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Sachs, Sebastian; Baloochi, Mostafa; Cierpka, Christian; König, Jörg;
On the acoustically induced fluid flow in particle separation systems employing standing surface acoustic waves - Part I. - In: Lab on a chip, ISSN 1473-0189, Bd. 22 (2022), 10, S. 2011-2027

By integrating surface acoustic waves (SAW) into microfluidic devices, microparticle systems can be fractionated precisely in flexible and easily scalable Lab-on-a-Chip platforms. The widely adopted driving mechanism behind this principle is the acoustic radiation force, which depends on the size and acoustic properties of the suspended particles. Superimposed fluid motion caused by the acoustic streaming effect can further manipulate particle trajectories and might have a negative influence on the fractionation result. A characterization of the crucial parameters that affect the pattern and scaling of the acoustically induced flow is thus essential for the design of acoustofluidic separation systems. For the first time, the fluid flow induced by pseudo-standing acoustic wave fields with a wavelength much smaller than the width of the confined microchannel is experimentally revealed in detail, using quantitative three-dimensional measurements of all three velocity components (3D3C). In Part I of this study, we focus on the fluid flow close to the center of the surface acoustic wave field, while in Part II the outer regions with strong acoustic gradients are investigated. By systematic variations of the SAW-wavelength λSAW and channel height H, a transition from vortex pairs extending over the entire channel width W to periodic flows resembling the pseudo-standing wave field is revealed. An adaptation of the electrical power, however, only affects the velocity scaling. Based on the experimental data, a validated numerical model was developed in which critical material parameters and boundary conditions were systematically adjusted. Considering a Navier slip length at the substrate-fluid interface, the simulations provide a strong agreement with the measured velocity data over a large frequency range and enable an energetic consideration of the first and second-order fields. Based on the results of this study, critical parameters were identified for the particle size as well as for channel height and width. Progress for the research on SAW-based separation systems is obtained not only by these findings but also by providing all experimental velocity data to allow for further developments on other sites.



https://doi.org/10.1039/D1LC01113H
Sachs, Sebastian; Cierpka, Christian; König, Jörg;
On the acoustically induced fluid flow in particle separation systems employing standing surface acoustic waves - Part II. - In: Lab on a chip, ISSN 1473-0189, Bd. 22 (2022), 10, S. 2028-2040

Particle separation using surface acoustic waves (SAWs) has been a focus of ongoing research for several years, leading to promising technologies based on Lab-on-a-Chip devices. In many of them, scattering effects of acoustic waves on suspended particles are utilized to manipulate their motion by means of the acoustic radiation force (FARF). Due to viscous damping of radiated waves within a fluid, known as the acoustic streaming effect, a superimposed fluid flow is generated, which additionally affects the trajectories of the particles by drag forces. To evaluate the influence of this acoustically induced flow on the fractionation of suspended particles, the present study gives a deep insight into the pattern and scaling of the resulting vortex structures by quantitative three-dimensional, three component (3D3C) velocity measurements. Following the analysis of translationally invariant structures at the center of a pseudo-standing surface acoustic wave (sSAW) in Part I, the focus in Part II turns to the outer regions of acoustic actuation. The impact of key parameters on the formation of the outer vortices, such as the wavelength of the SAW λSAW, the channel height H and electrical power Pel, is investigated with respect to the design of corresponding separation systems. As a result of large gradients in the acoustic fields, broadly extended vortices are formed, which can cause a lateral displacement of particles and are thus essential for a holistic analysis of the flow phenomena. The interaction with an externally imposed main flow reveals local recirculation regions, while the extent of the vortices is quantified based on the displacement of the main flow.



https://doi.org/10.1039/D2LC00106C
Vieweg, Philipp P.; Kolesnikov, Yuri; Karcher, Christian;
Experimental study of a liquid metal film flow in a streamwise magnetic field. - In: Magnetohydrodynamics, Bd. 58 (2022), 1/2, S. 5-11

Continuous wetting of a surface with liquid metal is indispensable in many applications, such as in fusion reactors. In the present study, we provide data on the suppression of free-surface instabilities of liquid metal film flows under the action of strong streamwise magnetic fields in analogy to the poloidal fields used in application. We have designed and built up an experimental test setup which allows studying the influence of magnetohydrodynamics on the dynamic behaviour of liquid metal GaInSn film flows in laminar, transient, and turbulent regimes. While the width and the length of the film are adjusted at w = 23 mm and l = 120 mm, respectively, we are able to apply strong uniform magnetic fields up to B = 5 T over the entire fluid-flow volume. Moreover, the setup allows to vary the Reynolds number within the range 200 ≤ Re ≤ 1700. The corresponding Hartmann and Stuart numbers are Ha ≤ 180 and N ≤ 40, respectively. This study shows that a streamwise magnetic field is capable of suppressing free-surface instabilities even in the turbulent regime of the film flow by dampening any motion perpendicular to the applied magnetic field. Plans for future studies include the quantitative investigation of the parameter space.



