Recent publications

This paper presents a methodology for the evaluation of waste heat distribution networks in industrial sites. In the context of ultra-efficient industrial sites, implementing these networks shows significant energy cost and emission reduction potential through cooperation between companies. To this end, a mathematical methodology for waste heat distribution network simulation was developed. In this approach, a differential equation relating radial heat losses and internal temperature was derived to include temperature changes inside the pipe system. Furthermore, relevant parameters such as diameter, insulation thickness, and temperature at source and sink were used to determine the distribution network’s technical and economic feasibility . Using time discrete, characteristic thermal energy supply and demand data in the analysis framework, the technical and economic assessment of waste heat distribution networks was carried out via the use case of an industrial park. In this use case, the economic and ecological advantages were quantified and shown.

The experimental evaluation of melt surface flow in blown laser cladding and additive manufacturing usually employs tracer particles added to the powder feed. This paper presents details of a high-speed imaging (HSI) and image processing technique, which can directly monitor the flow of standard (nontracer) particles on the surface of the melt. This technique should improve the accuracy of flow observations as no foreign bodies are added to the melt. To verify the technique, 316 Stainless steel powder was laser clad onto a substrate of the same material, and an HSI frame rate of 40 000 frames/s was employed with a specific illumination system. The images were subsequently image processed and the trajectories of the powder feed particles on the melt surface are analyzed and discussed here. As would be expected in this case, the observed melt surface flow was in agreement with what would be expected of a melt with a negative surface tension gradient as a function of temperature.

The present research work aims in investigating the large-scale jet flapping instability during the primary breakup process and its effects on the liquid shedding and the resulting flame. For this purpose, a modified coaxial burner is operated over wide range of flow conditions with different liquids in the central passage. Time-resolved images of the liquid jet breakup process and the resulting flame during spray combustion are captured. A Proper orthogonal decomposition (POD) technique is used to investigate the jet flapping and calculate the time-scale corresponding to this mode of instability. Fast-Fourier Transform (FFT) analysis is performed over the average intensity of the interrogation window in an image ensemble to measure the time-scale corresponding to liquid shedding and flame fluctuations. A strong dependence of the frequency corresponding to jet flapping, liquid shedding and flame fluctuations is observed with respect to the dispersion gas velocity. Jet flapping indicates a strong influence on the liquid shedding from the liquid jet, which then influences the resulting flame in the downstream region of the burner.

The spherical shell and spherical zonal band are two elemental geometries that are often used as benchmarks for gravity field modeling. When applying the spherical shell and spherical zonal band discretized into tesseroids, the errors may be reduced or cancelled for the superposition of the tesseroids due to the spherical symmetry of the spherical shell and spherical zonal band. In previous studies, this superposition error elimination effect (SEEE) of the spherical shell and spherical zonal band has not been taken seriously, and it needs to be investigated carefully. In this contribution, the analytical formulas of the signal of derivatives of the gravitational potential up to third order (e.g., V , $$V_{z}$$ V z , $$V_{zz}$$ V zz , $$V_{xx}$$ V xx , $$V_{yy}$$ V yy , $$V_{zzz}$$ V zzz , $$V_{xxz}$$ V xxz , and $$V_{yyz}$$ V yyz ) of a tesseroid are derived when the computation point is situated on the polar axis. In comparison with prior research, simpler analytical expressions of the gravitational effects of a spherical zonal band are derived from these novel expressions of a tesseroid. In the numerical experiments, the relative errors of the gravitational effects of the individual tesseroid are compared to those of the spherical zonal band and spherical shell not only with different 3D Gauss–Legendre quadrature orders ranging from (1,1,1) to (7,7,7) but also with different grid sizes (i.e., $$5^{\circ }\times 5^{\circ }$$ 5 ∘ × 5 ∘ , $$2^{\circ }\times 2^{\circ }$$ 2 ∘ × 2 ∘ , $$1^{\circ }\times 1^{\circ }$$ 1 ∘ × 1 ∘ , $$30^{\prime }\times 30^{\prime }$$ 30 ′ × 30 ′ , and $$15^{\prime }\times 15^{\prime }$$ 15 ′ × 15 ′ ) at a satellite altitude of 260 km. Numerical results reveal that the SEEE does not occur for the gravitational components V , $$V_{z}$$ V z , $$V_{zz}$$ V zz , and $$V_{zzz}$$ V zzz of a spherical zonal band discretized into tesseroids. The SEEE can be found for the $$V_{xx}$$ V xx and $$V_{yy}$$ V yy , whereas the superposition error effect exists for the $$V_{xxz}$$ V xxz and $$V_{yyz}$$ V yyz of a spherical zonal band discretized into tesseroids on the overall average. In most instances, the SEEE occurs for a spherical shell discretized into tesseroids. In summary, numerical experiments demonstrate the existence of the SEEE of a spherical zonal band and a spherical shell, and the analytical solutions for a tesseroid can benefit the investigation of the SEEE. The single tesseroid benchmark can be proposed in comparison to the spherical shell and spherical zonal band benchmarks in gravity field modeling based on these new analytical formulas of a tesseroid.

