@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{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} } @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{Hasanuzzaman2021, author = {Hasanuzzaman, Gazi}, title = {Experimental investigation of turbulent boundary layer with uniform blowing at moderate and high Reynolds numbers}, doi = {10.26127/BTUOpen-5566}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-55660}, school = {BTU Cottbus - Senftenberg}, year = {2021}, abstract = {Experimental investigation in turbulent boundary layer flows represents one of the canonical geometries of wall bounded shear flows. Utmost relevance of such experiments, however, is applied in the engineering applications in aerospace and marine industries. In particular, continuous effort is being imparted to explore the underlying physics of the flow in order to develop models for numerical tools and to achieve flow control. Within the scope of this Ph. D. topic, application of active control method such as micro-blowing effect in the incompressible, zero pressure gradient turbulent boundary layer was investigated. Turbulent boundary layer flow is particularly interesting as well as challenging due to the presence of different interacting scales which are increasingly becoming significant as the flow inertial conditions keeps growing. Therefore, energy content of the coherent structures in outer layer becomes stronger and necessitates measurements in relatively large Reynolds number. Present control experiments in turbulent boundary layer can be split into two different work segments, where one is objected towards the data measurements in turbulent boundary layer over smooth surface with and without any external perturbation. Here, perturbation is applied in the form of wall normal blowing while keeping the magnitude of blowing very low compared to the free stream velocity. For the subsequent results reported here, magnitude of blowing ratio was varied between 0\%~6\%. In the first part of the present thesis e.g. 0.415×10e+3≤Reτ≤1.160×10e+3, measurements were performed at the Brandenburg University of Technology wind tunnel. Non-intrusive Laser Doppler Anemometry was applied to carry out a series of measurements on a zero pressure gradient flat plate turbulent boundary layer. Blowing ratio through the perforated surface was varied between 0.17\%~1.52\% of the free stream velocity. To a maximum of 50\% reduction in friction drag was achieved. For the measurements on the upper range of the stated Reynolds number, were conducted at the boundary layer wind tunnel. This boundary layer wind tunnel offers a spatially developed turbulent boundary layer over a flat plate within 2.2×10e+3≤Reτ≤5.5×10e+3 with an excellent spatial resolution. With the help of Stereo Particle Image Velocimetry technique, measurement of the velocity components were obtained covering entire boundary layer in streamwise wall normal plane. In addition, time resolved measurements were also obtained in spanwise and wall-normal plane in order to look into the morphology of turbulent structures immediately above the blowing area.}, subject = {Turbulent boundary layer; Drag reduction; Particle Image Velocimetry (PIV); Laser Doppler Anemometry (LDA); Friction drag; Turbulente Grenzschicht; Particle Image Velocimetry; Laser-Doppler-Anemometrie; Mikro-Ausblasen; Reibungswiderstand; Turbulente Grenzschicht; Particle-Image-Velocimetry; Reibungswiderstand; Laser-Doppler-Anemometrie; Ausblasen}, language = {en} } @phdthesis{Bacher2022, author = {Bacher, Christian}, title = {On the influence of electrohydrodynamically induced turbulence on momentum, heat and mass transport in electrostatic precipitators}, doi = {10.26127/BTUOpen-5994}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-59948}, school = {BTU Cottbus - Senftenberg}, year = {2022}, abstract = {Electrostatic precipitators (ESPs) belong to the most efficient devices for control of particle emissions in the process industry and environmental technologies in general. The operating principle of ESPs, relying on particle precipitation by electrical charging, leads to an enhanced turbulent motion by electrohydrodynamic (EHD) effects within the gaseous working fluid passing through the ESP. This enhanced turbulence is generally considered detrimental to the separation efficiency, because the additional shear forces acting on the collection electrodes promote re-entrainment of already precipitated particles. Further, the enhanced turbulent cross mixing diminishes the transport of charged particles by electric forces directed