Electrically driven free shear flows in a duct under a transverse uniform magnetic field. - In: Magnetohydrodynamics, Bd. 59 (2023), 1, S. 3-22
A mathematical model of two-dimensional electrically driven laminar plane free shear flows in a straight duct under the action of an applied spanwise uniform magnetic field is considered. The mathematical approach is like that used in the research of Hunt and Williams (J. Fluid. Mech., 31, 705, 1968) and Kolesnikov and Kalis (Magnetohydrodynamics, vol. 57, 2021, no. 2). A system of stationary partial differential equations with two unknown functions of velocity and induced magnetic field is solved. The electric current is injected into the liquid by means of two couples of linear electrodes located vis-à-vis on opposite duct walls, perpendicular to the magnetic field. Three cases are considered. One pair of electrodes is current supplied and, depending on the direction of electric current injection on the electrode pair, two coinciding or two counter flows are also driven. At Hartmann numbers Ha >> 1, quasi-potential cores are formed in these flows, bounded by lateral Shercliff free boundary layers parallel to the field and two Hartmann layers on the walls perpendicular to the field. As a result, almost all of the injected current passes through these layers. An increase of the magnetic field leads only to an internal rearrangement of the potential cores of the flows. The Hartmann number varies in the range from 1 to 100.
https://doi.org/10.22364/mhd.59.1.1
Thermal boundary condition studies in large aspect ratio Rayleigh-Bénard convection. - In: European journal of mechanics, ISSN 1873-7390, Bd. 101 (2023), S. 283-293
We study the influence of thermal boundary conditions on large aspect ratio Rayleigh-Bénard convection by a joint analysis of experimental and numerical data sets for a Prandtl number Pr=7 and Rayleigh numbers Ra=105−106. The spatio-temporal experimental data are obtained by combined Particle Image Velocimetry and Particle Image Thermometry measurements in a cuboid cell filled with water at an aspect ratio Γ=25. In addition, numerical data are generated by Direct Numerical Simulations (DNS) in domains with Γ=25 and Γ=60 subject to different idealized thermal boundary conditions. Our experimental data show an increased characteristic horizontal extension scale ÜÞλ of the flow structures for increasing Ra , which due to an increase of the convective heat transfer also leads to an increase of the Biot number (Bi) at the cooling plate. However, we find the experimental flow structure size to range in any case in between the ones observed for the idealized thermal boundary conditions captured by the simulations: On the one hand, they are larger than in the numerical case with applied uniform temperatures at the plates. On the other hand, they are smaller than in the case of an applied constant heat flux, the latter of which leads to a structure that grows gradually up to the horizontal domain size. We are able to link this observation qualitatively to theoretical predictions for the onset of convection. Furthermore, we study the effect of the asymmetric boundary conditions on the heat transfer. Contrasting experimental and numerical data reveals an increased probability of far-tail events of reversed heat transfer. The successive decomposition of the local Nusselt number Nuloc traces this effect back to the sign of the temperature deviation ÜÞΘ, eventually revealing asymmetries of the heating and cooling plate on the thermal variance of the generated thermal plumes.
https://doi.org/10.1016/j.euromechflu.2023.06.003
Data-informed reservoir computing for efficient time-series prediction. - In: Chaos, ISSN 1089-7682, Bd. 33 (2023), 7, 073109, S. 073109-1-073109-11
We propose a new approach to dynamical system forecasting called data-informed-reservoir computing (DI-RC) that, while solely being based on data, yields increased accuracy, reduced computational cost, and mitigates tedious hyper-parameter optimization of the reservoir computer (RC). Our DI-RC approach is based on the recently proposed hybrid setup where a knowledge-based model is combined with a machine learning prediction system, but it replaces the knowledge-based component by a data-driven model discovery technique. As a result, our approach can be chosen when a suitable knowledge-based model is not available. We demonstrate our approach using a delay-based RC as the machine learning component in conjunction with sparse identification of nonlinear dynamical systems for the data-driven model component. We test the performance on two example systems: the Lorenz system and the Kuramoto-Sivashinsky system. Our results indicate that our proposed technique can yield an improvement in the time-series forecasting capabilities compared with both approaches applied individually, while remaining computationally cheap. The benefit of our proposed approach, compared with pure RC, is most pronounced when the reservoir parameters are not optimized, thereby reducing the need for hyperparameter optimization.
