@article{SchopenNarayanBeckmannetal.2024, author = {Schopen, Oliver and Narayan, Sriram and Beckmann, Marvin and Najmi, Aezid-Ul-Hassan and Esch, Thomas and Shabani, Bahman}, title = {An EIS approach to quantify the effects of inlet air relative humidity on the performance of proton exchange membrane fuel cells: a pathway to developing a novel fault diagnostic method}, series = {International Journal of Hydrogen Energy}, volume = {58}, journal = {International Journal of Hydrogen Energy}, number = {8}, publisher = {Elsevier}, address = {Amsterdam}, isbn = {0360-3199 (print)}, issn = {1879-3487 (online)}, doi = {10.1016/j.ijhydene.2024.01.218}, pages = {1302 -- 1315}, year = {2024}, abstract = {In this work, the effect of low air relative humidity on the operation of a polymer electrolyte membrane fuel cell is investigated. An innovative method through performing in situ electrochemical impedance spectroscopy is utilised to quantify the effect of inlet air relative humidity at the cathode side on internal ionic resistances and output voltage of the fuel cell. In addition, algorithms are developed to analyse the electrochemical characteristics of the fuel cell. For the specific fuel cell stack used in this study, the membrane resistance drops by over 39 \% and the cathode side charge transfer resistance decreases by 23 \% after increasing the humidity from 30 \% to 85 \%, while the results of static operation also show an increase of ∼2.2 \% in the voltage output after increasing the relative humidity from 30 \% to 85 \%. In dynamic operation, visible drying effects occur at < 50 \% relative humidity, whereby the increase of the air side stoichiometry increases the drying effects. Furthermore, other parameters, such as hydrogen humidification, internal stack structure, and operating parameters like stoichiometry, pressure, and temperature affect the overall water balance. Therefore, the optimal humidification range must be determined by considering all these parameters to maximise the fuel cell performance and durability. The results of this study are used to develop a health management system to ensure sufficient humidification by continuously monitoring the fuel cell polarisation data and electrochemical impedance spectroscopy indicators.}, language = {en} } @article{SchopenShahEschetal.2024, author = {Schopen, Oliver and Shah, Neel and Esch, Thomas and Shabani, Bahman}, title = {Critical quantitative evaluation of integrated health management methods for fuel cell applications}, series = {International Journal of Hydrogen Energy}, volume = {70}, journal = {International Journal of Hydrogen Energy}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0360-3199}, doi = {10.1016/j.ijhydene.2024.05.156}, pages = {370 -- 388}, year = {2024}, abstract = {Online fault diagnostics is a crucial consideration for fuel cell systems, particularly in mobile applications, to limit downtime and degradation, and to increase lifetime. Guided by a critical literature review, in this paper an overview of Health management systems classified in a scheme is presented, introducing commonly utilised methods to diagnose FCs in various applications. In this novel scheme, various Health management system methods are summarised and structured to provide an overview of existing systems including their associated tools. These systems are classified into four categories mainly focused on model-based and non-model-based systems. The individual methods are critically discussed when used individually or combined aimed at further understanding their functionality and suitability in different applications. Additionally, a tool is introduced to evaluate methods from each category based on the scheme presented. This tool applies the technique of matrix evaluation utilising several key parameters to identify the most appropriate methods for a given application. Based on this evaluation, the most suitable methods for each specific application are combined to build an integrated Health management system.