as crucial as quorum sensing/quenching for these interactions. Particularly, syntrophic interactions under energy-deficiency stress seem to be an essential strategy for archaea. However, more research is urgently needed to discover how archaea sense their environment, compete with bacteria, and interact within complex microbiomes associated with multicellular organisms.
Purpose To assess the current use of drug-eluting devices for peripheral arterial disease (PAD) among interventional radiologists following the controversy caused by the 2018 meta-analysis suggesting an increased mortality risk for paclitaxel-eluting devices. Methods An anonymous survey was sent to 7035 CIRSE members via email; only complete responses were included and statistically analysed. Results Three hundred and seven members (4.4%) completed the survey. Among these, 95.8% indicated that they personally perform peripheral vascular procedures. Thirty-eight percentage of respondents did not see any change of practice since 2018, while 47% reported that the use of drug-eluting devices decreased; for 13%, the use stopped altogether, while it increased in 3% of responses. 45.6% of respondents also felt the impact of the controversy in terms of pricing, availability or directives from hospital administration. A large majority of respondents (83.7%) who perform peripheral vascular procedures consider the use of these devices as safe, 12.9% were undecided and 3.4% did not consider them as safe. Among the respondents who do not perform endovascular procedures, 77% considered these devices as safe and 23% were undecided. Conclusion Although the 2018 meta-analysis had a disruptive impact on the use of drug-eluting devices in PAD, with the increasing body of evidence available, a majority of respondents continue to believe in the safety of these devices for use in femoropopliteal disease.
Both the ecological and social dimensions of fisheries are being affected by climate change. As a result, policymakers, managers, scientists and fishing communities are seeking guidance on how to holistically build resilience to climate change. Numerous studies have highlighted key attributes of resilience in fisheries, yet concrete examples that explicitly link these attributes to social-ecological outcomes are lacking. To better understand climate resilience, we assembled 18 case studies spanning ecological, socio-economic, governance and geographic contexts. Using a novel framework for evaluating 38 resilience attributes, the case studies were systematically assessed to understand how attributes enable or inhibit resilience to a given climate stressor. We found population abundance, learning capacity, and responsive governance were the most important attributes for conferring resilience, with ecosystem connectivity, place attachment, and accountable governance scoring the strongest across the climate-resilient fisheries. We used these responses to develop an attribute typology that describes robust sources of resilience, actionable priority attributes and attributes that are case specific or require research. We identified five fishery archetypes to guide stakeholders as they set long-term goals and prioritize actions to improve resilience. Lastly, we found evidence for two pathways to resilience: (1) building ecological assets and strengthening communities, which we observed in rural and small-scale fisheries, and (2) building economic assets and improving effective governance, which was demonstrated in urban and wealthy fisheries. Our synthesis presents a novel framework that can be directly applied to identify approaches, pathways and actionable levers for improving climate resilience in fishery systems.
EEG microstate sequence analysis quantifies properties of ongoing brain electrical activity which is known to exhibit complex dynamics across many time scales. In this report we review recent developments in quantifying microstate sequence complexity, we classify these approaches with regard to different complexity concepts, and we evaluate excess entropy as a yet unexplored quantity in microstate research. We determined the quantities entropy rate, excess entropy, Lempel–Ziv complexity (LZC), and Hurst exponents on Potts model data, a discrete statistical mechanics model with a temperature-controlled phase transition. We then applied the same techniques to EEG microstate sequences from wakefulness and non-REM sleep stages and used first-order Markov surrogate data to determine which time scales contributed to the different complexity measures. We demonstrate that entropy rate and LZC measure the Kolmogorov complexity (randomness) of microstate sequences, whereas excess entropy and Hurst exponents describe statistical complexity which attains its maximum at intermediate levels of randomness. We confirmed the equivalence of entropy rate and LZC when the LZ-76 algorithm is used, a result previously reported for neural spike train analysis (Amigó et al., Neural Comput 16:717–736,, 2004). Surrogate data analyses prove that entropy-based quantities and LZC focus on short-range temporal correlations, whereas Hurst exponents include short and long time scales. Sleep data analysis reveals that deeper sleep stages are accompanied by a decrease in Kolmogorov complexity and an increase in statistical complexity. Microstate jump sequences, where duplicate states have been removed, show higher randomness, lower statistical complexity, and no long-range correlations. Regarding the practical use of these methods, we suggest that LZC can be used as an efficient entropy rate estimator that avoids the estimation of joint entropies, whereas entropy rate estimation via joint entropies has the advantage of providing excess entropy as the second parameter of the same linear fit. We conclude that metrics of statistical complexity are a useful addition to microstate analysis and address a complexity concept that is not yet covered by existing microstate algorithms while being actively explored in other areas of brain research.
