Defocus particle tracking : a comparison of methods based on model functions, cross-correlation, and neural networks. - In: Measurement science and technology, ISSN 1361-6501, Bd. 32 (2021), 9, 094011, insges. 14 S.
Defocus particle tracking (DPT) has gained increasing importance for its use to determine particle trajectories in all three dimensions with a single-camera system, as typical for a standard microscope, the workhorse of todays ongoing biomedical revolution. DPT methods derive the depth coordinates of particle images from the different defocusing patterns that they show when observed in a volume much larger than the respective depth of field. Therefore it has become common for state-of-the-art methods to apply image recognition techniques. Two of the most commonly and widely used DPT approaches are the application of (astigmatism) particle image model functions (MF methods) and the normalized cross-correlations between measured particle images and reference templates (CC methods). Though still young in the field, the use of neural networks (NN methods) is expected to play a significant role in future and more complex defocus tracking applications. To assess the different strengths of such defocus tracking approaches, we present in this work a general and objective assessment of their performances when applied to synthetic and experimental images of different degrees of astigmatism, noise levels, and particle image overlapping. We show that MF methods work very well in low-concentration cases, while CC methods are more robust and provide better performance in cases of larger particle concentration and thus stronger particle image overlap. The tested NN methods generally showed the lowest performance, however, in comparison to the MF and CC methods, they are yet in an early stage and have still great potential to develop within the field of DPT.
https://doi.org/10.1088/1361-6501/abfef6
Electromagnetic interaction between a permanent magnet and a sphere moving in liquid metal. - In: Experiments in fluids, ISSN 1432-1114, Bd. 62 (2021), 5, 109, S. 1-16
We present a series of model experiments where an electrically non-conductive solid sphere moves inside a vertical column of liquid alloy GaInSn. The experimental setup consists of the liquid metal container, the sphere driving system and the permanent magnet with the attached force sensor. The sphere moves at a controllable constant velocity U0 and follows a straight route, which in turn generates a liquid metal flow around the sphere. This flow interacts with the localized magnetic field of the permanent magnet, and thus a weak reaction force on the magnet is generated. The force sensor attached on the magnet has a resolution of the order 10^-6. Upon elimination of high frequency noise, reproducible time-dependent signals for the forces on the magnet are obtained in the experiments for several Reynolds numbers Re between 160 and 2000. The force component Fz on the magnet parallel to the direction of particle motion exhibits a typical two-peak structure with different peak heights, whereas the transverse force component Fx resembles an antisymmetric pulse. The results demonstrate that the force sensor can detect the presence of a moving particle in a quiescent conducting liquid. They also show that the structure of the Fx signal can be reproduced with less variation and is less sensitive to the Reynolds number than the Fz signal. Moreover, the structure and magnitude of time-dependent Lorentz force signals can be reasonably predicted by a numerical model.
https://doi.org/10.1007/s00348-021-03209-4
Electrical voltage by electron spin-vorticity coupling in laminar ducts. - In: Physical review fluids, ISSN 2469-990X, Bd. 6 (2021), 4, 043703, insges. 14 S.
We report a linear scaling law for an electrical voltage as a function of the pressure drop in capillary pipes and ducts. This voltage is generated by a process which is termed spin hydrodynamic generation (SHDG), a result of the collective electron spin-coupling to the vorticity field in the laminar flow in combination with an inverse spin-Hall effect. We study this phenomenon in laminar duct flows with different width-to-height aspect ratios ranging from 1 (square ducts) to infinite (two dimensional channels). First, we analytically solve the governing Valet-Fert spin diffusion equations for the SHDG by means of the method of small parameters together with proper boundary conditions for the set of inhomogeneous elliptic partial differential equations. Second, the proposed linear scaling law is validated through a series of experiments using capillary tubes with rectangular and square cross sections. The experimental results show very good agreement to the analytically found scaling law. A subsequent substitution of the bulk velocity of the laminar wall-bounded flows by the pressure drop reveals a universal scaling law for the electrical voltage that incorporates all pipe and duct geometries which we could study in our experiments. Finally, the efficiency of the system is estimated for circular pipes, rectangular and square ducts. This study shows that the efficiency of a spin hydrodynamic generator is the same for a circular pipe and a square duct with the same diameter and height, respectively. Hence, due to the ease of manufacturing and the possibility to scale the experiments up to parallel settings in a compact form, micro-channels with a square cross section seem to be the optimum for a spin hydrodynamic generator.