}, language = {en} } @article{DroopChenRadfordetal.2023, author = {Droop, Philipp and Chen, Shaohuang and Radford, Melissa J. and Paulßen, Elisabeth and Gates, Byron D. and Reilly, Raymond M. and Radchenko, Valery and Hoehr, Cornelia}, title = {Synthesis of 197m/gHg labelled gold nanoparticles for targeted radionuclide therapy}, series = {Radiochimica Acta}, volume = {111}, journal = {Radiochimica Acta}, number = {10}, publisher = {De Gruyter}, address = {Berlin [u.a.]}, issn = {2193-3405}, doi = {10.1515/ract-2023-0144}, pages = {773 -- 779}, year = {2023}, abstract = {Meitner-Auger-electron emitters have a promising potential for targeted radionuclide therapy of cancer because of their short range and the high linear energy transfer of Meitner-Auger-electrons (MAE). One promising MAE candidate is 197m/gHg with its half-life of 23.8 h and 64.1 h, respectively, and high MAE yield. Gold nanoparticles (AuNPs) that are labelled with 197m/gHg could be a helpful tool for radiation treatment of glioblastoma multiforme when infused into the surgical cavity after resection to prevent recurrence. To produce such AuNPs, 197m/gHg was embedded into pristine AuNPs. Two different syntheses were tested starting from irradiated gold containing trace amounts of 197m/gHg. When sodium citrate was used as reducing agent, no 197m/gHg labelled AuNPs were formed, but with tannic acid, 197m/gHg labeled AuNPs were produced. The method was optimized by neutralizing the pH (pH = 7) of the Au/197m/gHg solution, which led to labelled AuNPs with a size of 12.3 ± 2.0 nm as measured by transmission electron microscopy. The labelled AuNPs had a concentration of 50 μg (gold)/mL with an activity of 151 ± 93 kBq/mL (197gHg, time corrected to the end of bombardment).}, language = {en} } @article{MuesgenanntKoersMcNeilRadchenkoetal.2023, author = {Mues genannt Koers, Lucas and McNeil, S. W. and Radchenko, V. and Paulßen, Elisabeth and Hoehr, Cornelia}, title = {Production of Co-58m in a siphon-style liquid target on a medical cyclotron}, volume = {195}, number = {Art. 110734}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0969-8043}, doi = {10.1016/j.apradiso.2023.110734}, year = {2023}, abstract = {We present the production of 58mCo on a small, 13 MeV medical cyclotron utilizing a siphon style liquid target system. Different concentrated iron(III)-nitrate solutions of natural isotopic distribution were irradiated at varying initial pressures and subsequently separated by solid phase extraction chromatography. The radio cobalt (58m/gCo and 56Co) was successfully produced with saturation activities of (0.35 ± 0.03) MBq μA-1 for 58mCo with a separation recovery of (75 ± 2) \% of cobalt after one separation step utilizing LN-resin.}, language = {en} } @article{MuesgenanntKoersPrevostPaulssenetal.2023, author = {Mues genannt Koers, Lucas and Prevost, David and Paulßen, Elisabeth and Hoehr, Cornelia}, title = {Density reduction effects on the production of [11C]CO2 in Nb-body targets on a medical cyclotron}, volume = {199}, number = {Art. 110911}, publisher = {Elsevier}, address = {Amsterdam}, doi = {10.1016/j.apradiso.2023.110911}, year = {2023}, abstract = {Medical isotope production of 11C is commonly performed in gaseous targets. The power deposition of the proton beam during the irradiation decreases the target density due to thermodynamic mixing and can cause an increase of penetration depth and divergence of the proton beam. In order to investigate the difference how the target-body length influences the operation conditions and the production yield, a 12 cm and a 22 cm Nb-target body containing N2/O2 gas were irradiated using a 13 MeV proton cyclotron. It was found that the density reduction has a large influence on the pressure rise during irradiation and the achievable radioactive yield. The saturation activity of [11C]CO2 for the long target (0.083 Ci/μA) is about 10\% higher than in the short target geometry (0.075 Ci/μA).}, language = {en} } @article{KleefeldPieronek2020, author = {Kleefeld, Andreas and Pieronek, J.