The azulene moiety is a highly attractive building block in optoelectronic applications due to its unique properties. For high-performing devices, the molecular orientation is crucial and can be controlled through liquid-crystalline self-assembly. Recent work showed that liquid crystalline derivatives bearing the 2-phenyl-azulene-1-nitrile core formed broad de Vries-type SmA and SmC phases. For exact understanding of the structure-property relationship, a series of 2-(hetero)aryl-azulenes has been synthesized varying the chain linkage, the lateral substituent, and the aromatic ring. Small changes of the molecular structure determined whether the orthogonal SmA phase or the tilted SmC phase is predominant. Implementation of alkyne chains instead of alkoxy chains resulted in the reduction of phase transition temperatures and formation of mesophases at room temperature. Furthermore, de Vries-like behavior was investigated and reduction values between R = 0.35 and 0.74 were measured which supported the hypothesis that in this system de Vries-like behavior is caused by steric repulsion of the lateral substituent. The control of the phase geometry by the molecular structure might be used for improved molecular orientation in optoelectronic materials.

Near-surface negatively charged nitrogen vacancy (NV) centers hold excellent promise for nanoscale magnetic imaging and quantum sensing. However, they often experience charge-state instabilities, leading to strongly reduced fluorescence and NV coherence time, which negatively impact magnetic imaging sensitivity. This occurs even more severely at 4 K and ultrahigh vacuum (UHV, p = 2 × 10-10 mbar). We demonstrate that in situ adsorption of H2O on the diamond surface allows the partial recovery of the shallow NV sensors. Combining these with band-bending calculations, we conclude that controlled surface treatments are essential for implementing NV-based quantum sensing protocols under cryogenic UHV conditions.

Direct numerical simulations were performed to investigate the behavior of laminar separation bubbles subject to vertical gusts in an airfoil flow. Oscillating boundary-layer flows containing gusts—including unsteady pressure gradients—were prescribed via boundary conditions as well as forcing terms on a domain located in the upper rear section of a natural laminar flow airfoil. For this hybrid approach, unsteady Reynolds-averaged Navier-Stokes (URANS) simulations of the entire flowfield were carried out in conjunction with the so-called disturbance velocity approach, providing transient boundary conditions for the direct numerical simulation. A steady-state reference case with a Reynolds number of Re=880,000 is compared and validated with results from wind tunnel experiments. Results of simulations at four different gust frequencies κ at the same amplitude v'_gust, each with and without additional disturbances to the boundary layer introducing oblique resonance, are presented and discussed in this paper. The time-dependent behavior of convective instability modes is evaluated by using the continuous wavelet transform and linear stability theory with an unsteady extension. Furthermore, the contribution of the absolute instability—which appears to be larger in the case of oscillating flow compared to the steady-state case—is discussed. Lock-in effects are identified at high gust frequencies.

