Osamu Mochizuki

Turbulence properties of an axisymmetric separation-and-reattaching flow

The time-mean and root-mean-squared values, the integral time scale, the phase velocity, and the longitudinal and circumferential length scales of the velocity and the surface-pressure fluctuations are presented for a separation-and-reattaching flow formed by the boundary-layer separation at the leading edge of a blunt circular cylinder at Reynolds numbers of the order of 10 5 . The cross correlations of the surface-pressure fluctuations suggested that the flow in the reattachment region of the separated shear layerhas a cellular structure.

Micro-PIV (micro particle image velocimetry) visualization of red blood cells (RBCs) sucked by a female mosquito

A mosquitos pump is a highly effective system in the small suction domain. To understand a mosquitos blood suction mechanism, we analysed the characteristics of red blood cells (RBCs) in human blood during and after suction by a female mosquito. Focussing on the flow patterns of the RBCs in human blood being sucked by a mosquito, we visualized blood flow by using a micro-particle image velocimetry (μ-PIV) system, which combines an optical microscope and a PIV method. In an ex vivo experiment, a female mosquito was supplied diluted blood at the tip of the proboscis. We examined the blood flow around the tip of the proboscis and observed that RBCs were periodically sucked towards a hole around the tip. The sucked RBCs then homogeneously flowed parallel to the inner surface of the proboscis without adhering to the wall. Furthermore, using a bioelectric recording system, we directly measured electrical signals generated during suction by the pump muscles located in the mosquitos head. We found that the electrical signal power was synchronized with the acceleration of the RBCs in the sucking phase. A histological stain method was adapted for the observation of the form and internal structure of RBCs in the mosquito. Although the blood flow analysis revealed that the RBCs underwent shear stress during suction, RBCs in the mosquitos stomach maintained their original shape.

Pressure variation on a flat wall induced by an approaching vortex ring

Variation of pressure was measured by using a condenser microphone, when a vortex ring was approaching a flat wall. Considerably large negative pressure was observed even at the stagnation point on the wall. This negative pressure was found to occur when the ring “rebounded” from the wall. Before the ring rebounding, the pressure variation could be predicted approximately by the potential flow theory. After the rebounding, it showed negative values, entirely different from the theory. However, this fact can be illustrated by the potential theory applied to a simple, modified model of trajectory of the ring during the rebounding, which takes account of the viscosity effect.

Splash Formation by a Spherical Body Plunging into Water

Splashes caused by a spherical body plunging into water were investigated experimentally using a high speed CMOS camera. We categorized types of splash according to impact velocities of the sphere. Three types of splash were found: Type-I is a thin spire-type splash, Type-II is a mushroom-type splash with many droplets, and Type-III is a crown-type splash with many droplets. The reaction to the concave water surface attached to the sinking sphere is a cause of the Type-I splash. The film flow climbing up the sphere is a dominant cause of the Type-II splash. The velocity of the film flow, which is proportional to the impact velocity of the sphere, affects the fingers of the film flow, detaching of droplets, and maximum height of the Type-II splash. The Type-III crown-type splash is characterized by water jets with many droplets. A bulky air column in water is formed behind the sinking sphere, and longitudinal ridges and ripples on the surface of the air column were observed.

Some remarks on surface conditions of solid body plunging into water with particle method

BackgroundThe water splash patterns strongly depend on the surface conditions of the solid object. The present paper discusses the influence of the surface conditions of the solids falling into the water on the formation of the splashes, employing the experimental method with the high speed video camera and the numerical approach by the particle method.MethodsWe propose two engineering models for calculating the Navier–Stokes flows to distinguish the different surface conditions of the solids falling into the water. One is to add the attractive or repulsive force between the water and the surfaces of the solids, and the other is to consider the swelling ratio as the slip condition, which causes the different wall shear stresses at the interfaces between the solids and the water.ResultsThe above models are successfully employed to study the effects of the differences of the surface conditions of the falling objects on the splash formations.ConclusionsWe have successfully calculated the splashes caused by the different surface conditions of spheres using the MPS method.

