@phdthesis{Zaussinger2021, author = {Zaussinger, Florian}, title = {Modeling and simulation of thermo-electro hydrodynamics}, doi = {10.26127/BTUOpen-5517}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-55178}, school = {BTU Cottbus - Senftenberg}, year = {2021}, abstract = {The GeoFlow (Geophysical Flow) experiment on the International Space Station (ISS) and the AtmoFlow (Atmospherical Flow) experiment are designed to study convective processes under microgravity conditions in the spherical gap geometry. By applying a high voltage field between two concentric spherical shells and utilizing a dielectric working fluid it is possible to maintain an artificial radial force field that is comparable to a planetary gravitational field. This makes it possible to study convection such as known from the Earth's outer core, the Earth's mantle, or planetary atmospheres. The radial force field is based on the dielectrophoretic effect and is described by thermo-electro hydrodynamics (TEHD). This habilitation thesis presents a comprehensive view on modeling TEHD and the numerical simulation of the governing equations with a focus on GeoFlow and AtmoFlow. The GeoFlow experiment investigated thermal convection with and without dielectric (internal) heating under long-time micro-gravity conditions on the ISS. This unique experimental setup consisted of a bottom heated and top cooled spherical gap, filled with the silicon oil M5 or 1-Nonanol. Rotation, varying voltage, and temperature differences across the gap could be applied, to spread the experimental parameter space. The main focus of GeoFlow was the investigation of flow properties such as the convective onset, the transition from laminar to turbulent flows, and the influence of rotation on convection. Experimental outcomes were compared with theoretical and numerical results via advanced post-processing techniques. This includes pattern recognition algorithms and statistical evaluation of the numerical simulations. The TEHD model was validated on the onset of convection through linear stability analysis, on properties of columnar cells and global convective structures such as regular laminar flows. It is shown that TEHD based convection is comparable with Rayleigh-Benard convection and that is can be described by the quasi-normal approximation. For rotating cases and low super-criticalities the Proudman-Taylor theorem dominated the fluid flow which resulted in global columnar cells. In summary, the presented TEHD model is able to explain certain aspects of convective flows observed in GeoFlow. It is the first validation for such a model at all stages. The AtmoFlow experiment is based on GeoFlow but is designed to investigate global cells and planetary waves which are known from planetary atmospheres. Its unique feature are the atmospheric-like boundary conditions. Understanding the interaction between atmospheric circulation and a planet's climate, be it Earth, Mars, Jupiter, or a distant exoplanet, contributes to various fields of research such as astrophysics, geophysics, fluid physics, and climatology. AtmoFlow is currently under construction and is planned for operation on the ISS in 2024.}, subject = {Hydrodynamics; Numerics; Micro-gravity; Str{\"o}mungsmechanik; Numerik; Mikrogravitation; Str{\"o}mungsmechanik; Mikrogravitation}, language = {en} } @phdthesis{Sharma2019, author = {Sharma, Sparsh}, title = {Stochastic modelling of leading-edge noise in time-domain using vortex particles}, doi = {10.26127/BTUOpen-5085}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-50858}, school = {BTU Cottbus - Senftenberg}, year = {2019}, abstract = {The conceptual designing of rotating machines such as fans, wind turbines, contra-rotating open rotors and helicopter blades require low-cost, easy-to-run tools which allow quick noise assessments and optimization analyses underlying this phenomenon. The