Today’s internet is shaped by privately operated platforms that not only organize economic processes but also coordinate and regulate broader societal contexts. Against this background, this conceptual paper develops a sociological notion of platform companies and the platforms they operate as a new type of enterprise that consists not only of economic features (business and revenue models, exploitation patterns, market relations) but also of action-orienting rules, institutional infrastructures and social relations between a great variety of individual, corporate and collective actors that clearly reach beyond economic contexts and far into society. To this end, we specify the often fuzzy talk of ‘the platforms’ by drawing an analytical distinction between (1) the platform-operating companies as organizing cores whose goal is to operate a profitable business; (2) the platforms belonging to them as technically mediated market and social action spaces that provide the basis for not only economic but also genuine social activities on today’s internet; and (3) the institutionalized coordination, control and exploitation mechanisms implemented by the platform operators, linking these two constitutive levels of the platform architecture.

We present a first-principles approach for the calculation of solvation energies and enthalpies with respect to different ion pair combinations in various solvents. The method relies on the conceptual density functional theory (DFT) of solvation, from which detailed expressions for the solvation energies can be derived. In addition to fast and straightforward gas phase calculations, we also study the influence of modified chemical reactivity descriptors in terms of electronic perturbations. The corresponding phenomenological changes in molecular energy levels can be interpreted as the influence of continuum solvents. Our approach shows that the introduction of these modified expressions is essential for a quantitative agreement between the calculated and the experimental results.

The shape of the laser beam used for fusion cutting significantly influences the geometry of both the cutting front and the cutting kerf. The angle of the cutting front in turn impacts the local absorptivity, while the width of the kerf defines the amount of material, which has to be molten. The kerf’s geometry therefore determines the maximum possible cutting speed at which a successful cut is feasible with a given available laser power. The absorptivity, the width of the kerf, and the maximum possible cutting speed can be estimated from a simple model considering the conservation of energy and rough geometrical approximations. In order to verify the prediction of the model, the geometry of the cutting front and kerf resulting from different processing conditions was observed by means of online high-speed X-ray diagnostics. The geometry of the interaction zone was recorded with a framerate of 1000 Hz during fusion cutting of 10-mm-thick samples of stainless steel. Comparing the results obtained with different shapes of the laser beam, it was found that the absorptivity is increased when the beam’s longitudinal cross-section (parallel to the feed) is enlarged. Reducing the width of the beam in the transversal direction normal to the feed reduces the cross-sectional area of the cutting kerf. The findings show a good agreement with the geometric model which enabled the prediction of the absorptivity and the cross-sectional area of the cutting kerf and hence allows to reliably estimate the maximum cutting speed for different shapes of the laser beam, laser power, and sheet thicknesses.

Physically consistent coupling conditions at the fluid–porous interface with correctly determined effective parameters are necessary for accurate modeling and simulation of various applications. To describe single-fluid-phase flows in coupled free-flow and porous-medium systems, the Stokes/Darcy equations are typically used together with the conservation of mass across the interface, the balance of normal forces and the Beavers–Joseph condition on the tangential velocity. The latter condition is suitable for flows parallel to the interface but not applicable for arbitrary flow directions. Moreover, the value of the Beavers–Joseph slip coefficient is uncertain. In the literature, it is routinely set equal to one that is not correct for many applications, even if the flow is parallel to the porous layer. In this paper, we reformulate the generalized interface condition on the tangential velocity component, recently developed for arbitrary flows in Stokes/Darcy systems, such that it has the same analytical form as the Beavers–Joseph condition. We compute the effective coefficients appearing in this modified condition using theory of homogenization with boundary layers. We demonstrate that the modified Beavers–Joseph condition is applicable for arbitrary flow directions to the fluid–porous interface. In addition, we propose an efficient two-level numerical algorithm based on simulated annealing to compute the optimal Beavers–Joseph parameter.
Article Highlights
A modification of the Beavers–Joseph condition is proposed based on recently developed generalized coupling conditions.
The Beavers-Joseph parameter can be found only for unidirectional flows.
An efficient numerical algorithm to determine the optimal Beavers-Joseph parameter is developed.

Interkommunales Gewerbeflächenmanagement ist durch spezifische Konfliktfelder geprägt. Daher scheitern Kooperationsversuche häufig. Gleichzeitig gibt es aber auch zahlreiche erfolgreiche Kooperationsbeispiele. Unsere Fragestellung lautet daher: Wie und wodurch gelingt bzw. scheitert interkommunale Kooperation im Gewerbeflächenmanagement? Mögliche Erfolgsfaktoren und Hemmnisse von der Planung bis zur Nutzung werden auf Basis eines Literaturreviews diskutiert. Zudem werden Thesen formuliert, unter welchen Bedingungen Gewerbeflächenmanagement perspektivisch auch eine stadtregionale Raumorientierung aufweisen könnte.