towards the collection electrodes. In other industrial applications however, many process units that are based on heat or mass transport would benefit from enhanced turbulent mixing. These include heat exchangers for energy recuperation or evaporation and absorption units such as flue gas scrubbers. A combination of these with an ESP possibly allows for versatile new applications in the process industry. The motivation of this thesis is to study the effects of electrohydrodynamically generated motions onto the turbulent cross mixing within gaseous flows over a wide parameter space including variations in geometry and operating conditions. In order to do so, the influence of EHD effects on the three major transport properties, i.e. thermal energy, mass and momentum, were experimentally investigated. Based on analogies caused by similar transport mechanisms, the experimental results were compared despite different methods being used. In order to help with the comparison a simplified eddy viscosity model is applied. The study for each transport property was conducted with a different setup of a tube type ESP. These setups include axial and radial pressure drop measurements to quantify the momentum transport, a self-built local heat transfer sensor to determine heat transfer coefficients and a wetted wall-column to study the enhancement of mass transport phenomena. Experiments on momentum and heat transport include a large variation of different geometries and operating variables such as discharge electrode design, tube diameter, flow velocity, as well as magnitude and polarity of operating voltage. In addition, selected measurements with aerosol particles were conducted, to gain additional insights on the effects of EHD induced turbulence generation by particle bound space charge compared to ion bound space charge in clean gas flows. The results do not only provide transport coefficients for the hands-on application by plant designers, but also feature numerous experimental data that can be used as reference values for ongoing research in the numerical simulation of electrohydrodynamically enhanced flow and therefore significantly increase the available amount of data provided by current literature.}, subject = {Electrohydrodynamics; Electrostatic precipitation; Heat and mass transfer intensification; Elektrohydrodynamik; Elektroabscheider; Verbesserung des W{\"a}rme- und Stofftransports; Elektrofilter; Elektrohydrodynamik; Numerische Str{\"o}mungssimulation}, language = {en} } @phdthesis{Kessler2024, author = {Keßler, Raphael}, title = {Entwicklung eines raumflugf{\"a}higen Moduls zur optischen Untersuchung weicher Materie unter Schwerelosigkeit}, doi = {10.26127/BTUOpen-6696}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-66960}, school = {BTU Cottbus - Senftenberg}, year = {2024}, abstract = {Mit dem Begriff „weiche Materie" wird eine Stoffklasse bezeichnet, die uns im Alltag st{\"a}ndig und nahezu {\"u}berall begegnet. Dennoch z{\"a}hlt die Erforschung solcher Materialsysteme zu einer noch recht jungen Disziplin moderner Materialforschung. Dem Ziel, ein besseres Verst{\"a}ndnis und Vorhersehbarkeit mechanischer Eigenschaften zu gewinnen, gehen Kenntnisse {\"u}ber die innere Dynamik voraus. Die Dynamik in solchen Systemen wird dabei von vergleichsweise geringen Kr{\"a}ften getrieben, die bei Untersuchungen auf der Erde von dominant erscheinenden schwerkraftgetriebenen Ph{\"a}nomenen wie beispielsweise der Sedimentation {\"u}berlagert werden k{\"o}nnen. In der vorliegenden Arbeit wird die Entwicklung einer Anlage beschrieben, die eine modulare Experimentplattform auf einer H{\"o}henforschungsrakete darstellt und Experimente zur Untersuchung weicher Materie unter nahezu g{\"a}nzlicher Ausschaltung der Schwerkraft erm{\"o}glicht. Ein druckdichter und wiederverwendbarer Rumpf stellt den Experimenten auch w{\"a}hrend des Fluges atmosph{\"a}rische Bedingungen zur Verf{\"u}gung, wobei durch eine Luke nicht langzeitstabile Proben kurz vor Start der Rakete integriert werden k{\"o}nnen. H{\"a}ufig genutzte Diagnostikelemente von Lichtstreuexperimenten und Servicesystemkomponenten zur Versorgung mit Energie und Informationen werden in eine Baugruppe geb{\"u}ndelt und {\"u}ber definierte Schnittstellen f{\"u}r Neuentwicklungen zug{\"a}nglich gemacht, sodass deren Entwicklungszeiten verk{\"u}rzt werden k{\"o}nnen. Exemplarisch werden zwei Lichtstreuexperimente entwickelt und in das Modul integriert: Ein kolloidaler Teil, der die Bewegung aktiver Mikroschwimmer untersucht, und einen granularen Teil, bei dem Licht an Teilchen und Gasblasen gestreut wird. Die phototaktisch getriebenen Mikroschwimmer zeigen ohne die auf der Erde unvermeidlichen Randschichteffekte unter Mikrogravitation eine messbar verschiedene Dynamik. Mit einer kritischen Bewertung werden Verbesserungspotentiale aufgelistet, die teils im Zweitflug umgesetzt werden. F{\"u}r zuk{\"u}nftige Anwendungen auf der Experimentplattform werden Ideen und Vorhaben im Kontext der M{\"o}glichkeiten aufgef{\"u}hrt, wobei der Nutzen und der Mehrwert eines wiederfliegenden Moduls an Hand von Ergebnissen aus den stattgefundenen Fl{\"u}gen manifestiert werden k{\"o}nnen.