state-of-the-art numerical and experimental methods are far more expensive to conduct an optimisation study, whereas inexpensive methods like the analytical ones can have significant errors in realistic geometries at high-frequency ranges, higher angles of attack. The response to large coherent disturbances and the statistical modeling of turbulence is required because turbulence, by far its nature, is stochastic. Determining the accurate unsteady response of airfoil is crucial for noise prediction. The primary goal of the project is to develop a new low-cost and easy-to-use numerical technique for aero-acoustic designs, focused primarily on airfoil-turbulence interaction. The development of the statistical method is divided into three sections; namely - 1) calculating the background flow, 2) modeling of statistically optimized inflow disturbance, 3) constructing a vortex database to predict the noise in multiple flow fields characterized by different values of turbulent intensities and length scales. In the framework of this work a new approach to model inflow turbulence, a significant noise-generating element, is suggested, which does not depend on heavy computations requiring supercomputers. Through this approach, the influence of turbulence parameters on the noise generated in turbomachinery can be quantified. The approach also considers the geometrical parameters of the airfoil in the noise prediction. The background flow is numerically simulated via solving the vorticity transport equations in the Lagrangian form (vortex methods). The acoustic influence of a finite number of vortices, characterized by all the possible combinations of size, circulation and injection position/time defined using the ranges of probability distribution functions, released from injection points upstream of the airfoil are precomputed and stored in a matrix. The method is computationally inexpensive compared to classical vortex methods since the effect due to particles are precomputed, stored in a/an matrix/array. The matrix can be called as a library while predicting the noise from a specific airfoil.}, subject = {Leading-edge noise; Aeroacoustics; Ffowcs-Williams Hawkings; Broadband noise; Airfoil-turbulence interaction; Tragfl{\"u}gelvorderkantenschalls; Breitbandger{\"a}usch; Tragfl{\"a}chenger{\"a}usch; Aeroakustik; Zeitbereich; Tragfl{\"u}gel; Vorderkante; Tragfl{\"u}gelumstr{\"o}mung; Str{\"o}mungsakustik}, language = {en} } @phdthesis{Motuz2014, author = {Motuz, Vasyl}, title = {Gleichm{\"a}ßiges Mikro-Ausblasen zur Beeinflussung einer turbulenten Grenzschicht}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-31242}, school = {BTU Cottbus - Senftenberg}, year = {2014}, abstract = {Das gleichm{\"a}ßige Ausblasen ist eine effektive Methode zur Beeinflussung einer turbulenten Grenzschicht, n{\"a}mlich zur Reduzierung des Reibungswiderstands einer umstr{\"o}mten Wandoberfl{\"a}che. In der vorliegenden Arbeit wurde turbulente Grenzschicht an einer ebenen Platte hinsichtlich ihrer Beeinflussbarkeit mittels gleichm{\"a}ßigem kontinuierlichen Mikro-Ausblasen in einer Str{\"o}mung ohne Druckgradient untersucht. Die durchgef{\"u}hrte Untersuchung umfasst das Str{\"o}mungsverhalten in einer viskosen Unterschicht {\"u}ber einer durchl{\"a}ssigen Wand ohne und mit Ausblasen, der Einfluss der Ausblasrate und der Temperatur der ausgeblasenen Luft auf die Grenzschichtparameter sowie eine Visualisierung und qualitative Analyse des Temperaturfeldes der Oberfl{\"a}che der durchl{\"a}ssigen Wand ohne und mit Ausblasen unter verschiedenen Bedingungen. Die Untersuchung wurde in einem geschlossenen Unterschall Windkanal f{\"u}r einen Reynoldszahlen-Bereich von 1x10³ < Reₓ < 1x10⁶ und f{\"u}r eine Ausblasrate von 0 < F < 0.0173 durchgef{\"u}hrt. Die Ermittlung der Profile der Str{\"o}mungsgeschwindigkeit im wandnahen Bereich sowie des Einflusses der Ausblasrate und der Temperatur