}, title = {Elastic transmission eigenvalues and their computation via the method of fundamental solutions}, series = {Applicable Analysis}, volume = {100}, journal = {Applicable Analysis}, number = {16}, publisher = {Taylore \& Francis}, address = {London}, issn = {1563-504X}, doi = {10.1080/00036811.2020.1721473}, pages = {3445 -- 3462}, year = {2020}, abstract = {A stabilized version of the fundamental solution method to catch ill-conditioning effects is investigated with focus on the computation of complex-valued elastic interior transmission eigenvalues in two dimensions for homogeneous and isotropic media. Its algorithm can be implemented very shortly and adopts to many similar partial differential equation-based eigenproblems as long as the underlying fundamental solution function can be easily generated. We develop a corroborative approximation analysis which also implicates new basic results for transmission eigenfunctions and present some numerical examples which together prove successful feasibility of our eigenvalue recovery approach.}, language = {en} } @article{SaretzkiBergmannDahmannetal.2021, author = {Saretzki, Charlotte and Bergmann, Ole and Dahmann, Peter and Janser, Frank and Keimer, Jona and Machado, Patricia and Morrison, Audry and Page, Henry and Pluta, Emil and St{\"u}bing, Felix and K{\"u}pper, Thomas}, title = {Are small airplanes safe with regards to COVID-19 transmission?}, series = {Journal of Travel Medicine}, volume = {28}, journal = {Journal of Travel Medicine}, number = {7}, publisher = {Oxford University Press}, address = {Oxford}, issn = {1708-8305}, doi = {10.1093/jtm/taab105}, year = {2021}, language = {en} } @article{Bung2024, author = {Bung, Daniel Bernhard}, title = {Kamerabasierte Fließtiefen- und Geschwindigkeitsmessungen}, series = {Wasserwirtschaft}, volume = {114}, journal = {Wasserwirtschaft}, number = {4}, publisher = {Springer Vieweg}, address = {Wiesbaden}, issn = {0043-0978}, pages = {47 -- 53}, year = {2024}, abstract = {In der wasserbaulichen Forschung werden neben klassischen Messinstrumenten zunehmend kamerabasierte Verfahren genutzt. Diese erlauben neben der Bestimmung von Fließgeschwindigkeiten auch die Detektion der freien Wasseroberfl{\"a}che oder zeitliche Vermessung von Kolken. Durch die hohen r{\"a}umlichen und zeitlichen Aufl{\"o}sungen, welche neueste Kamerasensoren liefern, k{\"o}nnen neue Erkenntnisse in turbulenten, komplexen Str{\"o}mungen gewonnen werden. Auch in der Praxis k{\"o}nnen diese Verfahren mit geringem Aufwand wichtige Daten liefern.}, language = {de} } @article{OertelBung2021, author = {Oertel, Mario and Bung, Daniel Bernhard}, title = {Hochwasserschutz - eine Aufgabe f{\"u}r eine nachhaltige Wasserwirtschaft}, series = {Wasserwirtschaft}, volume = {111}, journal = {Wasserwirtschaft}, number = {9-10}, publisher = {Springer Vieweg}, address = {Wiesbaden}, issn = {0043-0978}, pages = {3 -- 19}, year = {2021}, language = {de} } @article{Bung2021, author = {Bung, Daniel Bernhard}, title = {Extreme flooding in Western Germany: some thoughts on hazards, return periods and risk}, series = {Hydrolink}, journal = {Hydrolink}, number = {4}, publisher = {International Association for Hydro-Environment Engineering and Research (IAHR)}, address = {Madrid}, pages = {108 -- 113}, year = {2021}, abstract = {The low-pressure system Bernd involved extreme rainfalls in the Western part of Germany in July 2021, resulting in major floods, severe damages and a tremendous number of casualties. Such extreme events are rare and full flood protection can never be ensured with reasonable financial means. But still, this event must be starting point to reconsider current design concepts. This article aims at sharing some thoughts on potential hazards, the selection of return periods and remaining risk with the focus on Germany.}, language = {en} } @article{AyalaHarrisKleefeld2024, author = {Ayala, Rafael Ceja and Harris, Isaac and Kleefeld, Andreas}, title = {Direct sampling method via Landweber iteration for an absorbing scatterer with a conductive boundary}, series = {Inverse Problems and Imaging}, volume = {18}, journal = {Inverse Problems and Imaging}, number = {3}, publisher = {AIMS}, address = {Springfield}, issn = {1930-8337}, doi = {10.3934/ipi.2023051}, pages = {708 -- 729}, year = {2024}, abstract = {In this paper, we consider the inverse shape problem of recovering isotropic scatterers with a conductive boundary condition. Here, we assume that the measured