The transition of the flow behind bluff bodies has been the main topic of research for many decades. Despite the efforts of many scientist and engineers, understanding of the transition mechanism of wake flows behind both streamlined and bluff bodies is still a challenge. The focus in this research is the modified flow regime in the wake of a circular cylinder. The modification is obtained by placing a very thin wire at a particular position in the cylinder wake. The occurring transitional flow is denoted as Mode-C, in comparison to Mode-A and Mode-B transition for the non-wired cylinder. The flow structures have been investigated both experimentally and numerically for different Reynolds numbers (Re=100-250) using flow visualizations based on tin-precipitation method, velocity measurements using Particle Image Velocimetry and numerical simulations based on Spectral Element method. In the laminar two-dimensional flow regime Re=100, it is observed, both numerically and experimentally, that the wake of the cylinder is taking different trajectories with respect to the wire position. A hypothesis is formulated about the reasons of the wake deflection using a Point Vortex Model. The hypothesis is supported with the assessment of vortex trajectories, strengths, lift and drag characteristics. It is concluded that the deflection of the wake is primarily caused by a modification of the vortex arrangement in the wake. This modified vortex arrangement is caused by different formation times of the upper and lower vortices, by different vortex strengths or by both. A three-dimensional transition of the wired cylinder flow is observed for Re>170. This transition is characterized by the so-called Mode-C instability. The analysis of the experimental results shows that this Mode-C instability consists of secondary vortices with a period-doubling character, ie. the secondary vortices alternate sign from one shedding cycle to the next. It is shown that a feedback mechanism of the streamwise vortices between the two consecutively shed upper von Karman vortices causes the period-doubling character of the wake. The analysis of Mode-C transition is further extended using the data from comprehensive PIV experiments. The three-dimensional wake structure and vortex dynamics are investigated with a particular focus on the energy distribution of the wake, vortex strengths and vortex trajectories. The secondary vortices are shown to be counter rotating vortex pairs with a spanwise wavelength of ¿Z/D=2.16. In the final stage of the research, experiments are performed to evaluate the wake behind a rotating cylinder, particularly focusing on the so-called Shedding Mode II regime. In literature only numerical proof is found for the existence of this Shedding Mode II for which a single vortex is shed with a much lower frequency compared to non-rotating case. Both flow visualization and PIV techniques are used to investigate this kind of flow. Shedding Mode II is experimentally detected for a Reynolds number of Re=100 in the same rotation rate regime as in the numerical studies.
|Kwalificatie||Doctor in de Filosofie|
|Datum van toekenning||27 jun 2011|
|Plaats van publicatie||Eindhoven|
|Status||Gepubliceerd - 2011|