On the benefits and limitations of Echo State Networks for turbulent flow prediction. - In: Measurement science and technology, ISSN 1361-6501, Bd. 34 (2022), 1, 014002, S. 1-18
The prediction of turbulent flow by the application of machine learning (ML) algorithms to big data is a concept currently in its infancy which requires further development. It is of special importance if the aim is a prediction that is good in a statistical sense or if the vector fields should be predicted as good as possible. For this purpose, the statistical and deterministic prediction of the unsteady but periodic flow of the von Kármán Vortex Street (KVS) was examined using an Echo State Network (ESN) which is well suited for learning from time series due to its recurrent connections. The experimental data of the velocity field of the KVS were collected by Particle Image Velocimetry (PIV). Then, the data were reduced by Proper Orthogonal Decomposition (POD) and the flow was reconstructed by the first hundred most energetic modes. An ESN with 3000 neurons was optimized with respect to its three main hyperparameters to predict the time coefficients of the POD modes. For the deterministic prediction, the aim was to maximize the correct direction of the vertical velocities. The results indicate that the ESN can mimic the periodicity and the unsteadiness of the flow. It is also able to predict the sequence of the upward and downward directed velocities for longer time spans. For the statistical prediction, the similarity of the probability density functions of the vertical velocity fields between the predicted and actual flow was achieved. The leaking rate of the ESN played a key role in the transition from deterministic to statistical predictions.
Analysis of an unsteady quasi-capillary channel flow with time-resolved PIV and RBF-based super-resolution. - In: Journal of coatings technology and research, ISSN 1935-3804, (2022), insges. 14 S.
We investigate the interface dynamics in an unsteady quasi-capillary channel flow. The configuration consists of a liquid column that moves along a vertical 2D channel, open to the atmosphere and driven by a controlled pressure head. Both advancing and receding contact lines were analyzed to test the validity of classic models for dynamic wetting and to study the flow field near the interface. The operating conditions are characterized by a large acceleration, thus dominated by inertia. The shape of the moving meniscus was retrieved using Laser-Induced Fluorescence-based image processing, while the flow field near was analyzed via Time-Resolved Particle Image Velocimetry (TR-PIV). The TR-PIV measurements were enhanced in the post-processing, using a combination of Proper Orthogonal Decomposition and Radial Basis Functions to achieve super-resolution of the velocity field. Large counter-rotating vortices were observed, and their evolution was monitored in terms of the maximum intensity of the Q-field. The results show that classic contact angle models based on interface velocity cannot describe the evolution of the contact angle at a macroscopic scale. Moreover, the impact of the interface dynamics on the flow field is considerable and extends to several capillary lengths below the interface.
A combined velocity and temperature measurement with an LED and a low-speed camera. - In: Measurement science and technology, ISSN 1361-6501, Bd. 33 (2022), 11, 115301, S. 1-12
Microfluidic devices are governed by three-dimensional velocity and temperature fields, and their boundary conditions are often unknown. Therefore, a measurement technique is often desired to measure both fields in a volume. With astigmatism particle tracking velocimetry (APTV) combined with luminescence lifetime imaging, the temperature and all velocity components in a volume can be measured with one optical access. While the three-dimensional particle position is determined by evaluating the shape of the corresponding particle image, the temperature measurement relies on estimating the temperature-dependent luminescence lifetime derived from particle images on two subsequent image captures shortly after the photoexcitation. For this, typically a high-energetic pulsed laser is required to ensure a high signal-to-noise ratio. However, it can also cause additional heating of the fluid. We show that this problem is solved by replacing the pulsed laser with an LED. To compensate for the lower power provided by the LED, we adapted the timing schedule and vastly extended the illumination time and the exposure time for both image captures. In addition, we were able to replace the typically used high-speed camera with an ordinary double-frame camera. In this way, very low measurement uncertainties on all measured quantities can be achieved while keeping the temperature of the fluid unaffected. Random errors dominate within the two focal planes of APTV, yielding a standard deviation of the temperature of individual particles of about 1 only. The measurement error caused by the movement of tracer particles during the much longer illumination and exposure time were found to be acceptable when the measured velocity is low. With the circumvention of light-source induced heating and the lower cost of hardware devices, the adapted approach is a suitable measurement technique for microfluidic related research.