far-field data is known at a fixed wave number. Motivated by recent work, we study a new direct sampling indicator based on the Landweber iteration and the factorization method. Therefore, we prove the connection between these reconstruction methods. The method studied here falls under the category of qualitative reconstruction methods where an imaging function is used to recover the absorbing scatterer. We prove stability of our new imaging function as well as derive a discrepancy principle for recovering the regularization parameter. The theoretical results are verified with numerical examples to show how the reconstruction performs by the new Landweber direct sampling method.}, language = {en} } @article{ClausnitzerKleefeld2024, author = {Clausnitzer, Julian and Kleefeld, Andreas}, title = {A spectral Galerkin exponential Euler time-stepping scheme for parabolic SPDEs on two-dimensional domains with a C² boundary}, series = {Discrete and Continuous Dynamical Systems - Series B}, volume = {29}, journal = {Discrete and Continuous Dynamical Systems - Series B}, number = {4}, publisher = {AIMS}, address = {Springfield}, issn = {1531-3492}, doi = {10.3934/dcdsb.2023148}, pages = {1624 -- 1651}, year = {2024}, abstract = {We consider the numerical approximation of second-order semi-linear parabolic stochastic partial differential equations interpreted in the mild sense which we solve on general two-dimensional domains with a C² boundary with homogeneous Dirichlet boundary conditions. The equations are driven by Gaussian additive noise, and several Lipschitz-like conditions are imposed on the nonlinear function. We discretize in space with a spectral Galerkin method and in time using an explicit Euler-like scheme. For irregular shapes, the necessary Dirichlet eigenvalues and eigenfunctions are obtained from a boundary integral equation method. This yields a nonlinear eigenvalue problem, which is discretized using a boundary element collocation method and is solved with the Beyn contour integral algorithm. We present an error analysis as well as numerical results on an exemplary asymmetric shape, and point out limitations of the approach.}, language = {en} } @article{HarrisKleefeld2022, author = {Harris, Isaac and Kleefeld, Andreas}, title = {Analysis and computation of the transmission eigenvalues with a conductive boundary condition}, series = {Applicable Analysis}, volume = {101}, journal = {Applicable Analysis}, number = {6}, publisher = {Taylor \& Francis}, address = {London}, issn = {1563-504X}, doi = {10.1080/00036811.2020.1789598}, pages = {1880 -- 1895}, year = {2022}, abstract = {We provide a new analytical and computational study of the transmission eigenvalues with a conductive boundary condition. These eigenvalues are derived from the scalar inverse scattering problem for an inhomogeneous material with a conductive boundary condition. The goal is to study how these eigenvalues depend on the material parameters in order to estimate the refractive index. The analytical questions we study are: deriving Faber-Krahn type lower bounds, the discreteness and limiting behavior of the transmission eigenvalues as the conductivity tends to infinity for a sign changing contrast. We also provide a numerical study of a new boundary integral equation for computing the eigenvalues. Lastly, using the limiting behavior we will numerically estimate the refractive index from the eigenvalues provided the conductivity is sufficiently large but unknown.}, language = {en} } @article{ChwallekNawrathKrastinaetal.2024, author = {Chwallek, Constanze and Nawrath, Lara and Krastina, Anzelika and Bruksle, Ieva}, title = {Supportive research on sustainable entrepreneurship and business practices}, series = {SECA Sustainable Entrepreneurship for Climate Action}, journal = {SECA Sustainable Entrepreneurship for Climate Action}, number = {3}, publisher = {Lapland University of Applied Sciences Ltd}, address = {Rovaniemi}, isbn = {978-952-316-514-4 (pdf)}, issn = {2954-1654 (on-line publication)}, pages = {67 Seiten}, year = {2024}, language = {en} } @article{KleefeldZimmermann2022, author = {Kleefeld, Andreas and Zimmermann, M.