http://mhd.sal.lv/contents/2022/1/MG.58.1.1.R.html
Moller, Sebastian;
Experimental characterization of turbulent superstructures in large aspect ratio Rayleigh-Bénard convection. - Ilmenau : Universitätsbibliothek, 2021. - 1 Online-Ressource (xii, 187 Seiten)
Technische Universität Ilmenau, Dissertation 2021

Die Untersuchung von thermisch induzierten Strömungen hat in den letzten Jahrzehnten eine enorme Aufmerksamkeit erfahren, um geophysikalische und astrophysikalische Systeme besser verstehen zu können. Hierfür hat sich das sogenannte Rayleigh-Bénard Modell als eines der meist untersuchten fluidmechanischen Systeme etabliert, da es die kaum abzubildende Komplexität von natürlichen Systemen in ihrer Mannigfaltigkeit auf ein Fluidvolumen reduziert, welches von unten isotherm erwärmt und von oben isotherm gekühlt wird. Trotz dieser Reduzierung an Komplexität können mit diesem Modell die wesentlichen Eigenschaften von thermischer Konvektion abgebildet werden. Die Strömung in einem solchen System, welche als Rayleigh-Bénard Konvektion bekannt ist, weist Strömungsstrukturen auf unterschiedlichsten Längenskalen auf. In der vorliegenden Arbeit werden die sogenannten Superstrukturen untersucht. Diese sich in horizontaler Richtung weit erstreckenden Strukturen treten in Erscheinung, wenn die horizontale Dimension der Fluidschicht wesentlich größer als der vertikale Abstand zwischen der erwärmten Unterseite und der gekühlten Oberseite ist. Da die Superstrukturen bisher im Wesentlichen anhand von numerischen Simulationen untersucht wurden, soll in dieser Arbeit erstmals vom experimentellen Standpunkt ein besserer Eindruck gewonnen werden. Zur Untersuchung der Superstrukturen wird eine Rayleigh-Bénard Zelle mit den Abmessungen l × w × h = 700 mm × 700 mm × 28 mm und folglich mit einem Aspektverhältnis von [Gamma] = l/h =25 aufgebaut. Bei allen Experimenten wird diese Zelle mit Wasser als Arbeitsmedium befüllt. Um die Rayleigh-Bénard Strömung zu untersuchen, werden thermochrome Flüssigkristalle als Impfpartikel der Strömung beigefügt, sodass simultane Messungen des Temperatur- und Geschwindigkeitsfeldes in horizontalen Ebenen der Zelle vorgenommen werden können. Während das Geschwindigkeitsfeld mittels der Bewegung der thermochromen Flüssigkristalle im zeitlichen Verlauf anhand der etablierten Partikelbild-Geschwindigkeitsmessung (Particle Image Velocimetry) bestimmt wird, basiert die Messung des Temperaturfelds auf der farblichen Erscheinung der thermochromen Flüssigkristalle, welche unter der Beleuchtung von Weißlicht temperaturabhängig ist. Im Hinblick auf die genaue Bestimmung der Temperatur wird diese Messtechnik umfänglich charakterisiert, wobei die wesentlichen Einflussfaktoren auf die Messunsicherheit diskutiert werden. Da die Untersuchung der turbulenten Superstrukturen mittels dieser Messtechnik den optischen Zugang zur flachen Rayleigh-Bénard Zelle erfordert, ist der Aufbau speziell konstruiert und ermöglicht die Beobachtung der Strömung durch eine transparente Kühlplatte. Der Entwicklungsprozess wird in der Arbeit aus ingenieurstechnischer Sicht genauestens erklärt. Bei der Auswertung der Messungen kommen die großskaligen Strukturen sowohl im Temperaturfeld als auch im Geschwindigkeitsfeld zum Vorschein. Die Größe der Superstrukturen wird untersucht in Abhängigkeit der Rayleigh-Zahl Ra, welche den thermischen Antrieb der Strömung beschreibt und in der vorliegenden Arbeit etwa im Bereich 2 × 10^5 ≤ Ra ≤ 2 × 10^6 variiert wird. Auf der Basis dieser Messungen, welche jeweils einen großen Zeitraum abdecken, wird das Langzeitverhalten der Superstrukturen analysiert, womit deren langsam voranschreitende Umstrukturierung gezeigt wird. Da die kombinierte Messung des Temperatur- und Geschwindigkeitsfeldes in den horizontalen Messebenen die Berechnung des lokalen Wärmestroms ermöglicht, wird diese Möglichkeit ebenfalls demonstriert. Um die experimentellen Ergebnisse dieser Arbeit bewerten zu können, werden jene mit den Resultaten aus numerischen Simulationen verglichen.