https://doi.org/10.1103/PhysRevFluids.6.043703
Long-time experimental investigation of turbulent superstructures in Rayleigh-Bénard convection by noninvasive simultaneous measurements of temperature and velocity fields. - In: Experiments in fluids, ISSN 1432-1114, Bd. 62 (2021), 4, 64, S. 1-18
Large-scale mean patterns in Rayleigh-Bénard convection, also referred to as turbulent superstructures, have mainly been studied by means of numerical simulations so far, but experimental investigations are still rare. However, the analysis of turbulent superstructures, which are of great importance due to their effect on the local transport of heat and momentum, require both numerical and experimental data. Therefore, within the scope of this study measurements were performed in the horizontal mid plane and in a horizontal plane closer to the top of a Rayleigh-Bénard cell with an aspect ratio of [Gamma]=l/h=25, thereby showing the initial formation of turbulent superstructures and their long-time rearrangement. The turbulent superstructures are investigated experimentally by noninvasive simultaneous measurements of temperature and velocity fields, using the color signal of thermochromic liquid crystals (TLCs) for the evaluation of the temperature and their temporal displacement for the determination of all three velocity components in the measurement planes via stereoscopic particle image velocimetry (stereo-PIV). Applying this measuring technique it is demonstrated that the time-averaging of instantaneous temperature and velocity fields uncovers the turbulent superstructures in both fields. Furthermore, the combination of the temperature and velocity data is used to characterize the local heat flux quantified by the local Nusselt number, which confirms that the turbulent superstructures strongly enhance the heat transfer in Rayleigh-Bénard convection.
https://doi.org/10.1007/s00348-020-03107-1
Energy-efficient operation of vapor compression systems applied to the battery thermal management of electric buses. - Düren : Shaker Verlag, 2021. - XIV, 187 Seiten. - (Schriftenreihe des MAHLE Doktorandenprogramms ; Band 8)
Technische Universität Ilmenau, Dissertation 2021
ISBN 978-3-8440-8009-4
In dieser Arbeit wird eine energieeffiziente Betriebsstrategie eines Batterie-Thermomanagementsystems von Elektrobussen untersucht. Dabei wird sowohl die flüssigkeitsbasierte Batterietemperaturregelung als auch der ideale Betrieb von Dampfkompressionssystemen im Allgemeinen betrachtet. Um das thermische Verhalten der Batterie und die erforderliche Kühllast zu untersuchen, wird eine neuartige transiente Berechnungsmethode des thermischen Batteriesystems vorgeschlagen. Die Ergebnisse zeigen eine ausreichende Temperierung der Zelltemperatur über eine quasistationäre Regelung. Folglich kann die Batteriekühlanlage unter stationären Bedingungen betrieben werden, um die Anforderungen an die Batterietemperatur zu erfüllen. Zur Optimierung der Energieeffizienz der Batteriekühlanlage wird eine detaillierte theoretische und experimentelle Analyse des idealen stationären Betriebs der Kompressionskältemaschine (KKM) für eine Serienkühleinheit der Mahle GmbH durchgeführt. Basierend auf den Ergebnissen wird eine neue modellbasierte Sollwertoptimierungsmethode vorgeschlagen, um die KKM bei idealen Einstellungen zu betreiben. Für quasistationäre Anwendungen stellt die vorgeschlagene Methode eine vielversprechende Alternative zur Extremwertoptimierung oder anderen fortschrittlichen Methoden dar, die zwar genauer sind, aber eine hohe Konvergenzzeit benötigen.
Stereoscopic PIV measurements using low-cost action cameras. - In: Experiments in fluids, ISSN 1432-1114, Bd. 62 (2021), 3, 57, S. 1-16
Recently, large progress was made in the development towards low-cost PIV (Particle Image Velocimetry) for industrial and educational applications. This paper presents the use of two low-cost action cameras for stereoscopic planar PIV. A continuous wave laser or alternatively an LED was used for illumination and pulsed by a frequency generator. A slight detuning of the light pulsation and camera frame rate minimizes systematic errors by the rolling shutter effect and allows for the synchronization of both cameras by postprocessing without the need of hardware synchronization. The setup was successfully qualified on a rotating particle pattern in a planar and stereoscopic configuration as well as on the jet of an aquarium pump. Since action cameras are intended to be used at outdoor activities, they are small, very robust and work autarkic. In conjunction with the synchronization and image pre-processing scheme presented herein, those cameras enable stereoscopic PIV in harsh environments and even on moving experiments.
https://doi.org/10.1007/s00348-020-03110-6
Suppression of free convection effects for spherical 1 kg mass prototype. - In: International journal of heat and mass transfer, ISSN 1879-2189, Bd. 170 (2021), 121037, insges. 13 S.
