Transition of wire-disturbed cylinder wake flow
Başlık çevirisi mevcut değil.
- Tez No: 402986
- Danışmanlar: PROF. DR. ANTON VAN STEENHOVEN, DR. CAMILO RINDT
- Tez Türü: Doktora
- Konular: Fizik ve Fizik Mühendisliği, Physics and Physics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2011
- Dil: İngilizce
- Üniversite: Technische Universiteit Eindhoven
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 139
Özet
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Özet (Çeviri)
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 transition, 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 the tin-precipitation method, velocity measurements using Particle Image Velocimetry(PIV), and numerical simulations based on the Spectral Elementmethod. In the laminar two-dimensional flow regime (Re = 100), it was observed, both numerically and experimentally, that the wake of the cylinder is taking different trajectories with respect to the wire position. A hypothesis was formulated about the reasons of thewake deflection using a Point VortexModel. 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 was observed for Re > 170. This transition is characterized by the so-called Mode-C instability. Analysis of the experimental results shows that this Mode-C instability consists of secondary vortices with a period-doubling character, i.e. 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 Kármán vortices causes the period-doubling character of the wake. The analysis ofMode-C transitionwas further extended using the data fromcom prehensive PIV experiments. The three-dimensional wake structure and vortex dynamics were 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 have been 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 the non-rotating case. Both flow visualization and PIV techniques were used to investigate this kind of flow. Shedding Mode II has experimentally been detected for a Reynolds number of Re = 100 in the same rotation rate regime as in the numerical studies.
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