}, subject = {Mikrogravitation; H{\"o}henforschungsrakete; Weiche Materie; Microgravity; Sounding rockets; Soft matter; H{\"o}henforschungsrakete; Weiche Materie; Mikrogravitation}, language = {de} } @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{Shahirpour2024, author = {Shahirpour, Amir}, title = {A characteristic dynamic mode decomposition to detect transport-dominated large-scale coherent structures in turbulent wall-bounded flows}, doi = {10.26127/BTUOpen-6958}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-69583}, school = {BTU Cottbus - Senftenberg}, year = {2024}, abstract = {Large-scale energetic coherent structures are detected in turbulent pipe flow at shear Reynolds number of 181. They are distinguished by having long lifetimes, living on large scales and contributing prominently to the spectral peak in premultiplied spectra of streamwise velocity component. In order to investigate these structures in the absence of small scale perturbations, they are extracted from the underlying multi-scaled and complex turbulent flow. For this purpose, data-driven methods such as Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are suitable candidates as long as the structures are stationary in space and time. Nevertheless, the transport-dominated nature of structures in wall-bounded flows poses a major problem to application of such methods. Different instances of a structure travelling with a certain group velocity in space and time will be perceived as different modes. This results in poorly decaying singular values signifying that many modes will be required to describe a single structure. To remedy this issue a Characteristic DMD (CDMD) is developed showing that on a properly chosen frame of reference along the characteristics defined by the group velocity, a POD or DMD reduce the moving structures to a few modes. Reconstruction of the candidate modes in the spatio-temporal space and transforming them back to physical space gives the low rank model of the flow. The method is initially applied to the vortex head of a compressible starting jet as it offers a distinct coherent structure and therefore, can serve as a success measure of the method. The vortex head is described with a few modes only and it is shown that the dynamics associated with the modes are detected more accurately on a moving frame. In the next step the developed method is applied to the data from Direct Numerical Simulations (DNS) and a low dimensional subspace is extracted out of highly complex turbulent pipe flow. The essential features of the flow such as spectral energy and Reynolds stresses are captured in a subspace with only 3\% of the modes. The structures living in this subspace have long lifetimes, possess wide range of length-scales and travel at group velocities close to that of the moving frame of reference. Having a significantly lower degree of freedom, the detected low rank subspace offers a more clear basis for capturing large-scale persistent structures. Aiming at separating the scales, a second spatio-temporal decomposition is applied to the low rank subspace, normal to the direction of characteristics. A secondary subspace is formed comprising of the modes with large scales using only 10\% of the new modes. Investigating the spectral and turbulent properties of the mentioned subspace, shows that it accommodates the near-wall streaks. The captured streaks show a significant contribution to the spectrum of streamwise velocity component and very low contributions to the spectra ofradial and azimuthal components. The developed CDMD proves to be an effective tool to detect and identify the large-scale coherent structures in wall-bounded turbulent flows.}, subject = {Coherent structures; Dynamische Modenzerlegung; Turbulent wall-bounded flows; Dynamic Mode Decomposition; Koh{\"a}rente Stukturen; Turbulente wandbegrenzte Str{\"o}mungen; Turbulente Str{\"o}mung; Rohrstr{\"o}mung; POD-Methode; Direkte numerische Simulation}, language = {en} }