色画像処理計測の高度化による熱対流研究
| Přispěvatelé: | 田坂, 裕司, 村井, 祐一, 大島, 伸行, 渡部, 正夫, 染矢, 聡 |
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| Jazyk: | angličtina |
| Rok vydání: | 2021 |
| Popis: | Study on thermal convection has been continued more than a century since Henri B´enard’s systematic experiments. Thermal convection has attracted worldwide interests not only as a key phenomenon in the field of thermo-fluid physics, but also as a large-scale fluid motion in the fields of geo- and astro-physics. Further, thermal convection is a significant platform when considering a way of prediction and control of a heat transport system from the perspective of engineering. Since the driving force of fluid flows originates in thermal buoyancy for relaxing density inhomogeneity of the fluid, temperature field measurements are sometimes required in addition to velocity field measurements for comprehensive understandings, even though temperature field measurements require much effort than velocity field measurements. Flow field measurements, such as particle image velocimetry (PIV) and particle tracking velocimetry (PTV), have been developed as monochrome digital image processing techniques, as velocity fields can be easily measured by tracking particle displacements across different image frames. Meanwhile, rich color information which can be acquired by widespread color cameras has not been fully utilized in flow measurements to date. Addition of color information has the potential to improve the measurement ability of optical measurement techniques by keeping the optical system simple. Thus, this thesis aims to develop color-based optical measurements which append a function to measure physical quantities, such as temperature, in addition to velocity fields for the purpose of comprehensive understandings of various thermal convection phenomena. This thesis comprises four parts involving the introduction (Part I) and the summary (Part IV). In Part II, four different color-based optical measurement techniques are established. Chapter 1, 2, and 3 demonstrate ways of utilization of color information as out-of-plane physical coordinates. Color-based optical measurements are systematized as two different problem settings; classification problem and regression problem. The classification problem treats color information as thresholds for separating distinct differences identifiable on color images; the use of two differently colored laser sheets with different illuminating planes enables to separate a color image into two different planes only from the color information. In Chapter 1, this color image separation technique is utilized to simultaneously measure convective motions in two immiscible fluid layers. With the simultaneous velocity field measurements, coupling structures of convection cells formed in each layer can be identified as two-dimensional interfatial interactions. Chapter 2 demonstrates a possibility to measure out-of-plane velocity component only by a single camera and two different laser sheets. The method named “dualplane ensemble correlation” and color image separation enable pixelwise out-of-plane velocity field measurements with such a simple optical system. The regression problem considers color information as raw data representing certain physical quantities. Color PTV method established in Chapter 3 utilizes continuous rainbow color changes as continuous out-of-plane coordinate changes. With a combination of artificial neural network (ANN), originally recorded ambiguous color information can be calibrated to out-of-plane coordinates as a multi-regression problem without unreasonable assumptions. Chapter 4 proposes a novel technique that can identify fluid deformation by coloration originated in birefringence. The results suggest a new insight leading to direct and pixelwise fluid deformation measurements only by utilizing color information. In Part III, multi-timescale physics of various thermal convection phenomena are studied utilizing thermochromic liquid crystals (TLC). Efficacy of the TLC particles is carefully studied and subsequent special image processing schemes are provided in Chapter 1. The TLC particles can be utilized not only for temperature field visualization but also for velocity field measurements in thermal convection studies thanks to their high traceability and high dispersibility. In Chapter 2, developing stages of horizontal convection (HC) are examined through long-time laboratory experiments. It is found that the degree of background stable temperature stratification plays an important role in determining roll structures formed during transient evolution from conduction to thermally-equilibrated states. A novel nondimensional parameter representing balance of braking effect by the stable temperature stratification and driving effect by the baroclinic torque describes well the temporal evolution of the developing HC. Transient states and equilibrated states of rotating Rayleigh–B´enard convection (RBC) are discussed deeply in Chapter 3 and 4, respectively. On the transient states during spin-up of rotating RBC studied in Chapter 3, it is revealed that break down of thermally coherent structures, toroidal roll formations, azimuthally aligned vortex formations, and plume emissions forming columnar vortices appear in order. Mechanisms of these stepwise transitions are studied in detail. Such beautiful stepwise transitions are found to be realized only when multiple timescales, e.g., Ekman spin-up and viscous timescales, are balanced in a certain manner. Dynamics of columnar vortices formed in equilibrated rotating RBC are studied in Chapter 4. In order to measure local and instantaneous heat transport carried by a single columnar vortex, simultaneous three-dimensional velocity and temperature measurement systems are constructed. Stereoscopic PIV and coloration of the TLC particles enable such complicated measurements by employing two color cameras, a light source, and a scanning system. Horizontal motions of the columnar vortices are quantified as diffusive motions with inelastic collisions, and its motion seems to be motivated by thermal forcing. Relationship between the heat transport characteristics and the dynamics of a single columnar vortex is elucidated by statistical analysis. The results suggest the source of random-walk-like horizontal diffusive motions of the columnar vortices, which has been an open issue in past studies. (主査) 准教授 田坂 裕司, 教授 村井 祐一, 教授 大島 伸行, 教授 渡部 正夫, 染矢 聡 (産業総合研究所・省エネルギー研究部門） 工学院（エネルギー環境システム専攻） |
| Databáze: | OpenAIRE |
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