Influence of head shape of solid body plunging into water on splash formation

We experimentally investigated the influence of a head shape of a solid body plunging into water on splash formation. Three different head shapes were tested: a hemisphere, cone, and circular cylinder. A hemisphere as a tail shape is common to all three head shapes. We captured images of splash formation using a high-speed CMOS camera. We found that a film flow generated at an early stage when a body impacts the water surface influences subsequent events until the splash sequence is completed. We explain the origin of the film flow according to the principle of conservation of momentum. The film flow as the primary splash originates from water displaced by the head. The meridian line, which connects the head to the tail of the body, affects separation of the film flow and causes the secondary splash. The air cavity generated when the body plunges into the water is also influenced by the head shape. The tertiary splash is formed by a reaction of the air cavity, which is detached from the body. We found that the secondary dome-type splash obstructs growth of the tertiary splash. Thus, we conclude that the head shape affects all events of the splash.Graphical Abstract

The Flow Induced by a Jellyfish

The purpose of this study is to understand the propulsion mechanism of a jellyfish during its swimming. We observed the motion of a jellyfish (Aurelia aurita) by a motion-capture camera, and measured the vector field of flow around a jellyfish by using a PIV (Particle Image Velocimetry) measurement. A jellyfish is considered to be principally propelled by a jet at the contracting phase of its motion. If that is true, it is interesting that a jellyfish never stops traveling even at the expanding phase. We found that a vortex ring with the opposite vorticity to shed vortex ring was inside a jellyfish body in the expanding phase. We discussed a cause of an increase in thrust force and keeping constant speed in the expanding phase.

Micro PIV measurement of slip flow on a hydrogel surface

Slip flow on a hydrogel surface was investigated in order to clarify the effect of drag reduction on the aqueous surface of living things. Thin-film flow along the hydrogel surface was measured by using a micro PIV (particle image velocimetry) system for comparison with theoretical velocity distribution which satisfied the non-slip condition on a solid surface. The slip flow on the hydrogel was found to be related to the degree of swelling and molecular weight of the hydrogel materials. This shows the possibility of a reduction in wall shear stress as a result of the decrease in the velocity gradient near a wall surface.

The Effect of Particles on Electrolytically Polymerized Thin Natural MCF Rubber for Soft Sensors Installed in Artificial Skin

The aim of this study is to investigate the effect of particles as filler in soft rubber sensors installed in artificial skin. We examine sensors made of natural rubber (NR-latex) that include magnetic particles of Ni and Fe3O4 using magnetic compound fluid (MCF). The 1-mm thickness of the electrolytically polymerized MCF rubber makes production of comparatively thin rubber sensors feasible. We first investigate the effect of magnetic particles Ni and Fe3O4 on the curing of MCF rubber. Next, in order to adjust the electric properties of the MCF rubber, we adopt Al2O3 dielectric particles. We investigate the effect of Al2O3 particles on changes in electric current, voltage and temperature of electrolytically polymerized MCF rubber liquid, and on the electric properties under the application of normal and shear forces. By adjusting the ratio of Ni, Fe3O4, Al2O3 and water in MCF rubber with Al2O3, it is possible to change the electric properties.

Velocity profile of thin film flows measured using a confocal microscopy particle image velocimetry system with simultaneous multi depth position

In this paper, we report a technique for simultaneously visualizing flows near walls at nano-depth positions. To achieve such a high interval of depth gradient, we developed a tilted observation technique in a particle image velocimetry (PIV) system based on confocal microscopy. The focal plane along the bottom of the flow channel was tilted by tilting the micro-channel, enabling depth scanning in the microscopic field of view. Our system is suitable for measuring 3D two-component flow fields. The depth interval was approximately 220 nm over a depth range of 10 μm, depending on the tilt angle of the micro-channel. Applying the proposed system, we visualized the near-wall flow in a drainage film flow under laminar conditions to the depth of approximately 30 μm via vertical scanning from the bottom to the free surface. The velocity gradient was proportional to the distance from the wall, consistent with theoretical predictions. From the measured near-wall velocity gradient, we calculated the wall shear stress. The measurement accuracy was approximately 1.3 times higher in our proposed method than in the conventional confocal micro-PIV method.