Antibiotics, by definition, reduce bacterial growth rates in optimal culture conditions; however, the real-world environments bacteria inhabit see rapid growth punctuated by periods of low nutrient availability. How antibiotics mediate population decline during these periods is poorly understood. Bacteria cannot optimize for all environmental conditions because a growth-longevity tradeoff predicts faster growth results in faster population decline, and since bacteriostatic antibiotics slow growth, they should also mediate longevity. We quantify how antibiotics, their targets, and resistance mechanisms influence longevity using populations of Escherichia coli and, as the tradeoff predicts, populations are maintained for longer if they encounter ribosome-binding antibiotics doxycycline and erythromycin, a finding that is not observed using antibiotics with alternative cellular targets. This tradeoff also predicts resistance mechanisms that increase growth rates during antibiotic treatment could be detrimental during nutrient stresses, and indeed, we find resistance by ribosomal protection removes benefits to longevity provided by doxycycline. We therefore liken ribosomal protection to a “Trojan horse” because it provides protection from an antibiotic but, during nutrient stresses, it promotes the demise of the bacteria. Seeking mechanisms to support these observations, we show doxycycline promotes efficient metabolism and reduces the concentration of reactive oxygen species. Seeking generality, we sought another mechanism that affects longevity and we found the number of doxycycline targets, namely, the ribosomal RNA operons, mediates growth and longevity even without antibiotics. We conclude that slow growth, as observed during antibiotic treatment, can help bacteria overcome later periods of nutrient stress.
A new class of lanthanide mixed carboxylate ligands compounds with formula {[Ln 2 (phthgly) 4 (bdc)(H 2 O) 6 ]·(H 2 O) 4 } ∞ , labeled as Ln ³⁺ : Eu ( 1 ) and Gd ( 2 ) coordination polymers (CP), were synthesized at mild reaction conditions between lanthanide nitrate salts and a solution of N ‐phthaloylglycine (phthgly) and terephthalic (bdc) ligands. The ( 1) and ( 2) coordination polymers are formed by symmetric binuclear units, in which phthgly and bdc carboxylate ligands are coordinated to the lanthanide ions by different coordination modes. Surprisingly, all organic ligands participate in hydrogen bonding interactions, forming an extremally rigid crystalline structure. The red narrow emission bands from the ⁵ D 0 → ⁷ F J transitions of the Eu ³⁺ ion show a high color purity. The intramolecular energy transfer process from L→Eu ³⁺ ion has been discussed. The experimental intensity parameters (Ω 2,4 ) reflect lower angular distortion and polarizability of the chemical environment around the metal ion compared with other Eu ³⁺ compounds reported in the literature. This novel class of coordination polymer offers a more attractive platform for developing luminescent functional materials for different applications.