https://doi.org/10.1063/5.0152311
3D passive microfluidic valves in silicon and glass using grayscale lithography and reactive ion etching transfer. - In: Microfluidics and nanofluidics, ISSN 1613-4990, Bd. 27 (2023), 8, 55, S. 1-12
A fabrication strategy for high-efficiency passive three-dimensional microfluidic valves with no mechanical parts fabricated in silicon and glass substrates is presented. 3D diffuser-nozzle valve structures were produced and characterized in their added value in comparison to conventional diffuser-nozzle valve designs with rectangular cross sections. A grayscale lithography approach for 3D photoresist structuring combined with a proportional transfer by reactive ion etching allowed to transfer 3D resist valve designs with high precision into the targeted substrate material. The efficiency with respect to the rectification characteristics or so-called diodicity of the studied valve designs is defined as the ratio of the pressure drops in backward and forward flow directions. The studied valve designs were characterized experimentally as well as numerically based on finite element simulations with overall matching results that demonstrate a significantly improved flow rectification of the 3D valves compared to the corresponding conventional structure. Our novel 3D valve structures show, for instance, even without systematic optimization a measured diodicity of up to 1.5 at low flow rates of only about 10 μl/s.
https://doi.org/10.1007/s10404-023-02663-2
Numerical simulation of melt flow, heat transfer and solidification in CSP continuous casting mold with vertical-combined electromagnetic braking. - In: Metallurgical and materials transactions, ISSN 1543-1916, Bd. 54 (2023), 4, S. 1646-1664
During continuous casting, electromagnetic braking (EMBr) is a widely used technology to improve the quality of steel product. The EMBr technology takes benefit of the generation of Lorentz forces that are induced by the interactions of melt flow with externally applied magnetic fields. In the present paper we propose and investigate a new type of EMBr, named vertical-combined electromagnetic braking (VC-EMBr) in application to the Compact Strip Production (CSP) thin slab continuous casting mold. The unique characteristic of the VC-EMBr lies in the fact that two new pairs of vertical magnetic poles (VMPs) are located adjacent to the mold narrow faces on the basis of Ruler-EMBr. To determine the braking effect of the VC-EMBr, the influence of the installation position of the VMPs on the flow, heat transfer and solidification behaviors of ultra-low carbon steel in a 1500 × 70 mm CSP funnel-type mold is numerically solved. The fluid-flow-related phenomena of three casting cases in the CSP mold, i.e., No-EMBr, Ruler-EMBr, and VC-EMBr, are further investigated numerically to evaluate the metallurgical capability of the VC-EMBr, including the quantitative evaluation of level fluctuation, heat transfer, and shell growth at a casting speed of 4.5 m/min. The parametric study shows that for the CSP mold with width of 1500 mm, the optimal braking effect of the VC-EMBr can be obtained when the VMPs are located at 50 mm from the narrow face of the mold. With this adjustment, the magnitude of the maximum surface velocity is reduced by 70 pct when compared to the case of p1 = 0 mm. This reduction can decrease the heat loss in the upper recirculation region of the CSP mold and promote the homogeneity of the temperature field therein. In addition, the evaluation results show that the newly proposed VC-EMBr provides more obvious technological advantages than the traditional Ruler-EMBr in application to the CSP mold with a bifurcated nozzle. For the VC-EMBr, the horizontal magnetic poles (HMPs) keep the same advantage as the Ruler-EMBr in providing a good protection against excessive downward impact of the molten steel. On the other hand, the VMPs overcome the disadvantage that the Ruler-EMBr cannot well suppress the upward backflow in the CSP mold. For instance, by applying a magnetic flux density of 0.3 T, the VC-EMBr has a better capability to reduce the maximum amplitude of the level fluctuation by 83.8 pct and increase the average surface temperature of the molten steel from 1803.6 K to 1804.5 K when compared to the case of Ruler-EMBr. This variation can well prevent surface defects related to the level fluctuation, such as slag entrapment and mold powder freezing. On this basis, it can be seen that the industrial application of the VC-EMBr in the CSP mold can benefit from these findings.
https://doi.org/10.1007/s11663-023-02784-7
Deformability and collision-induced reorientation enhance cell topotaxis in dense microenvironments. - In: Biophysical journal, ISSN 1542-0086, Bd. 122 (2023), 13, S. 2791-2807
In vivo, cells navigate through complex environments filled with obstacles such as other cells and the extracellular matrix. Recently, the term “topotaxis” has been introduced for navigation along topographic cues such as obstacle density gradients. Experimental and mathematical efforts have analyzed topotaxis of single cells in pillared grids with pillar density gradients. A previous model based on active Brownian particles (ABPs) has shown that ABPs perform topotaxis, i.e., drift toward lower pillar densities, due to decreased effective persistence lengths at high pillar densities. The ABP model predicted topotactic drifts of up to 1% of the instantaneous speed, whereas drifts of up to 5% have been observed experimentally. We hypothesized that the discrepancy between the ABP and the experimental observations could be in 1) cell deformability and 2) more complex cell-pillar interactions. Here, we introduce a more detailed model of topotaxis based on the cellular Potts model (CPM). To model persistent cells we use the Act model, which mimics actin-polymerization-driven motility, and a hybrid CPM-ABP model. Model parameters were fitted to simulate the experimentally found motion of Dictyostelium discoideum on a flat surface. For starved D. discoideum, the topotactic drifts predicted by both CPM variants are closer to the experimental results than the previous ABP model due to a larger decrease in persistence length. Furthermore, the Act model outperformed the hybrid model in terms of topotactic efficiency, as it shows a larger reduction in effective persistence time in dense pillar grids. Also pillar adhesion can slow down cells and decrease topotaxis. For slow and less-persistent vegetative D. discoideum cells, both CPMs predicted a similar small topotactic drift. We conclude that deformable cell volume results in higher topotactic drift compared with ABPs, and that feedback of cell-pillar collisions on cell persistence increases drift only in highly persistent cells.
