@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for the study of two scenarios, in which the nonlinear effects become significant and a traditional linear solution is no longer valid.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{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} } @misc{Hasanuzzaman2014, type = {Master Thesis}, author = {Hasanuzzaman, Gazi}, title = {Experimental investigation and CFD analysis of wind energy estimation considering building integrated ducts}, doi = {10.26127/BTUOpen-6192}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-61928}, school = {BTU Cottbus - Senftenberg}, year = {2014}, abstract = {Wind energy is a growing concern over the present awareness of lethal impact of green house gas emission. This energy source has been proven a promising alternative to fossil fuel based energy. Increased onshore wind capacity and decreased amount of low roughness wind sites has inspired the wind energy researchers to explore the possibilities of wind energy from high roughness sites such as urban area. Moreover, exhausted grid capacity between the wind energy producer from remote area and the consumer at city is also a major constrain for wind energy expansion. Driven by such motivation, this thesis has explored possibilities of wind energy conversion from buildings where energy is needed the most. Urban topography is known to be highly turbulent region considering its roughness characteristics. Wind energy yield from urban aerodynamics is a vast arena of experimental research. Within the time frame of the thesis period and available opportunities, a brief description about the wind energy assessment modelling approach from urban flow was outlined. There are several possibilities of wind energy yield from the built structure, but only building integrated duct was focused in this thesis. Time-averaged and global wind speed on the building integrated ducts, flow around the buildings was measured from wind tunnel and numerical analysis. Available wind energy yield and turbulence present in the locations measured from the flow was calculated based on the wind tunnel data and summarized with the pros and cons of the particular geometry. Elliptical duct configuration was found to achieve maximum energy yield from the omnidirectional free stream flow. However, simple rectangular duct configuration was determined as most efficient and optimized considering its simplicity, financial feasibility and relative energy yield with other duct configuration. The thesis also showed that on roof configuration is also very promising for wind energy exploration from the omnidirectional free stream flow. Necessary recommendations were made based on available result for future development of the research approach. Scope and opportunities was mentioned. This investigation has proved that it is possible to extract limited amount of wind energy from building augmented ducts using concentrator effect of the building exterior. Thus, the thesis concluded that the wind energy yield from building augmented ducts using the concentrator effect of the building exterior is a promising renewable energy source.}, subject = {Atmospheric boundary layer; Wind energy; Laser Doppler Anemometry; Numerical simulation; Computational Fluid Mechanics; Atmosph{\"a}rische Grenzschicht; Windenergie; LDA; Numerische Simulation; Windkanal; Windenergie; Windkanal; Atmosph{\"a}rische Grenzschicht; Laser-Doppler-Anemometrie; Numerische Str{\"o}mungssimulation}, language = {en} } @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{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} }