Youhei Takagi

Composition-dependent shape changes of self-propelled droplets in a phase-separating system

Chemical potential gradient in a multicomponent fluid system undergoing phase separation acts as a driving force for transporting a fluid. We have experimentally shown the self-propelled motion of the droplet undergoing phase separation and its shape changes using the aqueous two-phase system. The droplet behavior depended on the composition of the continuous phase. For higher concentrations of the continuous phase than the equilibrium concentration, a droplet moved unidirectionally even in a homogeneous concentration field with a constant radius of the droplet. However, for slightly lower concentrations, a droplet shrunk in proportion to time while moving. For much lower concentrations, the shape of the droplet periodically changed from a sphere to a bullet shape, and then to a parachute shape. The droplet has deformability like a biological cell. The self-propelled motion is due to the coupling between mass and momentum transfer via Korteweg force which arises from minimizing the free energy of the system. For investigating the composition-dependent shape changes, we carried out the visualization experiments of concentration distributions inside and outside the droplets.

Numerical investigation for the effect of the liquid film volume on thermocapillary flow direction in a thin circular liquid film

NASA Astronaut Dr. Pettit carried out a thermocapillary flow experiment onboard the International Space Station in 2003. In this experiment a thin water film containing milk powder was formed in a stainless-steel wire ring. Heating a section of the ring by a soldering iron induced in the water film a thermocapillary flow towards the heated section of the ring (outward flow: cold to hot). This flow was in the opposite direction of the usually observed thermocapillary flows (inward flow: hot to cold). To shed light on this interesting phenomenon observed in the space experiment, we have conducted a three-dimensional numerical simulation study. Simulation results showed that the film geometry of the water film is a key factor determining flow direction and flow strength. When the liquid film free surfaces are convex, i.e., the water film volume is larger than that when the free surfaces are flat, an outward flow develops in the film as observed in the space experiment. However, when the free surfaces are con...

Numerical Investigation of the Thermal Behavior in a Hydrogen Tank During Fast Filling Process

To investigate the thermal behavior during fast hydrogen filling process, the simple one-dimensional analysis considering the heat conduction in tank wall and the three-dimensional numerical simulation dealing with inner gas region were carried out. The numerical analyses were subject to the fast filling test of 35 MPa hydrogen gas into 34 litter tank for 80 seconds. The one-dimensional analysis predicted the temperature rise and the heat loss into surrounding air qualitatively and the averaged temperature of tank wall was underestimated. On the other hand, the three-dimensional simulation overestimated the temperature distribution because of using adiabatic wall condition. However, the effects of buoyant force and convective flow on local thermal profile were fully explained from our numerical results.© 2011 ASME

Numerical investigation of oscillatory thermocapillary flows under zero gravity in a circular liquid film with concave free surfaces

NASA astronaut Pettit has conducted thermocapillary flow experiments in water films suspended in a solid ring onboard the International Space Station (ISS) in 2003 and 2011. In one of these experiments, an oscillatory thermocapillary flow was observed. The developed flow broke its symmetry along the centerline of the film. To the best of our knowledge, there are no studies on such oscillatory thermocapillary flows in thin films, and the flow-mechanism giving rise to such oscillatory flows is also not well understood. In order to shed light on the subject, we have carried out a numerical simulation study. The simulation results have shown that the water film geometry (film surface shape; being concave) is an important parameter and give rise to three oscillatory flow structures in the film, namely, a hydrothermal wave developing near the heated section, a symmetric oscillatory flow due to temperature variations, and a symmetry breaking flow due to the hydrodynamic instability along the free boundary layer ...