Highly efficient passive Tesla valves for microfluidic applications. - In: Microsystems & nanoengineering, ISSN 2055-7434, Bd. 8 (2022), 1, 97, S. 1-12
A multistage optimization method is developed yielding Tesla valves that are efficient even at low flow rates, characteristic, e.g., for almost all microfluidic systems, where passive valves have intrinsic advantages over active ones. We report on optimized structures that show a diodicity of up to 1.8 already at flow rates of 20 μl s^-1 corresponding to a Reynolds number of 36. Centerpiece of the design is a topological optimization based on the finite element method. It is set-up to yield easy-to-fabricate valve structures with a small footprint that can be directly used in microfluidic systems. Our numerical two-dimensional optimization takes into account the finite height of the channel approximately by means of a so-called shallow-channel approximation. Based on the three-dimensionally extruded optimized designs, various test structures were fabricated using standard, widely available microsystem manufacturing techniques. The manufacturing process is described in detail since it can be used for the production of similar cost-effective microfluidic systems. For the experimentally fabricated chips, the efficiency of the different valve designs, i.e., the diodicity defined as the ratio of the measured pressure drops in backward and forward flow directions, respectively, is measured and compared to theoretical predictions obtained from full 3D calculations of the Tesla valves. Good agreement is found. In addition to the direct measurement of the diodicities, the flow profiles in the fabricated test structures are determined using a two-dimensional microscopic particle image velocimetry (μPIV) method. Again, a reasonable good agreement of the measured flow profiles with simulated predictions is observed.
Entwicklung und Charakterisierung einer membranlosen mikrofluidischen Brennstoffzelle. - Ilmenau : Universitätsbibliothek, 2022. - 1 Online-Ressource (viii, 119 Blätter)
Technische Universität Ilmenau, Dissertation 2022
Membranlose mikrofluidische Brennstoffzellen (MFCs) stellen aufgrund der theoretisch höheren Energiedichte eine potenzielle Alternative zu konventionellen Batterien dar und sind für die Anwendung in tragbaren elektronischen Geräten von großem Interesse. MFCs werden mit flüssigem Brennstoff und Oxidant betrieben, die in zwei getrennte Eintrittsöffnungen, in einen mit Elektroden ausgestatteten Mikrokanal eingeleitet werden. Bedingt durch die laminare Strömung im Mikrokanal fließen die beiden Fluide parallel zum Kanal, ohne sich konvektiv zu durchmischen. Allerdings sind MFCs aufgrund von geringen Stromdichten bei gleichzeitig niedrigem Brennstoffumsatz noch nicht kommerziell im Einsatz. Ein wesentlicher Grund für die geringen Stromdichten ist die Entstehung von Verarmungsschichten an den Elektrodenoberflächen aufgrund des diffusionsbegrenzten Massentransports. Aus diesem Grund wird zur Entwicklung einer leistungsfähigeren MFC der Fokus in dieser Arbeit auf die Steigerung des konvektiven Massentransports im Mikrokanal in Richtung der Elektroden gelegt. Es wird eine MFC mit gekrümmten Mikrokanal entwickelt. Durch die Krümmung entstehen in der Kurve des Kanals zwei entgegengesetzte Wirbel, die sogenannten Dean-Wirbel, die einen konvektiven Massentransport der Reaktanden an die Elektrodenoberfläche hervorrufen. Neben der Entwicklung einer MFC, werden in dieser Arbeit numerische und experimentelle Untersuchungen zum Einfluss des durch die Krümmung hervorgerufenen konvektiven Massentransports auf die Leistung der entwickelten MFC durchgeführt. Die dreidimensionale Strömung in gekrümmten Mikrokanälen wird mit Hilfe von numerischen Simulationen charakterisiert und die Ergebnisse mittels Astigmatismus Particle Tracking Velocimetry-Messungen (APTV) erfolgreich validiert. Weiterhin wird durch Simulationen und elektrochemische Experimente unter Verwendung eines Modell-Redoxsystems die negative Wirkung der sich bildenden Verarmungsschicht auf die Stromdichte im geraden Abschnitt des Mikrokanals aufgezeigt. In