Arbeitsgruppe Strömungsmechanik
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Item Experimental and numerical investigations of compressible, transient, and subcritical vessel outflows(2024) Fischer, Michael-David; Ehrhard, Peter; Janoske, UweDas Ausströmen eines Gases aus einem unter Druck stehenden Behälter gehört zu den klassischen strömungsmechanischen Grundlagenproblemen. Gerade der kompressible und unterkritische Fall ist hierbei komplexer als sich zunächst vermuten lässt und es gibt keine allgemeine analytische Lösung. Dieser Fall soll hier näher untersucht werden. So wird zunächst eine allgemeine selbstähnliche Lösung hergeleitet sowie eine dimensionslose Kennzahl identifiziert, mit der die Ausströmung ohne aufwändige numerische Berechnungen charakterisiert werden kann. Dieses selbstähnliche Modell wird anhand von numerischen Simulationen und experimentellen Untersuchungen validiert. Weiter bildet sich bei der Ausströmung aus dem Kessel in eine freie Umgebung ohne begrenzende Wände ein Freistrahl aus, welcher ebenfalls näher untersucht wird. Hierzu wird das transiente Geschwindigkeitsfeld von Luft-, Kohlenstoffdioxid- und Helium-Freistrahlen nach der schlagartigen Öffnung eines Kessels experimentell erstmalig mittels Laser Doppler Anemometrie und Phasen Doppler Anemometrie vermessen. Hierbei steht neben der Fragestellung nach der Selbstähnlichkeit des Geschwindigkeitsfeldes der Einfluss der Reynolds-Zahl und der Dichte des ausströmenden Gases im Fokus. Weiterführend werden die experimentellen Ergebnisse der transienten Freistrahlen mit numerischen Simulationen verglichen. Hierzu werden sowohl das etablierte 𝑘𝑘-𝜀𝜀 Turbulenzmodell als auch das relativ neue verallgemeinerte 𝑘𝑘-𝜔𝜔 (GEKO) Turbulenzmodell verwendet. Hierbei sollen potentielle Fehlerquellen wie Auftriebseffekte und schwankende Umgebungsbedingungen, wie sie in den experimentellen Untersuchungen auftauchen können, gezielt ausgeschaltet werden. Weiter wird untersucht, ob das GEKO Modell möglicherweise Vorteile gegenüber dem 𝑘𝑘-𝜀𝜀 Turbulenzmodell bei der Berechnung von Freistrahlen aufweist.Item Experimental investigation of high-speed rotary bell atomization by laser-optical methods(2023) Gödeke, Lutz; Ehrhard, Peter; Agar, David W.The coating process in the modern automotive industry is usually implemented using high-speed rotary atomizers. The requirements for coating qualities are on the one hand of a technical nature, on the other hand they have to satisfy the subjective perception of the consumer. For the application, it is therefore of great interest to fathom and understand the physical dependencies between the applied coating layer, the selectable process parameters and material properties by means of fundamental research. This enables targeted product development that meets financial, environmental and quality requirements while economizing on material during application. Within the scope of the present work, the basic theoretical background for the three topics of coating properties, measurement technology, as well as data preparation and processing will be evaluated in detail. The focus lies on highlighting necessary adjustments in the methodology and particularities during measurement, application and data processing. For this purpose, the experimental procedure including the derived methods is presented in detail. The results are then displayed and discussed with the help of mathematical modeling approaches with respect to their special characteristics. Finally, a comprehensive description of the relevant error sources and their individual influences is given.Item Experimental investigations on rising bubbles in stagnant water in vertical capillaries(2021-12-14) Grünendahl, Sabrina; Danila, Kevin; Brandner, Dean Martin; Ehrhard, PeterWe investigate how the walls of cylindrical capillaries affect the velocity of rising gas bubbles of various diameters. Of course, as the capillary diameter increases, the velocity of the rising bubble will approach the case of free rising. Such systematic experiments on the bubble rise in capillaries, in which the ratio of bubble diameter and capillary diameter is varied from one towards smaller values, can hardly be found in literature. Experiments within the system water/air have been conducted in stagnant water and will be discussed in this paper.Item Experimental investigations on rising bubbles in vertical capillaries(2021-01-25) Grünendahl, Sabrina; Brandner, Dean Martin; Erhard, PeterWe investigate how the walls of cylindrical capillaries affect the velocity of rising gas bubbles of various diameters. Of course, as the capillary diameter increases, the velocity of the rising bubble will approach the case of free rising. Such systematic experiments on bubble rise in capillaries, in which the ratio of bubble diameter and capillary diameter is varied from one towards smaller values, can hardly be found in literature. First orienting experiments within the system water/air have been conducted and will be discussed in this paper.Item Dimensional analysis of droplet size and ligament length during high-speed rotary bell atomization(2020-10-01) Gödeke, Lutz; Oswald, Walter; Willenbacher, Norbert; Ehrhard, PeterModern spray-coating processes are based on high-volume, low-pressure, airless atomization or high-speed rotary bell atomization, often assisted by electrostatic charging to increase the transfer efficiency. The process from the liquid film flow beneath the bell, through ligament formation and consecutive disintegration to droplet deposition, has been constantly explored during the evolution of automotive spray coating. This work proposes a set of dimensionless groups that fully describe the process from film flow to ligament disintegration, including shear and elongational flow effects during atomization of particle laden, shear thinning, viscoelastic fluids.Item Analysis of paint flow pulsations during high-speed rotary bell atomization(2019-10-17) Oswald, Walter; Lauk, Jan; Gödeke, Lutz; Erhard, Peter; Willenbacher, NorbertHigh-speed rotary bell atomization is the preeminent coating technique in the automotive industry. It is widely accepted that a narrow droplet size distribution and constant spray are necessary in order to guarantee uniform film thickness and high-quality appearance. This may be deteriorated by paint flow pulsations. So far, however, no studies exist regarding such fluctuations quantitatively for this type of atomizers. We fill this gap using image analysis of high-speed recordings close to the bell edge. We could show that the fundamental pulsation frequency increases linearly with rotational speed. A ratio of pulsation frequency and true rotational speed of about 3 was found, indicating that pulsations were initiated mainly by the three struts of the distributor disc. The coefficient of variation, i.e., the amplitude of fluctuation increased with decreasing liquid volume rate and rotational speed. Beyond that, we could show that the formation of droplets larger than 100 μm, which are assumed to cause paint defects, is promoted by the degree of fluctuation. These findings may stimulate development of bell cups showing less paint flow pulsations.Item Spatially-resolved mass transport in a liquid/liquid slug-flow micro-capillary reactor(2019) Heckmann, Christian; Ehrhard, Peter; Agar, David W.