Combined Numerical and Experimental Study on Drag Torque in a Wet Clutch

The effect of flow field on drag torque in a wet clutch was examined through a combined numerical and experimental study. Three-dimensional hydrodynamic numerical simulations were carried out, and the drag torque was measured experimentally for a single wet clutch pack. Two-phase flow induced by aeration was visualized in the experiment. In the present drag torque test, the main section was consisted of two parallel circular plates. The plate with the frictional material was rotated. The frictional material was divided into some sections, and radial or circumferential grooves were made on the rotating disk. Automatic Transmission Fluid (ATF) was supplied from the axial center, and ejected into the surrounding open boundary. At low rotation speeds, it was found that the oil flow is of single-phase, and the drag torque is linearly proportional to the rotation speed since the shear stress on the clutch plate increased monotonically. In the single-flow regime, the slope of drag torque curve was controlled with the clearance between the clutch plates. The drag torque reached a peak value at a certain rotation speed, and it decreased gradually after the peak. These observed phenomena were due to the aeration from the inner gap on the disk, and the bubble volume fraction was directly related to the drag torque. The peak of drag torque was controlled by both the flow rate of supplied ATF and the arrangement of grooves on the frictional material. It also was found that the smooth ejection of ATF and the enhancement of aeration led to a reduction in the drag torque.Copyright

Effect of Liquid-Gas Interaction on Plume Structure in Blowout Flow

Multiphase flow simulation for oil and gas blowout in water is performed to investigate the effect of liquid-gas interaction on the plume structure. The mass and momentum balance equations were solved for each phase, and the interactions were considered with the Euler-Euler approach. To consider the interphase interaction between two dispersed phases in the oil-gas-water three-phase system, we applied the drag force between oil and gas in the momentum equation. A series of numerical results showed that the interaction between the two dispersed phases affected the velocity distribution of the gas and oil phases in the multiphase plume. These results suggested that the gas-oil interaction might affect the traveling time of gas and oil in an actual marine environment.

Numerical Simulation of Oil and Gas Blowout from Seabed in Deep Water

To simulate oil and gas blowouts from seabed in deep water, we developed two numerical models based on conventional plume and Lagrangian approaches. The simulations were carried out for the Deepspill experiment at the Norwegian Sea, and the two test cases of oil and gas discharges were considered. It was shown that the simple oil-tracking model only with Lagrangian particle tracking could predict the field experiment if the size distribution of oil droplet at the inlet nozzle remains steady during the rising process. The methane gas spill behavior was solved with the hybrid plume/Lagrangian model, and the methane hydrate model was considered in both the near and far fields. Compared with the non-hydration simulation, it was found that the methane hydration has a significant effect on the fate of spilled methane. Furthermore, the parameter determining growth rate in the methane hydrate model plays an important role in obtaining accurate numerical predictions.

Numerical Investigation of Drag Reduction by Hydrogel with Trapped Water Layer

Hydrogels are novel materials that exhibit drag reduction properties in marine applications. It is suggested that they can reduce drag by trapping water between the dimples present on a rough coating but the precise mechanism hardly understood. In the present study, a series of direct numerical simulations (DNS) of turbulent channel flow were performed. The water trap effect of the hydrogel was modelled as Navier slip boundary condition on a flat surface with periodically varying slip length in the streamwise and spanwise directions. The results showed that drag reduction effect increases with larger slip lengths and that wavelengths comparable with the size of the vortex structures are effective for drag reduction.

Numerical simulation model by volume averaging for the dissolution process of GaSb into InSb in a sandwich system

ABSTRACT The volume-averaging continuum technique has been utilized to obtain numerical predictions for the transport phenomena occurring during the dissolution process of GaSb into InSb melt in a sandwich system. Dissolution and subsequent growth in this system are achieved by the application of a temperature gradient. The developed model was first verified for two test cases [(i) fluid/solid conjugate heat transfer and (ii) the solidification process of the binary system]. The code was then utilized to simulate the dissolution process of GaSb into InSb in the GaSb/InSb/GaSb sandwich system. The present results show that the developed volume-averaging model provides accurate predictions.

Numerical Simulation of InGaSb Crystals Growth under Microgravity Onboard the International Space Station

InxGa1-xSb bulk crystal was grown using a GaSb(seed)/InSb/GaSb(feed) sandwich-structured system onboard the International Space Station (ISS). In order to investigate the transport phenomena especially in terms of interface shapes and dissolution heights, the dissolution process was simulated under a micro-gravity level of the ISS. Simulation results showed that the seed/melt interface was concave towards the seed due to the temperature distribution of the system. This prediction is in good agreement with the results of our previous experimental study.