Wide field of view stereoscopic PIV measurements in a Rayleigh-Bénard cell. - In: Experimentelle Strömungsmechanik - 29. Fachtagung, 6.-8. September 2022, Ilmenau, (2022), 44
Entwicklung eines magnetohydrodynamischen Pumpsystems für die Mikrofluidik. - In: Experimentelle Strömungsmechanik - 29. Fachtagung, 6.-8. September 2022, Ilmenau, (2022), 38
Kombinierte Geschwindigkeits- und Temperaturmessungen mittels LED und einer Doppelbildkamera. - In: Experimentelle Strömungsmechanik - 29. Fachtagung, 6.-8. September 2022, Ilmenau, (2022), 3
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.
https://doi.org/10.1088/1361-6501/ac93a4
Experimentelle Strömungsmechanik - 29. Fachtagung, 6.-8. September 2022, Ilmenau. - Karlsruhe : GALA, 2022. - verschiedene Seitenzählungen ISBN 978-3-9816764-8-8
Literaturangaben
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.
https://doi.org/10.1088/1361-6501/ac82da
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.
https://doi.org/10.1038/s41378-022-00437-4
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.
https://doi.org/10.22032/dbt.53021
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.
https://doi.org/10.1039/D2LC00200K
Combined particle image velocimetry and thermometry of turbulent superstructures in thermal convection. - In: Journal of fluid mechanics, ISSN 1469-7645, Bd. 945 (2022), A22, 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.
https://doi.org/10.1017/jfm.2022.538