@phdthesis{Koenig2015, author = {K{\"o}nig, Franziska}, title = {Investigation of high Reynolds number pipe flow - CoLaPipe experiments}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-35392}, school = {BTU Cottbus - Senftenberg}, year = {2015}, abstract = {Investigations of high Reynolds number pipe flow is up to now a great challenge due to the complex mechanisms which appear in pipe flow turbulence. Hence, suitable experimental facilities are necessary to resolve turbulent dynamics and therewith to provide the knowledge for the understanding of such a simple shear flow. For this reason the recent thesis deals with conceptual design and setup of a new high Reynolds number pipe test facility further on named CoLaPipe - Cottbus Large Pipe. It also comprises first investigations on pipe flow obtained from the new CoLaPipe, which can be classified into 1.)calibration measurements to put the facility into service and 2.)continuative measurements to provide experimental results helping to understand pipe flow. The first results within the CoLaPipe show that this new experimental facility is suitable to investigate turbulence at high Reynolds numbers, where this conclusion can be drawn from intensive investigations on the development length of the flow either for natural and artificial transition. From further experiments on the evaluation of the wall friction velocity using different estimation methods great difficulties and variations in the calculated values are obtained. These deviations are directly related to the scaling behavior of the mean and fluctuating velocity, which is also shown within this thesis and intensively discussed. Among the discussion of the setup of the new CoLaPipe and the first experimental results this thesis contains a broad literature review with the focus on high and very high Reynolds numbers. Nevertheless, pipe flow at low and moderate Reynolds numbers is described as well.}, subject = {Turbulence; Pipe flow; High Reynolds number; Wind tunnel; Fluid dynamics; Turbulenz; Rohrstr{\"o}mung; Hohe Reynoldszahlen; Windkanal; Str{\"o}mungsmechanik; Windkanal; Rohrstr{\"o}mung; Turbulente Str{\"o}mung; Reynolds-Zahl}, 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} } @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} } @phdthesis{Xu2021, author = {Xu, Wenchao}, title = {Experiments on nonlinear waves in homogeneous flows with free upper surface and time-dependent forcing}, doi = {10.26127/BTUOpen-5386}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-53860}, school = {BTU Cottbus - Senftenberg}, year = {2021}, abstract = {The linear-theory assumption is a fundamental approach for the study of waves in fluids. The governing equations are linearized by assuming the perturbations are small so that the consequences of nonlinear terms are negligible. Nevertheless, if a wave approaches a critical level, in which the wave amplitude grows so as to create an instability of the background flow, the assumption of linearity may not hold any longer. In this case, the nonlinear terms are required to be taken into consideration. In this thesis, two experimental setups have been proposed for the study of two scenarios, in which the nonlinear effects become significant and a traditional linear solution is no longer valid. The first experiment focuses on an inertially oscillating rotating fluid. In the thesis, we present experimental results from a system that is simpler than classical precession experiments but still shows very similar wave interactions and a collapse to turbulence. This system consists of a partly filled rotating annulus that rotates about its symmetry axis slightly tilted with respect to the gravity vector. In the experiments, we find a resonant collapse when the forcing frequency corresponds with a resonant frequency of the rotating tank. Two types of instability can be triggered: a parametric triadic instability, in which two free Kelvin modes arise and form a triad with the forced Kelvin mode, and a shear-type instability related to the nonlinearly excited geostrophic flow. The latter instability gives rise to a barotropic mode that interacts with the forced mode and generates secondary modes. We also observed dependency of the mode frequencies on the Ekman number, which can, at least partly, be explained by a Doppler shift due to the mean flow. Finally, we try to connect our data to a low-order dynamical system based on the weakly nonlinear theory that describes the main features of single triad interaction in precession experiments. The second experiment concerns the study of undular bores (or tidal bores), in which the nonlinearity plays an important role. An experiment has been performed in which undular bores are produced in an open circular channel. More specifically, two different cases have been investigated: a single bore case with a rigid boundary setup and a bore colliding case with a periodic lateral boundary setup. Bores are generated by abruptly releasing a barrier that separates fluids with different surface levels. Up to our knowledge, this is the first experimental study of undular bores in a circular channel. For a setup without barriers, this geometry accomplishes in a natural way the periodic lateral boundary conditions, which is very often used in numerical simulations. The experimental results have been compared with the nonlinear numeric simulations and achieved an excellent agreement.}, subject = {Nonlinear waves; Flow instability; Rotating geophysical flows; Inertial waves; Weak turbulence; Tidal bore; Nichtlinearit{\"a}t; Rotierende Str{\"o}mung; Instabilit{\"a}t; Tr{\"a}gheitswellen; Gezeitenwellen; Flutwelle; Tr{\"a}gheitswelle; Instabile Str{\"o}mung; Rotationsstr{\"o}mung}, 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} } @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{Pizzi2023, author = {Pizzi, Federico}, title = {Numerical studies of a fluid-filled precessing cylinder : a framework for the DRESDYN precession experiment}, doi = {10.26127/BTUOpen-6421}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-64218}, school = {BTU Cottbus - Senftenberg}, year = {2023}, abstract = {Precession driven flows are believed to play a relevant role in planetary dynamics, such as in atmospheric phenomena, and as a complementary energy source for homogeneous dynamo action, i.e. the self-generation of planetary magnetic fields. Precessional motion occurs when a body rotates around an axis, which itself is rotating around another axis. The main influence of this forcing mechanism is a gyroscopic effect on the fluid flow which gives rise to a wavy dynamics even in the laminar regime. If the forcing magnitude is strong enough the flow goes through a series of phenomena such as instabilities, resonant interactions between waves, and transition to turbulence whose occurrence depends on the container shape and the angle between the two axis. Although many phenomena have a satisfactory explanation, others still remain elusive and merit further investigations. The interest in moderate to large forcing is particularly motivated by the need of theoretical supports for the upcoming DRESDYN (DREsden Sodium facility for DYNnamo and thermohydraulic studies) precession experiment, whose main purpose is to test the capability of a precessing fluid system to achieve a dynamo effect. Here, the possibility to generate a magnetic field is connected to the emergence of three large scale structures in the bulk flow: a directly forced standing wave, poloidal vortices, and a geostrophic axisymmetric flow. In this thesis we use numerical simulations to study and understand the flow behavior in a fluid-filled precessing cylinder. We use two types of approaches: a global study to investigate large scale phenomena and the resulting magnetohydrodynamics behavior, and a local model to analyze and unveil the properties of turbulence forced by precession. The bulk flow behavior present different responses with respect to the sense of motion: while prograde precession shows a steep transition to turbulence when increasing the forcing magnitude with a marked breakdown of the directly forced mode, retrograde precession presents a much smoother change. A related distinction has been found also for the dynamo action, which is more likely to occur for perpendicular and retrograde precession. The precession driven turbulence is a complex scenario determined by the coexistence of geostrophic vortices (called also condensates), a typical feature of rotating turbulence prone to an inverse cascade of energy, and small scale 3D waves characterized by a direct energy cascade. We observe the interaction of these two structures as being governed by a clear hierarchy.}, subject = {Inertial waves; Precession-driven flows; Rotating turbulcence; Dynamo action; Pr{\"a}zessionsgetriebene Str{\"o}mungen; Inertialwellen; Dynamo-Aktion; Rotierende Turbulenzen; Turbulente Str{\"o}mung; Atmosph{\"a}rische Turbulenz; Inertialsystem; Dynamotheorie; Gyroskop}, 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{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{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} }