京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第19955号 / 理博第4222号 / 新制||理||1607(附属図書館) / 33051 / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 釜井 俊孝, 准教授 王 功輝, 教授 林 愛明 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20925号 / 理博第4377号 / 新制||理||1628(附属図書館) / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 田上 高広, 教授 下林 典正, 准教授 河上 哲生 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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One approach recently proposed for reducing the frictional resistance to liquid flow in microchannels is the patterning of micro-ribs and cavities on the channel walls. When treated with a hydrophobic coating, the liquid flowing in the microchannel wets only the top surfaces of the ribs, and does not penetrate into the cavities, provided the pressure is not too high. The net result is a reduction in the surface contact area between channel walls and the flowing liquid. For micro-ribs and cavities that are aligned normal to the channel axis (principal flow direction), these micropatterns form a repeating, periodic structure. This thesis presents numerical results of a study exploring the momentum and thermal transport in a parallel plate microchannel with such microengineered walls. The liquid-vapor interface (meniscus) in the cavity regions is approximated as flat in the numerical analysis. Two conditions are explored with regard to the cavity region: 1) The liquid flow at the liquid-vapor interface is treated as shear-free (vanishing viscosity in the vapor region), and 2) the liquid flow in the microchannel core and the vapor flow within the cavity are coupled through the velocity and shear stress matching at the interface. Predictions reveal that significant reductions in the frictional pressure drop (as large as 80%) can be achieved relative to the classical smooth channel Stokes flow. In general, reductions in the friction factor-Reynolds number product (fRe) are greater as the cavity-to-rib length ratio is increased (increasing shear-free fraction), as the relative module length (length of a rib-cavity module over the channel hydraulic diameter) is increased, as the Reynolds number decreases, and as the vapor cavity depth increases. The thermal transport results predict lower average Nusselt (Nu) numbers as the cavity-to-rib length ratio is increased (increasing shear-free fraction), as the relative module length (is increased, and as the Reynolds number decreases with little dependence on cavity depth. The ratio of Nu to fRe was evaluated to characterize the relative change in heat transfer with respect to the reduction in driving pressure. Results show that the benefits of reduction in driving pressure outweigh the cost of reduction in heat transfer at higher Reynolds numbers and narrower relative channel widths.

There is significant interest in reducing the frictional resistance that occurs along a surface in contact with a liquid. A novel approach to reducing the frictional resistance across a liquid-solid interface is the use of superhydrophobic surfaces. superhydrophobic surfaces are created in this work by the use of micro-fabrication techniques where systematic roughness is fabricated on a substrate surface which is subsequently treated with a hydrophobic coating. This work reports an experimental study of superhydrophobic surfaces used to reduce drag in both laminar and turbulent channel flows. In the laminar flow regime reductions in frictional resistance greater than 55% were measured in microchannels consisting of superhydrophobic walls. The reduction in frictional resistance for laminar flow in microchannels with superhydrophobic walls was shown to be dependent on the rib/cavity orientation, with greater reduction achieved when the ribs/cavities were aligned parallel with the direction of the flow. Also, the ratio of the cavity width to the combined rib/cavity pitch and the ratio of the combined rib/cavity pitch to the microchannel hydraulic diameter exercise influence on the frictional resistance. The condition when the flowing liquid was allowed to completely "wet" the cavities was also explored. Generalized expressions enabling prediction of the classical friction factor-Reynolds number product as a function of the relevant governing parameters were also developed. The influence of superhydrophobic surfaces in turbulent flow was explored in macrochannels using particle imaging velocimetry (PIV). For the turbulent flow regime the time-averaged velocity profiles revealed no discernible slip velocity at the superhydrophobic wall. However, the results did show that the superhydrophobic surfaces exhibits an influence on the streamwise and wall-normal turbulence intensities, the turbulent shear stress, the total shear stress distributions, and the turbulence production in the channel. From the total shear stress distributions in the channel the coefficient of friction at the channel walls was determined. The results showed that for the superhydrophobic surface with ribs and cavities oriented parallel to the flow direction a reduction in the coefficient of friction as high as 16% was achieved compared to a smooth wall channel. Superhydrophobic surfaces with ribs and cavities oriented transverse to the flow direction showed a modest increase in the coefficient of friction. Differential pressure measurements in the turbulent flow channel were also acquired and used to calculate the channel average friction factor.

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京都大学 / 新制・課程博士 / 博士(工学) / 甲第23860号 / 工博第4947号 / 新制||工||1773(附属図書館) / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 岸田 潔, 教授 三村 衛, 教授 木村 亮 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

The design of skin friction gages has been explored analytically and experimentally for measuring skin friction in high-speed, short-duration flow. Several gage designs were considered. One promising gage design used a floating element, while another was microfabricated using sputtering techniques. All of the gages were physically modeled to determine the output caused by Mach 2 unheated flow. Frequency response analysis was also performed on the floating element and sputtered design to determine their ability to make measurements in the millisecond time range. Temperature and normal pressure effects were a source of measurement error, and techniques were developed for minimizing the error due these effects. Tests were made in Mach 2 flow and the results of these tests are discussed. Recommendations are provided as to how the gages can be improved for further testing. / Master of Science

This thesis seeks two objectives. One objective is to develop a means to estimate time invariant real contact areas and surface temperatures through thermal measurements in 1D/2D systems. This allows computationally easier models, resulting in faster simulations within acceptable convergence. The second objective is to provide experimental design guidance. The methods used are a modified cellular automata technique for the direct model and a Levenberg-Marquardt parameter estimation technique to stabilize inverse solutions. The modified cellular automata technique enables each piece of physics to be solved independently over a short time step, thus frequently allowing analytical solutions to those pieces. Overall, the method was successful. The major results indicate that appropriately selected measurement locations can determine the contact distribution accurately, and that the preferred measurement location of the sensor is not very sensitive to the contact distribution specifics. This is useful because it allows selection of measurement locations regardless of the specifics of the generally unknown contact distribution. Further results show the combined effects of the normalized length and the Stanton number have a significant impact on the estimation quality, and can change the acceptable sensor domain, if the loss is high. The effect of placing the sensor in the static body can, for low loss, provide a coarse image of the contact distribution. This is useful because the static body is easier to instrument than a moving body. Finally, the estimation method worked well for the most complex model utilized, even in a sub-optimal measurement location. / Master of Science