der ausblasenden Luft auf die Parameter der Grenzschicht wurde mit Hilfe des noninvasiven Laser Doppler- und bei einzelnen Experimenten mit Hilfe des hochaufl{\"o}senden Konstant Temperatur Anemometrie-Verfahrens durchgef{\"u}hrt. Zur Untersuchung des Temperaturfeldes sowie dessen {\"A}nderung wurde ein Thermografiesystem auf Basis einer Infrarot-Kamera eingesetzt. Die ermittelten Profile der Str{\"o}mungsgeschwindigkeit u(x,z) und besonders v(x,z) haben gezeigt, dass das Ausblasen durch eine durchl{\"a}ssige Wand, die gleichm{\"a}ßig verteilte Mikro-{\"O}ffnungen enth{\"a}lt, als ein „Ensemble" einzelner Freistrahlen zu betrachten ist. Im Fall einer Unterschallstr{\"o}mung darf diese Art des Ausblasens nur grob als ein gleichm{\"a}ßiges Ausblasen angenommen werden. Mit Hilfe des gleichm{\"a}ßigen Ausblasen wurde der Reibungsbeiwert im Bereich Reynoldszahlen Reₓ < 1x10⁶ im Vergleich zur festen Wand um bis zu 59 \% reduziert. Dabei wurde festgestellt, dass der Reibungsbeiwert bzw. die Wandschubspannung einer durchl{\"a}ssigen Wand mit Ausblasen von der Ausblasrate abh{\"a}ngt und deren Reduzierung nur bis zu einem bestimmten Wert der Ausblasrate stattfindet. Durch das Ausblasen der Luft verschiedener Temperatur wurde es gezeigt, dass auf die Eigenschaften der turbulenten Grenzschicht {\"u}ber einer durchl{\"a}ssigen Wand neben dem Ausblasen selbst, die Temperatur des ausblasenden Mediums auch eine wichtige Rolle spielt. Je nach dem, welche Temperatur das ausblasende Fluid besitzt, wird der Reibungsbeiwert reduziert oder erh{\"o}ht. Mittels einer Visualisierung des Temperaturfeldes der Oberfl{\"a}che der durchl{\"a}ssigen Wand mit Ausblasen wurde eine indirekte Visualisierung der Wirkung der {\"a}ußeren Str{\"o}mung auf die umstr{\"o}mte Wand realisiert. Da sich eine {\"a}ußere Str{\"o}mung in Wandn{\"a}he eine Grenzschicht bildet, kann diese Visualisierung als eine Visualisierung der Wirkung des Ausblasens auf die Grenzschicht betrachtet werden. Der Vergleich des Temperaturfelds der Oberfl{\"a}che der durchl{\"a}ssigen Wand ohne Ausblasen in einer {\"a}ußeren Str{\"o}mung mit derjenigen mit Ausblasen hat gezeigt, dass die Wirkung der {\"a}ußeren Str{\"o}mung auf die umstr{\"o}mte Wand durch das Ausblasen nicht direkt am Anfang des Ausblasbereichs, sondern in einem Abstand davon voll verhindert wird. Infolge Wechselwirkung zwischen {\"a}ußerer Str{\"o}mung und dem Ausblasen, bildet sich {\"u}ber dem Ausblasbereich eine Zwischenschicht bzw. eine D{\"a}mpferschicht, die einen Energieaustausch zwischen der {\"a}ußeren Str{\"o}mung und der Oberfl{\"a}che durchl{\"a}ssiger Wand blockiert. Diese Zwischenschicht bildet sich nicht sofort am Anfang des Ausblasbereichs, sondern {\"u}ber einer Strecke, deren L{\"a}nge von der Ausblasrate abh{\"a}ngt.}, subject = {Reduzierung des Reibungsbeiwerts; Viskose Unterschicht; Mikro-Ausblasen; Skin friction reduction; Viscous sub layer; Microblowing; Turbulent boundary layer; Turbulente Grenzschicht; Turbulente Grenzschicht; Ausblasen; Reibungskoeffizient}, language = {de} } @phdthesis{Butt2013, author = {Butt, Mohammad Usman}, title = {Experimental investigation of the flow over macroscopic hexagonal structured surfaces}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-30555}, school = {BTU Cottbus - Senftenberg}, year = {2013}, abstract = {The flow over macroscopic patterned/structured surfaces was investigated in a subsonic wind tunnel over Reynolds numbers ranging from 3.14 x 104 to 2.77 x 105 for cylinders and from 5.34 x 105 to 11.27 x 106 for plates. The investigations were accomplished by measuring local and global drag, velocity profiles and by visualization of the flow above the surface. The investigations on structured cylinders revealed that a cylinder with outwardly curved structures has a drag coefficient of about 0.65 times of a smooth one. Flow visualization was carried out by using oil-film