When small particles are suspended in a three-dimensional steady incompressible flow, Lagrangian coherent particle structures can be created by dissipative mechanisms which rely either on inertia, buoyancy or particle–boundary interactions. The dissipative effect particles experience when moving close to a wall or a free surface can lead to a particular rapid attraction of the particles to periodic, quasi-periodic or strange orbits. The particle–boundary interaction dominates the accumulation of particles in thermocapillary liquid bridges of millimetric size if the particles are small, giving rise to finite-size c oherent s tructures, d epending o n t he t opological t emplate o f t he u nderlying i ncompressible fl ow. The achievements obtained in understanding finite-size coherent structures in liquid bridges are reviewed, commenting the theoretical, experimental and numerical developments over the last two decades. Moreover, open questions and perspectives of the research on finite-size Lagrangian coherent structures are discussed.

View full abstract

Microgravity experiments on Marangoni convection in a liquid bridge of high-Prandtl-number fluid so-called Marangoni Experiment in Space (MEIS) and Dynamic Surf (DS) were performed on board the International Space Station (ISS) to understand the Marangoni convection instability. This study reports the internal flow patterns and velocity profiles of the axisymmetric steady Marangoni convection observed in these projects for various Prandtl numbers (Pr = 67, 112 and 207), and for various liquid bridge geometries. The classical three-dimensional particle tracking velocimetry (3-D PTV) is customized for the microgravity experiments, and the spatial structures of the Marangoni convection in liquid bridges are measured with good accuracy. The details of the measurement method currently used, and the results of the 3-D PTV are discussed in this study.

View full abstract

We experimentally investigate behaviors of a single spherical particle in a half-zone liquid bridge induced by the thermocapillary effect. It has been indicated that suspended particles show unique structures in time-dependent traveling flow state. This phenomenon is called ‘particle accumulation structure (PAS).’ In order to understand correlation between the identical particle motion and the coherent structures by the particles, we focus on a single particle behavior to illustrate the spatio-temporal correlation between PAS itself and individual particles on PAS. We indicate the Poincaré section for the particle forming PAS and that forming the toroidal core by preparing a thin light sheet. Through a long-period observation of the turnover motion of individual particle, we make comparison with the proposed topological flow fields accompanying with so-called Kolmogorov-Arnold-Moser (KAM) tori. The present work is conducted as a ground-based research for coming on-orbit international collaborative experiments aboard the Japanese Experiment Module ‘Kibo’ in the International Space Station by the project of Japanese-European Research Experiment on Marangoni Instability (JEREMI).

View full abstract

We investigate the effect of heat transfer through a free surface on the primary instability of thermocapillary-driven convection in a geometry of so-called half-zone liquid bridge of high Prandtl number fluid. The target liquid bridge is a straight whose aspect ratio Γ = H/R is mainly kept at 2.0, where H and R are the height and the radius of the bridge, respectively. We focus on the flow fields induced by the instability; it is found that the bifurcation diagram exhibits a significant difference between the cases of the Prandtl number Pr = 16 and 28. The effect of gravity level is also examined in order to discuss qualitatively the induced flow fields after the transition obtained in the ground-based experiments as well as in on-orbit experiments so-called ‘Dynamic Surf’ in the Japan Experiment Module ‘Kibo’ aboard the International Space Station.

View full abstract

A microgravity experiment utilizing sounding rocket is going to be held in 2021 to clarify cool flame occurrence from n-decane droplet array near spontaneous ignition limit. As a preliminary study for the experiment, the 2D numerical simulation is carried out. The almost identical numerical geometry to the actual furnace for the rocket experiment is used to predict the cool flame occurrence in the rocket experiment. The interference of the droplets is validated by calculation with different inter-droplet distance. The employed fuel is n-decane of 1.0 mm in diameter. The initial temperature and pressure are 550 K and 1 atm respectively. The results shows that the cool flame occurs from the outside of the fuel droplet array. A fuel concentration at inter-droplet is higher than the outer side, whereas the temperature at inter-droplet is lower than the outer side. It is thought that the lower temperature yields increase in the spontaneous ignition delay time surpassing the shorten effect of the ignition delay time due to higher fuel concentration.

View full abstract

Water movement plays an important role to grow crops under microgravity. Previous research reported that water hardly moved in porous media whereas water in capillary tubes rose to the top of the tubes under microgravity. Another study reported that water hardly moved on concave surfaces under microgravity. Our objective of this study was to evaluate the effects of surface shape and junction of capillary tubes on water movement driven by the capillary force under microgravity. We used several shapes of tube (straight, concave-convex, wide-narrow, narrow-wide, Y-shaped, T-shaped and spiral) and observed water movement in the tubes during drop–tower induced microgravity. Water in concave–convex, wide–narrow tubes moved beyond concave and convex surfaces under microgravity. Water in narrow–wide tubes, however, stopped on their concave surfaces. In the Y- and T-shaped glass tubes, water movement was restricted after the junction of glass tubes by viscous force.

View full abstract

Measurement of interfacial tension between molten slag and molten iron under microgravity in the International Space Station (ISS) is planned. An oscillating drop technique, in which interfacial tension is determined from the oscillation frequency of a levitated compound droplet, with an electrostatic levitator is going to be used in the interfacial tension measurement. In our previous work, a numerical model to simulate the oscillation behavior of a compound droplet was developed, and the effects of viscosity and radius ratios of shell to core phases in the droplet on the oscillation frequencies were investigated. However, the previous study used assumed values as physical properties of samples because of the insufficient data. In addition, the effect of temperature, which is one of the experimental conditions, was not investigated. Therefore, the objective of this work is to present proper experimental conditions in the ISS including the temperature and the radius ratio by numerical simulation with actually measured properties of samples. The first conclusion of this work is that the appropriate radius ratio is 1.2–1.4 depending on the experimental conditions. The second one is that all the measured interfacial tensions show low relative errors less than 10%, although the interfacial tension cannot be obtained under comparatively lower temperature, i.e., for higher viscous molten slag.

View full abstract