|
|
Hokkaido University Collection of Scholarly and Academic Papers >
Theses >
博士 （工学） >
Numerical investigation on large-sized bubble injection for control of turbulent boundary layer : Horizontal channel flow and bubble-induced drag reduction
|
Files in This Item:
|
|
|
Related Items in HUSCAP:
|
| Title: | Numerical investigation on large-sized bubble injection for control of turbulent boundary layer : Horizontal channel flow and bubble-induced drag reduction |
| Authors: | KIM, Sangwon1 Browse this author |
| Authors(alt): | 金, 相元1 |
| Issue Date: | 24-Sep-2021 |
| Publisher: | Hokkaido University |
| Abstract: | The control of turbulent boundary layers by injecting bubbles has many advantages because the bubbles consistently modify the boundary layer by traveling with the main flow. The characteristics of these bubbly flows, which can be divided into vertical and horizontal bubbly flows, change depending on the direction of buoyancy relative to that of the main flow. However, horizontal bubbly flows have received less attention than vertical bubbly flows because buoyancy acts in the wall-normal direction and hinders heat exchange and mass transfer between the main flow and wall, rendering horizontal bubbly flow unsuitable for industrial demands and more complex flows. The horizontal bubbly flow modifies the inner-layer structure of the turbulent boundary layer formed beneath a horizontal flat wall, thus reducing the frictional drag on the upper wall. This characteristic is called bubble drag reduction (BDR) and is applied to liquid transport in pipelines and ship surfaces in water. In particular, the energy efficiency of large vessels can be promoted by reducing the frictional drag, which accounts for 80% of the total drag. Many previous studies on horizontal bubbly flow have been based on microbubbles for inducing BDR. The typical micro-bubble method applied in industrial fields contains bubbles of a- few- millimeters, which frequently coalesce in the shear layer and become large-sized bubbles in the downstream region. Furthermore, air films, which are also used for drag reduction and implemented by injecting air on a superhydrophobic surface to separate the liquid phase of the flow from the wall, are broken up and separated into large-sized bubbles from the downstream; however, the critical size of these large-sized bubble has not yet been determined. From this perspective, understanding large-sized bubbles is essential to retain drag reduction from the downstream. Recently, it was discovered that large-sized bubbles provided a velocity gradient that calmed the wake region; furthermore, the drag reduction performance depended on the bubble length. Interestingly, large-sized bubbles show a common spatial relationship between drag modulation (drag increment and reduction) and bubble location. However, the dynamics of large-sized bubbles include some open questions, such as the mechanism of drag modulation, as bubbles of a certain size can increase, rather than decrease, drag, which results in larger skin friction than that under single-phase flow. Accordingly, the goals of the present study are to numerically investigate the above questions and achieve a comprehensive understanding of large-sized bubble dynamics.
This thesis comprises seven chapters, summarized as follows.
1. First, background research on bubbly flow for drag reduction and related numerical studies are
briefly described, and the objectives and strategies of the thesis are presented in Chapter 1.
2. The mathematical formulation and numerical details of the volume of fluid (VOF) methods and
two different types of interface sharpening methods are presented. Several series of verification
works as interface-sharpening methods are confirmed. [Chapter 2].
3. The modeling of a large-sized bubble interface and the vortical structures around the bubble are
validated. Before the validation work, the main part to be modeled is divided into three parts, and
the procedure of division is described in Chapter 3.
4. The dynamic behavior of the bubble interface and the drag modulation of large-sized bubbles in
the turbulent channel flow are investigated by exploring various bubble sizes. The mesh condition
of the numerical model is improved to resolve the coherent turbulent structure [Chapter 4].
5. The dynamic behavior of a large-sized bubble on turbulent Couette channel flow is examined to
investigate differences such as the deformation mechanism from that of turbulent Poiseuille
channel flow [Chapter 5].
6. The large-sized bubbly flow is investigated as a flow condition to explain the drag reduction
involved in bubble formation [Chapter 6].
7. Reviews of each chapter and the conclusions of this thesis are presented [Chapter 7]. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第14674号 |
| Degree Level: | 博士 |
| Degree Discipline: | 工学 |
| Examination Committee Members: | (主査) 教授 大島 伸行, 教授 渡部 正夫, 教授 村井 祐一, 准教授 寺島 洋史 |
| Degree Affiliation: | 工学院（機械宇宙工学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/86711 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 工学院(Graduate School of Engineering)
学位論文 (Theses) > 博士 （工学）
|
|