technique and velocity profile measurements to elucidate the observed effect, and hence present the mechanism responsible for the observed drag reduction. The near-wall velocity profiles above the surface revealed that a hexagonal bump induces local separation generating large turbulence intensity along the separating shear layer. Due to this increased turbulence, the flow reattaches to the surface with a higher momentum and become able to withstand the pressure gradient delaying the main separation significantly. Besides that, the separation does not appear to occur in a straight line along the length of the cylinder, but follow the curved path forming a wave with its crest at 115° and trough at 110°, in contrast to the laminar separation line at 85° on a smooth cylinder. Investigations on structured plates were performed with the help of hot wire anemometry and oil film interferometry. The main concern of the experiments on structured plates was to examine the effect of hexagonal structures on local and global drag of a structured plate. It was accomplished by determining and analyzing the boundary layer quantities like shear stress velocities, shear stress coefficients and momentum thicknesses over a selected Reynolds number range and various locations in streamwise direction. The results indicate that the values of shear stress coefficients measured by the conventional Clauser chart method are up to 13\% higher than the ones deduced by the Oil film Interferometry. Additionally, a maximum of 19\% reduction in shear stress coefficient behind the inwardly curved structured plate was observed. On the other hand, a dramatic increase of about 120\% in global drag coefficient supersedes the observed reduction in shear stresses at rear of the test plates. Investigations on individual hexagonal structures by measuring the shear stresses and the pressure distribution above the surface revealed that an uneven pressure distribution contributing in total drag force is responsible for a huge increase in global skin drag coefficient. Finally, a number of configurations of a wind turbine made of smooth and structured blades were investigated to compare their efficiencies at various flow velocities. No significant deviation in the efficiencies was observed.}, subject = {Aerodynamics; Oil film interferometry; Drag reduction; Hot wire anemometry; Aerodynamik; {\"O}l-Film Interferometrie; Widerstandsreduzierung; Hitzdraht Anemometrie; Hitzdrahtanemometer; Turbulente Grenzschicht; Str{\"o}mungsfeld; Str{\"o}mungsmesstechnik}, language = {en} } @phdthesis{Neben2019, author = {Neben, Matthias}, title = {3D-CFD der Gas-Partikel-Str{\"o}mung in einer Laval-D{\"u}se zur Vorhersage mechanischer Erosion}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-47976}, school = {BTU Cottbus - Senftenberg}, year = {2019}, abstract = {Die vorliegende Doktorarbeit betrachtet den erosiven Verschleiß von Laval-D{\"u}sen f{\"u}r den pneumatischen Stofftransport. Dazu wurden 3D-CFD-Simulationen der Gas-Partikel-Str{\"o}mung auf Basis des Softwarepaketes OpenFOAM durchgef{\"u}hrt und ein kompressibler 2-Wege-gekoppelter sowie 4-Wege-gekoppelter Str{\"o}mungsl{\"o}ser entwickelt. Zus{\"a}tzlich wurden die Partikelmethoden von OpenFOAM erheblich erweitert. So sind z.B. zus{\"a}tzliche stochastische Methoden f{\"u}r die Partikel-Partikel und Partikel-Wand-Kollisionen (Sommerfeld \& Huber, 1999) mit einer erweiterten translatorischen und rotatorischen Impulserhaltung implementiert worden. Die auf die disperse Phase wirkenden fluidmechanischen Kr{\"a}fte sind die Widerstandskraft nach Henderson (1976), die Kraft durch Druck- und Spannungstensor sowie die Auftriebskraft. Der erosive Verschleiß wird wahlweise mit dem Tabakoff-Modell (Grant \& Tabakoff, 1973) und dem Oka-Modell (Oka et al., 2005; Oka \& Yoshida, 2005) berechnet. Diese Arbeit entstand in Zusammenarbeit mit dem HKW