Fluid Physics

Fluid Physics

Title:	Experimental Assessment of Dynamic Surface Deformation Effects in Transition to Oscillatory Thermocapillary Flow in Liquid Bridge of High Prandtl Number Fluid
Team Coordinator:	Satoshi Matsumoto (Japan Aerospace Exploration Agency)
Principal Investigator:	Yasuhiro Kamotani (Case Western Reserve University / Japan Aerospace Exploration Agency)
Co-Investigator:	Koichi Nishino Hiroshi Kawamura Masahiro Kawaji Kazunori Kawasaki Nobuyuki Imaishi	Yokohama National University Science University of Tokyo University of Toronto IHI Aerospace Co., Ltd Kyushu University

Salient points

In the fluid science field, recent interest has focused on high-speed turbulent flows, which produce many small vortices, instead of gentle flow. Such turbulent flows are encountered in areas such as spacecraft design. At the same time, surface-driven flows like thermocapillary flows are not fully understood in spite of their importance from scientific and industrial points of view.

Microgravity experiments on thermocapillary flows are carried out frequently. Microgravity conditions would reveal the essence of the thermocapillary flow because buoyancy and convection are suppressed and a larger liquid bridge can be formed. Thermocapillary flows have been investigated to contribute the improvement of
materials processing. Convection driven by both buoyancy and thermocapillary flows are induced during material processing. Convection cannot be adequately controlled since we do not sufficiently understand thermocapillary flows. Therefore, fluid physics should be advanced by accumulating the results of thermocapillary flow experiments.

In Japan, thermocapillary flows have been systematically investigated from two points of view, advancing fluid physics by using a transparent model fluid and improving material processing techniques by using practical materials. The visualization techniques have been developed though Japanese sounding rocket experiments and have become some of the most advanced experimental methods. Both internal and surface flows can be stereoscopically visualized in a transparent model fluid. Therefore, detailed information of the complicated three-dimensional flow enables us to understand the oscillatory flow. Convection in an opaque fluid, such as molten metal, is usually invisible. However, even convection in a molten semiconductor can now be visualized since X-ray radiography observation equipment has been developed. Experiments utilizing sounding rockets played important roles in developing the observation system and starting the systematic investigation of oscillatory thermocapillary flows.

Complex flows such as oscillatory motion will be understood better because sophisticated experimental techniques have been developed. There are two questions to be resolved concerning thermocapillary flows. First, we must clarify the conditions that generate oscillatory flows. Second we must understand why a thermocapillary flow changes from steady to oscillatory. The microgravity experiment will resolve the first problem in the first KIBO utilization theme. The onset of complex oscillatory flows in both model fluid and molten semiconductor will be observed. This experiment to resolve the second problem.

The themes utilizing the International Space Station in the early stage allow understanding the transition behavior of complicated thermocapillary flows. The results will not only promote better understanding of thermocapillary flow but also contribute to the advancement of fluid physics in surface problems.

Brief summary

In this experiment, large liquid bridges are formed under microgravity conditions and flow motions and surface deformations are observed in detail to investigate the thermocapillary flow. The relation between surface deformation and flow will be clarified, and an innovative physical model of the oscillatory transition in a thermocapillary flow will be verified. The results are certain to contribute both to fluid physics and to applications in materials processing.

Under the influence of a temperature gradient, the surface is pulled from an area of lower surface tension to an area of higher surface tension. Consequently, convection called thermocapillary flow occurs (see Fig. 1). It is well known that a thermocapillary flow can be changed from a steady flow to a complicated oscillatory flow by changing the temperature difference (see Fig. 2). However, the conditions leading to the onset of oscillatory flow and the associated transition behavior have not been clarified yet.

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Fig. 1 Thermocapillary Flow		Fig. 2 Transition From steady to oscillatory flow.

Previous investigations to clarify the oscillatory transition revealed that Dynamic Surface Deformation (DSD),
which is a surface wave phenomena, is closely related to the flow motion (Fig. 2). A new physical model of the transition of thermocapillary flows has been developed to illustrate the important DSD roles that trigger the onset of oscillatory flow and maintain it. This model is essentially different from previous models. This model predicts that oscillatory flow occurs when the DSD amplitude exceeds a certain value.

An experiment with a larger liquid bridge is needed to verify the hypothesis. Microgravity conditions on board the International Space Station are utilized because only a small liquid bridge can be formed on the Earth due to gravity. This experiment uses eight liquid bridges with different diameters and heights. DSD is precisely observed, and flow and temperature fields are visualized in detail. The experiment results will be compared with an advanced numerical simulation taking into account surface motion. The behavior of DSD in the oscillatory transition and its roles will be clarified. As a result, this study is expected to clearly explain the unsolved phenomena related to the oscillatory transition of thermocapillary flow.

Last Updated : October 1, 2003