The review on various multi-phase flow phenomena on the ocean, ship and fuel injection process is presented. The multi-phase flows are on an oil spill in the ocean, the drag on a ship, the sloshing in a tank, the urea SCR system, the fuel injection system for Diesel engines and some others.

Cavitation occurring on propellers operating in ship wakes adversely affects vessel performance through hull vibration, thrust reduction, and erosion. With underwater radiated noise from ships under discussion at the International Maritime Organization for marine environmental protection, cavitation has been identified as a primary noise source. Accurate cavitation prediction methods at the propeller design stage require accumulating experimental data through model propeller tests. Quantitative evaluation of cavitation volume and bubble size distribution is particularly needed, as these parameters strongly correlate with pressure fluctuation and underwater radiated noise generation. This study presents two advanced cavitation visualization techniques developed to address these measurement requirements. First, a multi-view line sensing technique enables three-dimensional capture of tip vortex cavitation geometry, providing macroscopic cavity shape measurements. Second, a shadowgraph visualization method using a transparent airfoil model allows detailed observation and quantitative analysis of individual bubble characteristics, including bubble size distribution measurements. These complementary measurement techniques provide comprehensive characterization capabilities ranging from macroscopic cavity geometry to microscopic bubble properties. The developed methods contribute to improved cavitation prediction accuracy and support the development of quieter, more efficient marine propulsion systems.

Air lubrication is a technology that reduces frictional resistance by covering the bottom of a ship with a flow of air bubbles by releasing air from the bottom of a ship. It has attracted growing interest in recent years in efforts to achieve carbon neutrality in marine transport. While air lubrication reduces the power required for propulsion by reducing frictional resistance, it still requires power to pump air, and the released air can flow into the propeller at the stern, reducing propeller efficiency. There is a strong need for simulation technology to accurately predict these effects. This article explains simulations for predicting the reduction of frictional resistance on ships and simulations for predicting pressure loss at air injector.

To meet the International Maritime Organization's zero-emission targets, air lubrication systems are essential for reducing frictional drag on ships. This study focuses on bubble drag reduction and introduces a method to enhance its performance using artificial void waves. While natural void waves—spatiotemporal fluctuations in void fraction—are known to promote drag reduction, they occur uncontrollably in conventional constant air injection. We propose repetitive bubble injection, generating periodic high-density bubble clusters (artificial void waves) by pulsing the airflow. Experimental results from horizontal channel flows and a 36-m flat-bottomed model ship demonstrate that the artificial wave significantly improves drag reduction compared to continuous injection at equivalent average air flow rates. This improvement is attributed to the locally increased void fraction within the generated waves, which effectively modifies turbulent structures near the wall. Furthermore, when this method was applied to an actual ship, the net energy savings increased from 4% to 5%. To further optimize control parameters of this method, we also introduce novel laboratory-scale experimental setups—a belt-driven system and an expanding channel—that successfully reproduce void waves by simulating the developing boundary layer of a ship's bottom. These systems provide a platform for elucidating the detailed mechanism of void wave propagation and maximizing the efficacy of air lubrication.

Attempts have been made to convert the motion of automobiles and ships induced by road surface roughness and ocean waves into electricity using generators. Although wave-induced sea-surface roughness causes significant ship motion, it also offers considerable potential for wave energy harvesting, which can contribute to suppress such motion. Since 2008, the authors have been researching and developing small ships called WHzer (Wave Harmonizer) that both suppress motion and harvest wave energy. The development history of the small ships was summarized along with the functions of each configuration. The WHzer consists of a cabin, multiple floats, and a suspension system, in which the cabin and floats are connected by springs and suspension linkages. To suppress cabin motion, the springs are actively controlled to expand or contract based on the vertical velocity of the cabin. During power generation, electricity is produced when the springs return to their original length, driving a generator through a rack-and-pinion mechanism. The control model can be switched between the motion reduction mode and energy harvesting mode depending on whether people are onboard. A 1.6 m scale model, assuming an 8 m full-scale ship, was fabricated and tested under wave conditions in the IIS Ocean Engineering Basin. Subsequently, a 3.3 m prototype was constructed and tested in Yuya Bay off Nagato City in Yamaguchi Prefecture and off Hiratsuka City in Kanagawa Prefecture. The performance of motion reduction and energy harvesting was evaluated in real sea conditions. Based on the results obtained from the prototype experiments, the potential reduction in energy consumption was estimated for a fishing vessel equipped with this technology. In the future, further research and development are expected not only in wave energy harvesting but also in new energy-saving ships that utilize wave energy for motion suppression, including technologies that enhance propulsion through wave-energy-assisted propulsion systems.

We introduced a mass transfer calculation into the improved lattice Boltzmann method (LBM) for incompressible two-phase flows. To confirm the validity of the developed method, an unsteady mass transfer problem in a single droplet was simulated, and the numerical result was found to be in good agreement with the analytical solution. Subsequently, the mass transfer calculation method was applied to simulate collisions of binary droplets containing a very small amount of another miscible liquid, and the mass concentration distributions were investigated. In particular, we focused on the rotational separation phenomenon, i.e. colliding droplets coalesce, rotate generally by more than 180 degrees, and then separate from each other. It was found that the mass of the miscible liquid in each separated droplet is affected by the angular momentum at the collision. Furthermore, we defined and calculated the weighted angular momentum of the rotating droplets, which extracts the angular momentum in the high concentration region. The result showed that as the offset distance or the collision speed increases, the high concentration region of the droplets retains most of their pre-collisional angular momentum, which finally leads to enhancement of the mass transfer of the miscible liquid.

Ionic liquids are promising CO₂ absorbents due to their excellent CO₂ selective absorbability, non-volatile and chemically stable properties. In this study, a CO₂ absorption system using ionic liquids in a microchannel was developed, and the effect of the flow rate ratio between CO₂ gas and ionic liquid on the CO₂ absorption performance was investigated. Visualization of the two-phase flow revealed that CO₂ bubbles shrank in the flow owing to CO₂ absorption. Image analysis of the bubble size enabled the quantification of the amount of CO₂ absorbed and the absorption rate in this system. The results showed that increasing the flow rate ratio of CO₂ gas to ionic liquid enhanced the amount of CO₂ absorbed in ionic liquid. In addition, the CO₂ mass transfer coefficient was evaluated using the two-film theory. The CO₂ mass transfer coefficient exhibited an optimum value with respect to the flow rate ratio. Two-dimensional numerical simulation of the two-phase flow revealed that the effective circulation inside the liquid during advection was highest under the optimal condition.

The surface instability observed during the growth phase of a laser-induced bubble in agarose gels was examined utilizing the stability analysis method introduced by Yang et al. (2021). The agarose gel was modeled as the qKV model for viscoelastic fluids. Initially, the shear modulus G and the viscosity μ of the gel were determined by comparing the temporal evolution of bubble radii between experimental measurements and simulations based on the Rayleigh-Plesset equation for viscoelastic fluids. Utilizing G, μ, alongside experimentally obtained data on bubble radius, velocity, and acceleration, two types of instability parameters were computed for each surface oscillation mode, expanded using spherical harmonics. The analysis successfully predicted the instability of low-degree surface oscillation modes but was less accurate for high-degree modes. Additionally, the findings indicated that increasing shear modulus and viscosity tended to stabilize surface oscillation modes, with this effect being particularly significant for high-degree modes.