Cottbus, welches im Rauchgasreinigungssystem Laval-D{\"u}sen f{\"u}r den pneumatischen Abtransport von Asche verwendet. Aufgrund der hohen Gas- und Partikel-Geschwindigkeiten und der Kontur der D{\"u}sen sind diese massiv durch Erosion gesch{\"a}digt worden. Dadurch haben diese ihre Funktion als Blende verloren, was eine Sch{\"a}digung weiterer Komponenten des Rauchgasreinigungssystems zur Folge hatte. Zur Validierung des numerischen Str{\"o}mungsl{\"o}sers wurde das Experiment von Kumar et al. (1983) ausgew{\"a}hlt und ein qualitativ vergleichbares Ergebnis generiert (60\% des experimentellen Werts). Das charakteristische Verschleißbild des Experiments stellt sich nur unter Verwendung eines stochastischen Partikel-Wand-Kollisionsmodells mit hoher Wandrauigkeit ein. Im Fall glatter W{\"a}nde ist der erosive Verschleiß insgesamt geringer und es verschiebt sich das Maximum der lokalen Erosionsrate. Aufbauend auf diesen Erkenntnissen ist sowohl die Kontur der Laval-D{\"u}se des HKW Cottbus modifiziert worden als auch das Material der D{\"u}senwandung von 13CrMo44-Stahl zu Siliziumcarbid-Keramik ge{\"a}ndert worden. Die numerischen Simulationen des 4-Wege-gekoppelten Str{\"o}mungsl{\"o}sers zeigen, dass sich das lokale Maximum der Verschleißrate im kritischen Bereich der D{\"u}senkehle auf 1.5 \% des Ausgangswerts reduziert und zugleich von der D{\"u}senkehle weg verschiebt. In dieser Doktorarbeit ist im Gegensatz zu vergleichbaren numerischen Studien eine Validierung des Str{\"o}mungsl{\"o}sers an experimentellen Daten durchgef{\"u}hrt worden. Zudem ist die Bedeutung der Partikel-Partikel-Kollisionen und der Wandrauigkeiten f{\"u}r die Berechnung des erosiven Verschleißes bei Laval-D{\"u}sen aufgezeigt worden.}, subject = {Laval-D{\"u}se; Gas Partikel Str{\"o}mung; Mechanische Erosion; CFD; Laval nozzle; Gas particle flow; Mechanical erosion; Laval-D{\"u}se; Fluidpartikelstr{\"o}mung; Numerische Str{\"o}mungssimulation; Erosion}, language = {de} } @phdthesis{Schoen2025, author = {Sch{\"o}n, Franz-Theo}, title = {Transport and waves in parametrically excited fluid layers}, doi = {10.26127/BTUOpen-6995}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-69959}, school = {BTU Cottbus - Senftenberg}, year = {2025}, abstract = {The transport and waves in parametrically excited fluid layers play a significant role in an understanding of non-linear surface wave phenomena and tidal resonances. In this thesis, we study resonant waves occurring in a circular channel with various obstacles under external oscillatory excitation. Typically, such sloshing experiments are conducted in rectangular, straight channels. The external excitation is implemented using a rotating table on which the entire experiment, including measurement equipment, is mounted. The excitation is either sinusoidal or ratched motion. The obstacles include a fully blocking barrier and a symmetric or asymmetric hill. The channel circumference is 4.76 m, with water depths ranging from 2 cm to 6 cm. Wave displacements within the channel are measured using 17 ultrasonic sensors equidistantly distributed along half of the channel. Particle Image Velocimetry (PIV) is employed to measure the flow. We also consider a simplified numerical model capable of reproducing the experimental results. This model is based on a long-wave approximation and vertical integration using a profile function (K{\´a}rm{\´a}n-Pohlhausen approach). Additionally, we use a wave attractor model to quantitatively explain the development of resonances. These resonances are distributed in bands of the excitation frequency around the linear eigenfrequency. The experimental wave attractor and numerical results are consistent with each other. The waves observed within these resonant frequency bands appear as undular bores or solitary waves. In the fully blocking case, bands of constructive and destructive interference are observed, while