https://doi.org/10.22032/dbt.51488
Deng, Zichao; Kondalkar, V.; Weser, Robert; Schmidt, Hagen; Cierpka, Christian; König, Jörg;
Experimentelle Untersuchung eines SAW-Systems zur Parallelanalyse einzelner Zellen. - In: Experimentelle Strömungsmechanik - 28. Fachtagung, 7.-9. September 2021, Bremen, (2021), 35

Ghazijahani, Mohammad Sharifi; Heyder, Florian; Schumacher, Jörg; Cierpka, Christian;
The von Kármán Vortex Street, an archetype for Machine Learning in turbulence. - In: Experimentelle Strömungsmechanik - 28. Fachtagung, 7.-9. September 2021, Bremen, (2021), 29

Herzberg, Martin; Resagk, Christian; Cierpka, Christian;
Experimentelle Untersuchung von turbulenten Strömungsstrukturen in Mischkonvektion. - In: Experimentelle Strömungsmechanik - 28. Fachtagung, 7.-9. September 2021, Bremen, (2021), 18

Sachs, Sebastian; Cierpka, Christian; König, Jörg;
3D3C Strömungsmessungen zum Einfluss akustisch induzierter Wirbelstrukturen auf die Partikelfraktionierung in Mikrokanälen. - In: Experimentelle Strömungsmechanik - 28. Fachtagung, 7.-9. September 2021, Bremen, (2021), 2

Hübner, Maria; Miettinen, Mikael; Vainio, Valtteri; Cierpka, Christian; Theska, René; Viitala, Raine;
Behaviour of porous aerostatic bearings with various restrictor permeabilities. - In: Proceedings of the 21st International Conference of the European Society for Precision Engineering and Nanotechnology, (2021), S. 115-118

Aerostatic bearings are externally pressurized gas lubricated bearings. Aerostatic bearings are used in high speed and precision motion applications due to low friction and high accuracy. They use a restrictor to limit the flow of the gas into the bearing gap. The presence of the restrictor increases the stability of the bearing against self-excited vibrations. This study focuses on porous graphite restrictors and the effect of permeability on the behaviour of the bearing. The bearings were studied both experimentally and with a simulation model. Flat bearing pads with 37 mm diameter and different restrictor bulk permeability were manufactured and tested. Experimental measurements were conducted on a test setup allowing loading of the bearing against a ground steel plate. The load was supplied with a series of weights. The air gap was measured with a linear length gauge, measuring the displacement of the air bearing. The pressure was controlled with a regulator and the flow rate into the bearing was measured. In order to build an accurate simulation model, the permeability of the used material was calculated from the measured short circuit flow through each 4.5 mm thick sample. The flow in the porous material and in the restrictive layer follows Darcy's law, the flow in the air gap is described by the Navier-Stokes-equation. The simulation model was validated with experimental results. Measurement and simulation results include the air gap height, load and flow rate at a supply pressure of 0.4 MPa. According to previous research and preliminary results the surface restrictor layer has increased the resistance of the bearing to self-excited airhammer vibration, leading to a higher load capacity.



Otto, Henning;
Influence of thermal stratification on vertical natural convection - experimental investigations on the example of thermal energy storage systems. - In: Physics of fluids, ISSN 1089-7666, Bd. 33 (2021), 8, 083614, insges. 12 S.

Stratified thermal energy storages (TESs) are a promising solution for the large-scale energy storage problem of surplus renewable energy. Recent studies have shown parasitic convection occurring in near-wall regions inside such storage tanks, decreasing the working fluid's thermal stratification and reducing their exergy efficiency. This paper presents an experimental investigation of vertical convective flows in thermally stratified environments to complement the theoretical studies in this field. Specifically, we consider natural convection within a stratified laminar flow driven not by active heating but by the temperature gradient along a vertical wall, as is the case in real TES systems. The insights gained into the fundamental physical mechanisms of stratified vertical convection can promote efficiency improvements in TES systems. Therefore, we combine multiple particle image velocimetry and temperature measurements at different heights and thus obtain high-resolution vector fields of the entire wall jet flow and vertical temperature profiles for a TES model experiment. We appropriately modify scaling arguments found in the literature to develop a theory specifically suited to the experimental setup. The experimental data agree well with the modified theory. The results show two laminar counter-directed jets next to the vertical sidewall. In regions with high temperature gradients, the wall jets slow down, and flow reversals occur next to them. Moreover, the wall jets are asymmetric due to temperature-dependent fluid properties in conjunction with the ambient fluid stratification. In the stratification's upper, hot part, the wall jet is thinner and faster than the bottom jet in the cold region.



https://doi.org/10.1063/5.0056232