The aim of the present study is to create new technologies for solving problems such as ship fouling control and ballast water treatment in the field of maritime sciences. We report the investigation on underwater shock waves generated by the collapse of fine bubbles in cavitating flow fields. The cavitating jets are generated by four types of orifices and a triple-plunger pump. The schlieren method is used to clearly capturing the underwater shock waves. The schlieren images reveal that the local high-brightness region appeared in the shadow of the bubble flow moves downstream accompanied with shock wave generation. The relationship between the movement of the high-brightness region and the shock wave generation is correlated to the frequency of the chained collapse of cavitation bubbles. It is suggested that the movement of the high-brightness region depends on the jet flow velocity, the number density of bubbles, and the bubble contraction rate determined from the bubble diameter, the internal pressure of bubbles, and the strength of external shock wave pressures.

Membrane bioreactor (MBR) is a promising technology for future wastewater management. However, occurrence of membrane fouling in the process hinders widespread application of this technology. There are two types of membrane fouling: physically reversible fouling and physically irreversible fouling. The former is the fouling that can be cancelled by physical cleaning such as hydraulic backwashing, whereas the latter is the fouling that needs chemical cleaning to be cancelled. In the operation of MBRs, physically reversible fouling is usually dominant. Therefore, control of physically reversible fouling with a low energy consumption is important. In most of commercialized MBRs, control of physically reversible fouling is attempted by intensive aeration through aerators placed beneath the membrane modules. Various design factors including bubble size, air flow rate and arrangement of aerators and membranes would affect the efficiency of cleaning by aeration. Currently, however, it is totally unclear what is occurring in MBR tanks: multiphase flow in MBRs composed of biomass suspension with high viscosities, bubbles and membranes is too complex to monitor. As a result, no rational method for design of aeration for MBRs is available, which leads to considerable underestimation of the performance of MBRs. Application of advanced research methods of multiphase flow could change this situation: rational design of MBRs is enabled by the aid of cutting-edge knowledge on multiphase flow.

Membrane separation technologies has been widely applied to water treatment and importance of fouling mechanism of membrane and development of fouling mitigation methods is now one of the key issued in water treatment technologies using membrane separation. Outline of membrane bioreactor (MBR) technology, which uses membrane separation as solid-liquid separation is here explained and two examples of experimental research to investigate influence of air-water multiple phase flow characteristics on the performance of the MBR performance are shown.

Although water is a continuous phase, water treatment tanks often contain solid particles and bubbles, thereby adding a dispersed phase. Thus, water treatment tanks will often contain a multi-phase fluid. The solid particles and bubbles contribute significantly to the formation, promotion, and suppression of circulating flows that have a significant effect on water treatment performance. Even with just a small number of bubbles present in the dispersed phase, a density current is generated and a circulating flow is formed－nd a similar outcome is seen, when solid particles exist in the dispersed phase, due to the high bulk density. However, when the concentration of a solid is very high, friction between the solid particles increases, thereby greatly affecting the sludge viscosity. The increased viscosity suppresses the formation of circulating flows, which reduces the mixing performance of the solution. The circulating flow in an aeration tank or dissolved air flotation (DAF) tank and settling tank then have low oxygen transfer and solid separation efficiencies, respectively. This undesirable situation is due to the suppression of a circulating flow, which prevents deposition of the solid particles in the aeration tank and reduces the mixing efficiency of the anaerobic digester. Computational fluid dynamics (CFD) helps to investigate the effects of multi-phase flow on the performance of water treatment tanks. In this study, thermal convection in settling tanks, sponge-media deposition in an aeration tank, mixing performance in an anaerobic digester, and the rheology of dense anaerobically-digested sludge will be studied, and activated sludge model (ASM) and CFD coupled simulation for simultaneous nitrification and denitrification in shallow aeration tanks will be performed.

Acoustic levitation is a technique for the contactless manipulation of objects. In previous study, the contactless manipulation and coalescence of droplets were succeeded by using ultrasonic phased array. Additionally, a mixing of the droplet was enhanced by introducing oscillation on the interface of droplet. However, the mechanisms of the enhancement and the optimal conditions for mixing of the droplet are not yet understood. Therefore, we focused on the stretching of fluid particles that directly contributes to mixing and measured it experimentally. First, we performed the LIF method. It was confirmed that the mixing of droplet was enhanced by inducing interfacial oscillation. Moreover, the distribution of the fluorescent dye changed into a complex stripe pattern. Secondly, we visualized the internal flow of droplet by using fluorescent particles. As a result, the trajectories of the particles were sufficiently mixed. For quantitative evaluation of the mixing characteristics, we calculated the increase rate of the distance between nearest neighbor particles to reveal the agitation. The time averaged distribution demonstrated the agitation was changed by the interfacial oscillation. Finally, finite time Lyapunov exponent (FTLE) was introduced to characterize the mixing behavior of the droplet. FTLE was proportional to the amplitude of oscillation. From these results, it is considered that the mixing of droplet was enhanced by the chaotic advection occurred due to the interfacial oscillation of droplets. Furthermore, the stretching effect of the fluid particles inside the droplet increases in proportion to the amplitude of the interfacial oscillation relative to the droplet diameter.