Cuprate high-Tc superconductors are known for their intertwined interactions and the coexistence of competing orders. Uncovering experimental signatures of these interactions is often the first step in understanding their complex relations. A typical spectroscopic signature of the interaction between a discrete mode and a continuum of excitations is the Fano resonance/interference, characterized by the asymmetric light-scattering amplitude of the discrete mode as a function of the electromagnetic driving frequency. In this study, we report a new type of Fano resonance manifested by the nonlinear terahertz response of cuprate high-Tc superconductors, where we resolve both the amplitude and phase signatures of the Fano resonance. Our extensive hole-doping and magnetic field dependent investigation suggests that the Fano resonance may arise from an interplay between the superconducting fluctuations and the charge density wave fluctuations, prompting future studies to look more closely into their dynamical interactions.

The present paper investigates the feasibility of gust load alleviation at transonic speeds on a backward swept and a forward swept transport aircraft configuration. Spanwise-distributed control surfaces at the leading and trailing edges are employed to control gust-induced wing bending as well as wing torsion moments. The deflection amplitude and temporal flap actuation are determined by a novel scheme that builds on the aerodynamic strip theory. The aerodynamic effectiveness of the actuators is taken from a data base, computed from either 2D infinite swept wing simulations, or from yawed computations that take the effects of boundary-layer cross flow and the local sweep angle of the control surface into account. The present numerical flow simulations reveal that careful application of control laws at the trailing edge alleviates wing bending moments caused by strong vertical gusts by 85–90%, for both aircraft configurations. The application of leading-edge flaps introduces significant nonlinear aerodynamic interactions, that make the control of torsional moments comparably challenging. Here, the present results indicate that about 60% of wing torsion loads due to strong gusts can be removed.

Novel heterometallic quaternary tellurides EuGdCuTe3 and EuLuCuTe3 are reported for the first time. Both compounds were obtained from the elements as single crystals using the flux-assisted synthetic approach. The crystal structure of EuGdCuTe3 was solved in orthorhombic space group Pnma with the structural type Eu2CuS3, while the crystal structure of EuLuCuTe3 belongs to orthorhombic space group Cmcm with the structural type KZrCuS3. The 3D crystal structure of EuGdCuTe3 is constructed from EuTe7 capped trigonal prisms, GdTe6 distorted octahedra as well as CuTe4 tetrahedra. The octahedra form 2D layers, further strengthened by 1D polymeric chains (CuTe4)n. These layers are separated by 1D dimeric ribbons, formed by EuTe7 capped trigonal prisms and 1D free channels. The 3D crystal structure of EuLuCuTe3 is constructed from EuTe6 trigonal prisms, LuTe6 distorted octahedra and CuTe4 tetrahedra. The latter two polyhedra also form 2D layers, which are separated by alternating 1D polymeric chains (EuTe6)n and 1D free channels. Both tellurides were found to be paramagnetic with the transition to a ferrimagnetic state at about 8 K for EuGdCuTe3 and to a ferromagnetic state at about 3 K for EuLuCuTe3.

Das Feld der sozio-technischen Transitionsforschung befasst sich mit langfristigen, komplexen Transformationsprozessen in sozio-technischen Systemen wie Energieversorgung, Ernährung oder Mobilität. Der Beitrag stellt in einem ersten Schritt Grundbegriffe und Kernkonzepte der Transitionsforschung vor und diskutiert die im Feld bedeutsamsten theoretischen Bezugsrahmen. In einem zweiten Schritt werden diese Konzepte auf den historischen Fall der deutschen Energiewende angewandt und auf diesem Wege veranschaulicht. Abschließend werden die Grenzen der Übertragbarkeit der Erkenntnisse der Energiewende auf nachhaltige Transformationen im Mobilitäts- und Nahrungsmittelbereich diskutiert und die Potentiale und Schwachstellen der Transitionsforschung herausgearbeitet.

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Wolfram Ressel

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http://www.uni-stuttgart.de/en