in the presence of hills, all eigenfrequencies exhibit resonances of varying intensity. These non-linear wave phenomena are characterized by strong transport properties, which can be studied here due to the fact that the circular channel is not fully blocked. The ratched excitation generated asymmetric wave fields, which also induced asymmetric transport in the channel, leading to the emergence of a mean flow in the channel. A similar mean channel flow is observed for the asymmetric hill; however, wave-induced transport played a lesser role in this case. This is attributed to a large separation vortex on the steep side of the hill, which created a valve effect that rectified part of the oscillatory flow. These results are of interest not only for engineering applications but also for the understanding of tidal flows over seabed topography.}, subject = {Asymmetries; Asymmetrien; Bores; Sloshing; Transport; Waves; Bore; Schwappen; Transport; Wellen; Transport; Welle; Oberfl{\"a}chenwelle; Schwappende Fl{\"u}ssigkeit; Particle-Image-Velocimetry; Numerisches Modell}, language = {en} } @article{HasanuzzamanEivaziMerboldetal.2022, author = {Hasanuzzaman, Gazi and Eivazi, Hamidreza and Merbold, Sebastian and Egbers, Christoph and Vinuesa, Ricardo}, title = {Enhancement of PIV measurements via physics-informed neural networks}, doi = {10.1088/1361-6501/aca9eb}, year = {2022}, abstract = {Physics-informed neural networks (PINN) are machine-learning methods that have been proved to be very successful and effective for solving governing equations of fluid flow. In this work we develop a robust and efficient model within this framework and apply it to a series of two-dimensional three-component (2D3C) stereo particle-image velocimetry datasets, to reconstruct the mean velocity field and correct measurements errors in the data. Within this framework, the PINNsbased model solves the Reynolds-averaged-Navier-Stokes (RANS) equations for zeropressure-gradient turbulent boundary layer (ZPGTBL) without a prior assumption and only taking the data at the PIV domain boundaries. The TBL data has different flow conditions upstream of the measurement location due to the effect of an applied flow control via uniform blowing. The developed PINN model is very robust, adaptable and independent of the upstream flow conditions due to different rates of wall-normal blowing while predicting the mean velocity quantities simultaneously. Hence, this approach enables improving the mean-flow quantities by reducing errors in the PIV data. For comparison, a similar analysis has been applied to numerical data obtained from a spatially-developing ZPGTBL and an adverse-pressure-gradient (APG) TBL over a NACA4412 airfoil geometry. The PINNs-predicted results have less than 1\% error in the streamwise velocity and are in excellent agreement with the reference data. This shows that PINNs has potential applicability to shear-driven turbulent flows with different flow histories, which includes experiments and numerical simulations for predicting high-fidelity data.}, subject = {Machine learning; Particle image velocimetry; Turbulent boundary layer; Large Eddy Simulation; Measurement; Maschinelles Lernen; Turbulente Grenzschicht; Physikalisch informiertes neuronales Netz; Messung; Turbulente Grenzschicht; Neuronales Netz; Maschinelles Lernen; Particle-Image-Velocimetry}, language = {en} } @article{HasanuzzamanBuchwaldSchunketal.2024, author = {Hasanuzzaman, Gazi and Buchwald, Tom and Schunk, Christoph and Schr{\"o}der, Andreas and Egbers, Christoph and Hampel, Uwe}, title = {DATIV - Remote enhancement of smart aerosol measurement system using Raspberry Pi based distributed sensors}, series = {Sensors}, volume = {24}, journal = {Sensors}, number = {13}, publisher = {MDPI}, address = {Basel}, issn = {1424-8220}, doi = {10.3390/s24134314}, year = {2024}, abstract = {Enclosed public spaces are