Accelerated melting of the Greenland Ice Sheet is considered a tipping element in the freshwater balance of the subpolar North Atlantic (SPNA). The East Greenland Current (EGC) and Coastal Current (EGCC) are the major conduits for transporting Arctic‐sourced and Greenland glacial freshwater. Understanding freshwater changes in the EGC system and drivers thereof is crucial for connecting tipping elements in the SPNA. Using the eddy‐rich model VIKING20X (1/20°) and Copernicus GLORYS12 (1/12°), we find that from 1993–2019 freshwater remains close to the shelf with interannual extremes in freshwater content (FWC) attributable to the imprint of Greenland melt only in years 2010 and 2012. Runoff increased significantly from 1995–2005 and Arctic freshwater export after 2005. Overall, regional wind patterns, sea ice melt and increasingly glacial ice and snow meltwater runoff along with the Arctic‐sourced Polar Water set interannual FWC variations in the EGC system. We emphasize that these freshwater sources have different seasonal timing. South of 65°N sea ice melts year round and retreats to north of 65°N, where melt in summer prevails. Greenland runoff peaks in June–August with only some locations of year round discharge. Alongshore winds intensify in fall and winter where reduced onshore Ekman transport allows for freshwater to spread laterally in the EGC. We show that sea ice melt, runoff and wind can cause interannual variations of comparable magnitude. All of which makes attributing ocean freshening events to Greenland meltwater inflow at current magnitudes a major challenge.
In 1991 Hébrard introduced a factorization of words that turned out to be a powerful tool for the investigation of a word’s scattered factors (also known as (scattered) subwords or subsequences). Based on this, first Karandikar and Schnoebelen introduced the notion of k-richness and later on Barker et al. the notion of k-universality. In 2022 Fleischmann et al. presented at DCFS a generalization of the arch factorization by intersecting the arch factorization of a word and its reverse. While the authors merely used this factorization for the investigation of shortest absent scattered factors, in this work we investigate this new \(\alpha \)-\(\beta \)-factorization as such. We characterize the famous Simon congruence of k-universal words in terms of 1-universal words. Moreover, we apply these results to binary words. In this special case, we obtain a full characterization of the classes and calculate the index of the congruence. Lastly, we start investigating the ternary case, present a full list of possibilities for \(\alpha \beta \alpha \)-factors, and characterize their congruence.
The entomopathogenic nematode (EPN) Heterorhabditis bacteriophora is an effective biological-control agent of insect pests. The dauer juveniles (DJs) seek for, infect insects, and release cells of the carried symbiotic bacterium of the genus Photorhabdus. Inside the host, the DJs perceive signals from the insect’s haemolymph that trigger the exit from the arrested stage and the further development to mature adults. This developmental step is called DJ recovery. In commercial production, a high and synchronous DJ recovery determines the success of liquid-culture mass production. To enhance the understanding about genetic components regulating DJ recovery, more than 160 mutant- and 25 wild type inbred lines (WT ILs) were characterized for DJ recovery induced by cell-free bacterial supernatant. The mutant lines exhibited a broader DJ recovery range than WT ILs (4.6–67.2% vs 1.6–35.7%). A subset of mutant lines presented high variability of virulence against mealworm (Tenebrio molitor) (from 22 to 78% mortality) and mean time survival under oxidative stress (70 mM H2O2; from 10 to 151 h). Genotyping by sequencing of 96 mutant lines resulted in more than 150 single nucleotide polymorphisms (SNPs), of which four results are strongly associated with the DJ recovery trait. The present results are the basis for future approaches in improving DJ recovery by breeding under in vitro liquid-culture mass production in H. bacteriophora. This generated platform of EMS-mutants is as well a versatile tool for the investigation of many further traits of interest in EPNs. Keypoints • Exposure to bacterial supernatants of Photorhabdus laumondii induces the recovery of Heterorhabditis bacteriophora dauer juveniles (DJs). Both, the bacteria and the nematode partner, influence this response. However, the complete identity of its regulators is not known. • We dissected the genetic component of DJ recovery regulation in H. bacteriophora nematodes by generating a large array of EMS mutant lines and characterizing their recovery pheno- and genotypes. • We determined sets of mutants with contrasting DJ recovery and genotyped a subset of the EMS-mutant lines via genotyping by sequencing (GBS) and identified SNPs with significant correlation to the recovery trait.
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Reiner Jung
  • Department of Computer Science
Helmut Fickenscher
  • Institute for Infection Medicine
Georg Hörmann
  • Department of Hydrology and Water Resources Management
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