der Kurve kann durch die Dean-Wirbel die Stromdichte dagegen gesteigert werden. Zudem gelingt es mittels Simulationen den Einfluss der Dean-Wirbel auf die Stromdichte und die Leistung der MFC nachzuweisen und zu analysieren. Schließlich wird anhand von APTV-Messungen im Einlassbereich der MFC eine der Hauptströmung überlagerten Strömung festgestellt. Um den daraus resultierenden Brennstoff-Crossover zu verhindern, wird ein neues System mit einer dünnen Lippe im Einlassbereich des Mikrokanals gefertigt, welches den parallelen Fluss der beiden Fluide gewährleistet. Dies schafft die ideale Voraussetzung für die Verwendung des optimierten Mikrokanals als MFC und ebnet den Weg für die weitere Erforschung der MFC mit gekrümmten Mikrokanal.
Three-dimensional heating and patterning dynamics of particles in microscale acoustic tweezers. - In: Lab on a chip, ISSN 1473-0189, Bd. 22 (2022), 15, S. 2886-2901
Acoustic tweezers facilitate a noninvasive, contactless, and label-free method for the precise manipulation of micro objects, including biological cells. Although cells are exposed to mechanical and thermal stress, acoustic tweezers are usually considered as biocompatible. Here, we present a holistic experimental approach to reveal the correlation between acoustic fields, acoustophoretic motion and heating effects of particles induced by an acoustic tweezer setup. The system is based on surface acoustic waves and was characterized by applying laser Doppler vibrometry, astigmatism particle tracking velocimetry and luminescence lifetime imaging. In situ measurements with high spatial and temporal resolution reveal a three-dimensional particle patterning coinciding with the experimentally assisted numerical result of the acoustic radiation force distribution. In addition, a considerable and rapid heating up to 55 ˚C depending on specific parameters was observed. Although these temperatures may be harmful to living cells, counter-measures can be found as the time scales of patterning and heating are shown to be different.
Combined particle image velocimetry and thermometry of turbulent superstructures in thermal convection. - In: Journal of fluid mechanics, ISSN 1469-7645, Bd. 945 (2022), S. A22-1-A22-25
Turbulent superstructures in horizontally extended three-dimensional Rayleigh-Bénard convection flows are investigated in controlled laboratory experiments in water at Prandtl number Pr = 7. A Rayleigh-Bénard cell with square cross-section, aspect ratio Γ = l/h = 25, side length l and height h is used. Three different Rayleigh numbers in the range 10^5 < Ra < 10^6 are considered. The cell is accessible optically, such that thermochromic liquid crystals can be seeded as tracer particles to monitor simultaneously temperature and velocity fields in a large section of the horizontal mid-plane for long time periods of up to 6 h, corresponding to approximately 10^4 convective free-fall time units. The joint application of stereoscopic particle image velocimetry and thermometry opens the possibility to assess the local convective heat flux fields in the bulk of the convection cell and thus to analyse the characteristic large-scale transport patterns in the flow. A direct comparison with existing direct numerical simulation data in the same parameter range of Pr, Ra and Γ reveals the same superstructure patterns and global turbulent heat transfer scaling Nu(Ra). Slight quantitative differences can be traced back to violations of the isothermal boundary condition at the extended water-cooled glass plate at the top. The characteristic scales of the patterns fall into the same size range, but are systematically larger. It is confirmed experimentally that the superstructure patterns are an important backbone of the heat transfer. The present experiments enable, furthermore, the study of the gradual evolution of the large-scale patterns in time, which is challenging in simulations of large-aspect-ratio turbulent convection.
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.
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.
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.