https://doi.org/10.1016/j.bpj.2023.06.001
Boosting material utilization via direct growth of Zn2(V3O8)2 on the carbon cloth as a cathode to achieve a high-capacity aqueous zinc-ion battery. - In: Small, ISSN 1613-6829, Bd. 19 (2023), 46, 2303307, S. 1-10
Aqueous zinc-ion batteries (AZIBs) have attracted the attention of researchers because of their high theoretical capacity and safety. Among the many vanadium-based AZIB cathode materials, zinc vanadate is of great interest as a typical phase in the dis-/charge process. Here, a remarkable method to improve the utilization rate of zinc vanadate cathode materials is reported. In situ growth of Zn2(V3O8)2 on carbon cloth (CC) as the cathode material (ZVOCC) of AZIBs. Compared with the Zn2(V3O8)2 cathode material bonded on titanium foil (ZVO@Ti), the specific capacity increases from 300 to 420 mAh g−1, and the utilization rate of the material increases from 69.60% to 99.2%. After the flexible device is prepared, it shows the appropriate specific capacity (268.4 mAh g−1 at 0.1 A g−1) and high safety. The method proposed in this work improves the material utilization rate and enhances the energy density of AZIB and also has a certain reference for the other electrochemical energy storage devices.
https://doi.org/10.1002/smll.202303307
Calibration of low-temperature photoluminescence of boron-doped silicon with increased temperature precision. - In: Physica status solidi, ISSN 1521-3951, Bd. 260 (2023), 10, 2300300, S. 1-5
https://doi.org/10.1002/pssb.202300300
Three-dimensional light field fundus imaging: automatic determination of diagnostically relevant optic nerve head parameters. - In: Translational Vision Science & Technology, ISSN 2164-2591, Bd. 12 (2023), 7, 21, S. 1-16
Purpose: Morphological changes to the optic nerve head (ONH) can be detected at the early stages of glaucoma. Three-dimensional imaging and analysis may aid in the diagnosis. Light field (LF) fundus cameras can generate three-dimensional (3D) images of optic disc topography from a single shot and are less susceptible to motion artifacts. Here, we introduce a processing method to determine diagnostically relevant ONH parameters automatically and present the results of a subject study performed to validate this method. Methods: The ONHs of 17 healthy subjects were examined and images were acquired with both an LF fundus camera and by optical coherence tomography (OCT). The LF data were analyzed with a novel algorithm and compared with the results of the OCT study. Depth information was reconstructed, and a model with radial basis functions was used for processing of the 3D point cloud and to provide a finite surface. The peripapillary rising and falling edges were evaluated to determine optic disc and cup contours and finally calculate the parameters. Results: Nine of the 17 subjects exhibited prominent optic cups. The contours and ONH parameters determined by an analysis of LF 3D imaging largely agreed with the data obtained from OCT. The median disc areas, cup areas, and cup depths differed by 0.17 mm^2, -0.04 mm^2, and -0.07 mm, respectively. Conclusions: The findings presented here suggest the possibility of using LF data to evaluate the ONH. Translational Relevance: LF data can be used to determine geometric parameters of the ONH and thus may be suitable for future use in glaucoma diagnostics.
https://doi.org/10.1167/tvst.12.7.21
Ultrafast coupling of optical near fields to low-energy electrons probed in a point-projection microscope. - In: Nano letters, ISSN 1530-6992, Bd. 23 (2023), 12, S. 5528-5534
We report the first observation of the coupling of strong optical near fields to wavepackets of free, 100 eV electrons with <50 fs temporal resolution in an ultrafast point-projection microscope. Optical near fields are created by excitation of a thin, nanometer-sized Yagi-Uda antenna, with 20 fs near-infrared laser pulses. Phase matching between electrons and near fields is achieved due to strong spatial confinement of the antenna near field. Energy-resolved projection images of the antenna are recorded in an optical pump-electron probe scheme. We show that the phase modulation of the electron by transverse-field components results in a transient electron deflection while longitudinal near-field components broaden the kinetic energy distribution. This low-energy electron near-field coupling is used here to characterize the chirp of the ultrafast electron wavepackets, acquired upon propagation from the electron emitter to the sample. Our results bring direct mapping of different vectorial components of highly localized optical near fields into reach.
https://doi.org/10.1021/acs.nanolett.3c00738