the hotspots for airborne disease transmission. To measure and maintain indoor air quality in terms of airborne transmission, an open source, low cost and distributed array of Particulate Matter Sensors has been developed and named as Dynamic Aerosol Transport for Indoor Ventilation or DATIV system. This system can use multiple Particulate Matter Sensors (PMS) simultaneously and can be remotely controlled using a Raspberry Pie based operating system. The data acquisition system can be easily operated using the GUI within any common browser installed on a remote device such as a PC or Smartphone with corresponding IP address. The software architecture and validation measurements are presented together with possible future developments.}, subject = {Particulate matter; Aerosol; Low cost measurement system; Distributed sensors; Indoor ventilation}, language = {en} } @phdthesis{MalteseMelettideOliveira2021, author = {Maltese Meletti de Oliveira, Gabriel}, title = {High-performance computing and laboratory experiments on strato-rotational instabilities}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-54408}, school = {BTU Cottbus - Senftenberg}, year = {2021}, abstract = {Stratified vortices can be found from small to large scales in geophysical and astrophysical flows. On the one hand, tornadoes and hurricanes can lead to devastation and even a large number of casualties. On the other hand, vortices can distribute heat and momentum in the atmosphere which is important for a habitable environment on Earth. In the astrophysical context, accretion disks (from which solar systems are formed) can be seen as stratified vortices. In such systems, understanding the mechanisms that can result in an outward transport of angular momentum is a central problem. For a planet or star to be formed in a disk, angular momentum has to be carried away from its center to allow matter aggregation by gravity; otherwise, its rotation speed would be far too large, avoiding this matter aggregation (and the consequent star formation) to happen. In such gas systems, turbulence is the most likely mechanism to achieve such a large angular momentum transport. However, it was shown that the flow profile of accretion disks is stable with respect to purely shear instabilities, and the question arises about how the turbulence can be generated. Among other candidates, the strato-rotational instability (SRI) has attracted attention in recent years. The SRI is a purely hydrodynamic instability that can be modeled by a classical Taylor-Couette (TC) system with stable density stratification due to axial salinity or temperature gradients. In this thesis, a combined experimental and high-performance computing study of new specific behaviors of the strato-Rotational Instability (SRI) is performed. The density stratification causes a change in the marginal instability transition when compared to classical non-stratified TC systems, making the flow unstable in regions where - without stratification - it would be stable. This characteristic makes the SRI a relevant phenomenon in planetary and astrophysical applications, particularly in accretion disk theory. Despite many advances in the understanding of strato-rotational flows, the confrontation of experimental data with non-linear numerical simulations remains relevant, since it involves linear aspects and non-linear interactions of SRI modes which still need to be better understood. These comparisons also reveal new non-linear phenomena and patterns not yet observed in the SRI, that can contribute to our understanding of geophysical flows.