Drops propel in a particular direction on a heated surface if the surface has a fore-aft asymmetrical geometry. We have developed a new passive cooling technology using the drop propulsion. We, therefore, focus more on the propulsion at lower temperatures at which the boiling occurs at the heated surface. This study aims to evaluate the propulsion behavior with boiling by controlling the area of the hydrophilic region on the surface. In this study, we changed the area of the hydrophilic region with good reproducibility and high accuracy by using either a toothpick scratching or a plasma cleaning. As a result, we clarified that the acceleration of the drop marked the highest value with the width of the hydrophilic surface of about 0.3 mm because the propulsion force due to boiling overcame the force on the contact line the most with the width. We also clarified that the maximum climbing angle of the drop coincided with the angle at which the drop stayed in place.

The rapid cooling single-roll method is widely used to produce amorphous metal ribbons. To obtain more rapid and homogeneous cooling in the method, one needs to prevent the molten metal / solid interface from forming gas pores. We experimentally investigated the pore formation mechanism by directly observing the impact dynamics with the solidification of molten tin drops on a glass and sapphire substrate under an argon gas environment. We also measured the time variation in the temperature distribution on the substrate surface. As a result, we clarified that pores were formed mainly on the poorly conductive substrate, glass. In contrast, on the highly conductive substrate, sapphire, intermittent solidification layers were formed with thin gas layers between them. Furthermore, we derived two characteristic time scales for solidification relating to the sensible and the latent heats by employing one-dimensional thermal conduction analysis.

There are a few experimental studies on the flow past porous media due to its difficulties of measurement without disturbing the flow. In particular, there have been very few studies on the relationship between the shape of the objects constituting the porous media and the turbulence generated in the passing flow. In this study, the characteristics of the flow past a spheroid and arrays of spheroids were investigated by carrying out hydraulic experiments using the PTV technique. It was clarified that the complicated velocity field with the organized vortex and fluctuations with wide range of frequency occurs in the wake of them. According to a series of velocities, the different organized vortex is generated behind a spheroid and the rows of spheres.

Prediction of maximum spreading diameters of impacting drops is crucial for determining the quality and efficiency of many industrial applications using drop-on-demand or spray technology. In order to construct an exact theoretical model for predicting the maximum diameter, one needs to determine a time constant being required for the radial pressure gradient in a spreading drop to be disappeared, which is difficult to derive theoretically. We experimentally obtained the time constant by focusing on the energy budget during drop impact, especially on the viscous dissipation. By considering the time delay, we developed a theoretical model for predicting the maximum spreading diameters. The model shows good agreement with experimental results, revealing the importance of the internal velocity field in a drop.

The calculation results on the 3-D fluid motion induced by the oscillating circular cylinders through the air-water interface are presented. The boundary immersed method without tracking the gas-liquid interface by introducing the pressure divided by the fluid density is used. The calculation results were qualitatively verified by comparing them with the experimental studies. The results imply that the three-dimensional flow induced by the oscillating cylinders is extremely complex despite the simplicity of the system.

Wall functions are generally used in a numerical simulation of turbulent flows with computational fluid dynamics (CFD) codes. The logarithmic law is widely adopted for the wall functions, but its validity has not been confirmed for dimensionless profiles of temperature and steam mass fraction in condensation fields due to lack of data. In this study, we evaluated dimensionless profiles of temperature and steam mass fraction in flows of saturated steam and air mixtures in a vertical pipe with the diameter of 49.5 mm. The local Nusselt number Nu_y and Sherwood number Sh_y (which were proposed in our previous study) were used for defining the dimensionless temperature T⁺ and steam mass fraction Y_s⁺, respectively, because physical properties in computation cells in contact with the condensation surface are generally used in a CFD analysis. The T⁺ and Y_s⁺ values obtained from the measured temperature profiles were smaller than the existing logarithmic law due to condensation in the turbulent boundary layer (i.e. mist generation), and correlations for T⁺ and Y_s⁺ were obtained by using the least square method. The standard deviation of the correlations were 3.8 % and 0.75 ℃ for the steam mass fraction of X_s = 0.05-0.63 and the mixture temperature of T_g = 63-97 ℃, respectively.

The relationship between fluid permeability and pore structure in fibrous bed with large porosity has been investigated. Fibrous beds expressed by particle model and random particle beds were used for the permeation media having different pore structures and solid volume fractions. The permeability was calculated by Stokesian dynamics (SD) approach on the assumption of the Stokes flow. Pore structure analysis was performed on the basis of Voronoi tessellation, which enabled the extraction of pore bodies and their links. From these information, we suggested one way to estimate the hydraulic radius. The results of the hydraulic radius were uniquely related to the permeability calculated by SD method. The hydraulic radius suggested in the present study could be useful to predict fluid permeability by considering internal pore structure as well as solid volume fraction.