}, title = {Computing Elastic Interior Transmission Eigenvalues}, series = {Integral Methods in Science and Engineering}, journal = {Integral Methods in Science and Engineering}, editor = {Constanda, Christian and Bodmann, Bardo E.J. and Harris, Paul J.}, publisher = {Birkh{\"a}user}, address = {Cham}, isbn = {978-3-031-07171-3}, doi = {10.1007/978-3-031-07171-3_10}, pages = {139 -- 155}, year = {2022}, abstract = {An alternative method is presented to numerically compute interior elastic transmission eigenvalues for various domains in two dimensions. This is achieved by discretizing the resulting system of boundary integral equations in combination with a nonlinear eigenvalue solver. Numerical results are given to show that this new approach can provide better results than the finite element method when dealing with general domains.}, language = {en} } @article{Kleefeld2021, author = {Kleefeld, Andreas}, title = {The hot spots conjecture can be false: some numerical examples}, series = {Advances in Computational Mathematics}, volume = {47}, journal = {Advances in Computational Mathematics}, publisher = {Springer}, address = {Dordrecht}, issn = {1019-7168}, doi = {10.1007/s10444-021-09911-5}, year = {2021}, abstract = {The hot spots conjecture is only known to be true for special geometries. This paper shows numerically that the hot spots conjecture can fail to be true for easy to construct bounded domains with one hole. The underlying eigenvalue problem for the Laplace equation with Neumann boundary condition is solved with boundary integral equations yielding a non-linear eigenvalue problem. Its discretization via the boundary element collocation method in combination with the algorithm by Beyn yields highly accurate results both for the first non-zero eigenvalue and its corresponding eigenfunction which is due to superconvergence. Additionally, it can be shown numerically that the ratio between the maximal/minimal value inside the domain and its maximal/minimal value on the boundary can be larger than 1 + 10- 3. Finally, numerical examples for easy to construct domains with up to five holes are provided which fail the hot spots conjecture as well.}, language = {en} } @article{MartinVaqueroKleefeld2020, author = {Mart{\´i}n-Vaquero, J. and Kleefeld, Andreas}, title = {Solving nonlinear parabolic PDEs in several dimensions: Parallelized ESERK codes}, series = {Journal of Computational Physics}, journal = {Journal of Computational Physics}, number = {423}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0021-9991}, doi = {10.1016/j.jcp.2020.109771}, year = {2020}, abstract = {There is a very large number of very important situations which can be modeled with nonlinear parabolic partial differential equations (PDEs) in several dimensions. In general, these PDEs can be solved by discretizing in the spatial variables and transforming them into huge systems of ordinary differential equations (ODEs), which are very stiff. Therefore, standard explicit methods require a large number of iterations to solve stiff problems. But implicit schemes are computationally very expensive when solving huge systems of nonlinear ODEs. Several families of Extrapolated Stabilized Explicit Runge-Kutta schemes (ESERK) with different order of accuracy (3 to 6) are derived and analyzed in this work. They are explicit methods, with stability regions extended, along the negative real semi-axis, quadratically with respect to the number of stages s, hence they can be considered to solve stiff problems much faster than traditional explicit schemes. Additionally, they allow the adaptation of the step length easily with a very small cost. Two new families of ESERK schemes (ESERK3 and ESERK6) are derived, and analyzed, in this work. Each family has more than 50 new schemes, with up to 84.000 stages in the case of ESERK6. For the first time, we also parallelized all these new variable step length and variable number of stages algorithms (ESERK3, ESERK4, ESERK5, and ESERK6). These parallelized strategies allow to decrease times significantly, as it is discussed and also shown numerically in two problems. Thus, the new codes provide very good results compared to other well-known ODE solvers. Finally, a new strategy is proposed to increase the efficiency of these schemes, and it is discussed the idea of combining ESERK families in one code, because typically, stiff problems have different zones and according to them and the requested tolerance the optimum order of convergence is different.