}, subject = {Rotating flow; Stratified flow; Pattern formation; Particle image velocimetry; High performance computing; Rotierende Str{\"o}mungen; Geschichtete Str{\"o}mungen; Musterbildung; Particle image velocimetry; Direkte numerische Simulation; Rotationsstr{\"o}mung; Turbulente Str{\"o}mung; Schichtenstr{\"o}mung; Direkte numerische Simulation; Particle-Image-Velocimetry; Musterbildung}, language = {en} } @phdthesis{Hamede2023, author = {Hamede, Mohammed Hussein Haytham}, title = {The turbulent very wide-gap Taylor-Couette flow : experimental investigation}, doi = {10.26127/BTUOpen-6445}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-64456}, school = {BTU Cottbus - Senftenberg}, year = {2023}, abstract = {Die Taylor-Couette-Str{\"o}mung (TC), die Str{\"o}mung zwischen zwei konzentrischen, unabh{\"a}ngig voneinander rotierenden Zylindern, wird als perfektes Modell zur Untersuchung von Scherstr{\"o}mungen {\"u}ber konkaven Oberfl{\"a}chen verwendet und ist eines der paradigmatischen Systeme der Fluidphysik. In dieser Arbeit wird eine experimentelle Untersuchung der turbulenten TC-Str{\"o}mung in einer sehr breiten Spaltgeometrie mit einem Radiusverh{\"a}ltnis 𝜂 = 0,1 durchgef{\"u}hrt. Das physikalische und dynamische Verhalten der Str{\"o}mung wird in einer Geometrie untersucht, die vor der vorliegenden Studie kaum untersucht wurde, was diese Studie einzigartig macht. Ziel der Studie ist es, die Auswirkungen der Kr{\"u}mmung auf die TC-Str{\"o}mung zu verstehen, insbesondere in F{\"a}llen, in denen die Umfangsl{\"a}nge des inneren Zylinders kleiner ist als die Spaltbreite. Die Str{\"o}mung wird in den verschiedenen Rotationsregimen untersucht: gegenl{\"a}ufige, mitl{\"a}ufige und reine Innenzylinder-Rotationsregime bis zu Scher-Reynoldszahlen Re_s≤ 150000. Das Str{\"o}mungsfeld wurde mit Hilfe von Visualisierungstechniken qualitativ untersucht. Bei der Untersuchung der verschiedenen Str{\"o}mungsparameter zeigen sich bekannte koh{\"a}rente TC-Str{\"o}mungsmuster sowie neu beobachtete Muster, von denen wir annehmen, dass sie nur bei TC-Str{\"o}mungen mit sehr großem Spalt existieren. F{\"u}r eine detailliertere quantitative Untersuchung wurde eine zeitaufgel{\"o}ste Messung des Geschwindigkeitsfeldes mit der Hochgeschwindigkeits-Partikel-Image-Velocimetry-Technik durch die Endplatte des Systems durchgef{\"u}hrt. Die radialen und azimutalen Geschwindigkeitskomponenten in der horizontalen 2D-Ebene werden an verschiedenen axialen Positionen gemessen, um die axiale Varianz der Str{\"o}mung zu erfassen. Das aufgezeichnete Str{\"o}mungsfeld wird verwendet, um den Drehimpulstransport in Form der Quasi-Nusselt-Zahl (Nu_ω) zu berechnen. Die Ergebnisse zeigen ein Maximum der Nu_ω f{\"u}r niedrige gegenl{\"a}ufige Raten von -0,011 ≤ μ_max ≤ -0,0077, was mit großr{\"a}umigen Strukturen verbunden ist, die den gesamten Spalt {\"u}berspannen. Dar{\"u}ber hinaus nimmt Nu_ω f{\"u}r Gegenrotationsraten, die h{\"o}her als μ_max sind, ab, bis es einen Minimalwert erreicht, und steigt dann f{\"u}r h{\"o}here Gegenrotationsf{\"a}lle tendenziell wieder an. Das Raum-Zeit-Verhalten des turbulenten Str{\"o}mungsfeldes f{\"u}r die F{\"a}lle mit hoher Gegenrotation zeigt die Existenz neu beobachteter Muster neben der {\"a}ußeren Zylinderwand, die sich nach innen ausbreiten, den Drehimpulstransport verst{\"a}rken und zu einem zweiten Maximum im Transport f{\"u}r h{\"o}here Gegenrotationsraten f{\"u}hren. Das Raum-Zeit-Verhalten des turbulenten Str{\"o}mungsfeldes f{\"u}r die F{\"a}lle mit hoher Gegenrotation zeigt die Existenz von neu beobachteten Mustern neben der {\"a}ußeren Zylinderwand, die sich nach innen ausbreiten, den Drehimpulstransport verst{\"a}rken und zu einem zweiten Maximum des Transports f{\"u}r h{\"o}here Gegenrotationsraten f{\"u}hren. F{\"u}r den rein rotierenden inneren Zylinder skaliert der Impulstransport.}, subject = {Experimental fluid mechanics; Taylor-Couette flow; Rotating flow; Turbulence; Particle image velocimetry; Experimentelle Str{\"o}mungsmechanik; Taylor-Couette-Str{\"o}mung; Rotationsstr{\"o}mung; Turbulenz; Turbulente Str{\"o}mung; Str{\"o}mungsmechanik; Rotationsstr{\"o}mung; Taylor-Couette-Str{\"o}mung}, language = {en} }