We investigate the free convection processes in the vicinity of a spherical 1 kg mass standard by two- and three-dimensional direct numerical simulations using a spectral element method. Our focus is on the determination and suppression of updraft forces in a high-precision mass comparator which are caused by temperature differences between mass standard and its environment in the millikelvin range - a source of systematic uncertainties in the high-precison mass determination. A two-dimensional model is presented first, which obtains a good agreement with previous laboratory measurements for the smaller temperature differences up to 15 mK. The influence of different boundary conditions and side lengths of the square domain is discussed for the mass standard positioned in the center of the chamber. The complexity is increased subsequently in configurations with additional built-ins for counter heating in form of planar plates or hemispherical shells above the mass standard. The latter ones lead to a full compensation of the updraft force. Three-dimensional simulations in a closed cubic chamber confirm the two-dimensional findings and additionally reveal complex secondary flow patterns in the vicinity of the mass standard. The reduction of the heat transfer due to the built-ins is also demonstrated by a comparison of the Nusselt numbers as a function of the Rayleigh number in the chosen parameter range. Our simulations suggest that such additional constructive measures can enhance the precision of the mass determination by suppression of free convection and related systematic uncertainties.
https://doi.org/10.1016/j.ijheatmasstransfer.2021.121037
Liquid metal flat jet transformation under influence of transverse magnetic field. - In: ICNRP Volga 2020, (2020), 012002, insges. 7 S.
The article is devoted to an experimental study of a submerged flat jet flow in a transverse magnetic field. Two different approaches to the experimental study of jet flows are described. Detailed information about the experimental program and measuring methods presented here. The flow of a flat jet 6 mm high in a square channel with a side of 56 mm is considered. The channel is positioned so that the plane of the jet is perpendicular to the magnetic field induction. The results of measuring velocity profiles and waveforms by swivel-type probe with potential sensor are presented. Effects that can be interpreted in different ways are found: strongly unstationary flow regimes, mean flow reorganization, and development of near-wall jets. Additional experiments are prepared to obtain more detailed information about the restructuring and development of the jet. In particular, continuous measurements along the channel will be made in the presence of a slight main flow.
https://doi.org/10.1088/1742-6596/1689/1/012002
Model-based condenser fan speed optimization of vapor compression systems. - In: Energies, ISSN 1996-1073, Bd. 13 (2020), 22, 6012, S. 1-26
Vapor compression systems (VCS) cover a wide range of applications and consume large amounts of energy. In this context, previous research identified the optimization of the condenser fans speed as a promising measure to improve the energy efficiency of VCS. The present paper introduces a steady-state modeling approach of an air-cooled VCS to predict the ideal condenser fan speed. The model consists of a hybrid characterization of the main components of a VCS and the optimization problem is formulated as minimizing the total energy consumption by respectively adjusting the condenser fan and compressor speed. In contrast to optimization strategies found in the literature, the proposed model does not relay on algorithms, but provides a single optimization term to predict the ideal fan speed. A detailed experimental validation demonstrates the feasibility of the model approach and further suggests that the ideal condenser fan speed can be calculated with sufficient precision, assuming constant evaporating pressure, compressor efficiency, subcooling, and superheating, respectively. In addition, a control strategy based on the developed model is presented, which is able to drive the VCS to its optimal operation. Therefore, the study provides a crucial input for set-point optimization and steady-state modeling of air-cooled vapor compression systems.
https://doi.org/10.3390/en13226012
Liquid metal swirling flow affected by transverse magnetic field. - In: Magnetohydrodynamics, Bd. 56 (2020), 2/3, S. 121-129
In this work we study numerically liquid metal flow in a squareduct under the influence ofa transverse magnetic field applied in a spanwise direction (coplanar). The key interest of thepresent study is an attempt of passive control of flow regimesdeveloped under magnetic fieldand thermal loads by applying specially shaped conditions,such as swirling, at the duct inlet.In this paper, we report results of numerical simulations ofthe interaction of swirling flow andtransverse magnetic field in a square duct flow. Analysis of the obtained regimes might beimportant for the development of an experimental setup, in order to design corresponding inletsections.
https://doi.org/10.22364/mhd.56.2-3.3