}, language = {en} } @article{AsanteAsamaniKleefeldWade2020, author = {Asante-Asamani, E.O. and Kleefeld, Andreas and Wade, B.A.}, title = {A second-order exponential time differencing scheme for non-linear reaction-diffusion systems with dimensional splitting}, series = {Journal of Computational Physics}, volume = {415}, journal = {Journal of Computational Physics}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0021-9991}, doi = {10.1016/j.jcp.2020.109490}, year = {2020}, abstract = {A second-order L-stable exponential time-differencing (ETD) method is developed by combining an ETD scheme with approximating the matrix exponentials by rational functions having real distinct poles (RDP), together with a dimensional splitting integrating factor technique. A variety of non-linear reaction-diffusion equations in two and three dimensions with either Dirichlet, Neumann, or periodic boundary conditions are solved with this scheme and shown to outperform a variety of other second-order implicit-explicit schemes. An additional performance boost is gained through further use of basic parallelization techniques.}, language = {en} } @article{BreussKleefeld2020, author = {Breuß, Michael and Kleefeld, Andreas}, title = {Implicit monotone difference methods for scalar conservation laws with source terms}, series = {Acta Mathematica Vietnamica}, volume = {45}, journal = {Acta Mathematica Vietnamica}, publisher = {Springer Singapore}, address = {Singapore}, issn = {2315-4144}, doi = {10.1007/s40306-019-00354-1}, pages = {709 -- 738}, year = {2020}, abstract = {In this article, a concept of implicit methods for scalar conservation laws in one or more spatial dimensions allowing also for source terms of various types is presented. This material is a significant extension of previous work of the first author (Breuß SIAM J. Numer. Anal. 43(3), 970-986 2005). Implicit notions are developed that are centered around a monotonicity criterion. We demonstrate a connection between a numerical scheme and a discrete entropy inequality, which is based on a classical approach by Crandall and Majda. Additionally, three implicit methods are investigated using the developed notions. Next, we conduct a convergence proof which is not based on a classical compactness argument. Finally, the theoretical results are confirmed by various numerical tests.}, language = {en} } @article{HarrisKleefeld2018, author = {Harris, Isaac and Kleefeld, Andreas}, title = {The inverse scattering problem for a conductive boundary condition and transmission eigenvalues}, series = {Applicable Analysis}, volume = {99}, journal = {Applicable Analysis}, number = {3}, publisher = {Taylor \& Francis}, address = {London}, issn = {1563-504X}, doi = {10.1080/00036811.2018.1504028}, pages = {508 -- 529}, year = {2018}, abstract = {In this paper, we consider the inverse scattering problem associated with an inhomogeneous media with a conductive boundary. In particular, we are interested in two problems that arise from this inverse problem: the inverse conductivity problem and the corresponding interior transmission eigenvalue problem. The inverse conductivity problem is to recover the conductive boundary parameter from the measured scattering data. We prove that the measured scatted data uniquely determine the conductivity parameter as well as describe a direct algorithm to recover the conductivity. The interior transmission eigenvalue problem is an eigenvalue problem associated with the inverse scattering of such materials. We investigate the convergence of the eigenvalues as the conductivity parameter tends to zero as well as prove existence and discreteness for the case of an absorbing media. Lastly, several numerical and analytical results support the theory and we show that the inside-outside duality method can be used to reconstruct the interior conductive eigenvalues.}, language = {en} }