Ichiro Kumagai

On the fate of mantle plumes at density interfaces

Abstract The motion of plumes through compositionally layered systems is investigated by laboratory experiments in order to understand the ascent modes of deep mantle plumes. Experimental plumes entrain the ambient heavier fluid and the average density of the plume head ρP(t) increases with time. The degree of entrainment depends on the viscosity ratio of the matrix fluid μmatrix to the buoyant fluid μbuoyant, and the mass flux. Two types of plume penetration through the density interface are possible. The first, which can be characterized as a ‘pass-through mode’ (PTM), exhibits relatively little entrainment. Here the whole plume head passes directly through the interface because ρP(t) is smaller than the density of the upper layer ρu. In contrast, in the second type of interaction (named the ‘rebirth mode’, RBM), a large entrainment effect takes place. The plume is now greatly disturbed as it passes through the interface between the two fluid layers and entrained material remains at the density interface since ρP(t)>ρu. In addition, a new diapir emerges from the plume conduit and rises through the upper layer. The difference between these two types of interaction is reflected in the spatial and the temporal characteristics of geochemical data obtained from mantle plume products such as Ontong Java Plateau (OJP).

Experiments on bubble generation by a hydrofoil moving beneath the water surface for reducing ship drag

We have invented two types of hydrofoil bubble generator for drag reduction of ship that can reduce the energy for air bubble generation on the ship hull. Their fundamental process of air entrainment and subsequent bubble generation by the hydrofoil facility are described by a simple fluid dynamic model. We experimentally determined the critical velocity of the bubble generation and the relationship between air volume flow rate and the hydrofoil velocity. The magnitude of the negative pressure produced above the hydrofoil, which is a driving force of the air entrainment, depends on the shape of the hydrofoil, gap ratio (normalized depth of the hydrofoil), Reynolds number, Froude number, and angle of attack. Recent applications of the drag-reduction technology with air bubbles to a ship save about 10%–15% of the total energy consumption of the ship. The device works as a self-priming pump when the draft of the ship is shallow (< ∼5 m) as predicted by the theory. For ships of deeper draft, the device needs ...

Story visualization of novels with multi-theme keyword density analysis

A new method for visualization of stories in literary works was explored. Our story visualization method consists of two parts: keyword-based statistical analysis for multiple themes and imagery expression of the results for visual understanding. In this study, we focused on novels as the targets, and discussed ways in which complex structures can be simultaneously visualized using multiple themes. The method was applied for the comparison of Charles Dickens’ novels with Shakespeare’s plays in order to identify any existing evidence concerning literal interest created by the overlapping of multiple scenarios in a single story. We also applied the method to non-literary documents such as newspaper articles, and showed that these documents contain simple statistic patterns regarding a given theme, which contrasts with the case involving novels that include the dynamic fluctuation of individual story elements.Graphical Abstract

Lagrangian Frame PIV for capturing vortex shedding from a moving cylinder close to a free surface

We have conducted laboratory experiments on the flow past a circular cylinder moving beneath an air-water interface. The vortex street and the free surface profile are visualized for Reynolds number between 200 and 15000 and for Froude number between 0.01 and 3.2. For the lower Reynolds number (Re≦3000), the vortex shedding from a circular cylinder is suppressed by the free surface at the small gap ratio for Re 10000), the surface deformation becomes substantial in the downstream of the cylinder and intermittent bubble injection by wave breaking is observed. In order to investigate the relationship between the frequency of the vortex shedding and the free-surface wave behavior, Lagrangian Frame PIV measurement is conducted. From the results of Strouhal-Reynolds number relationship for gap ratios between 0.25 and 2.0, the period of the vortex shedding becomes long when the g...

Interaction of periodic wave trains in magma conduits

Abstract The interaction of two parallel periodic wave trains in vertical conduits has been studied through a simple laboratory analog experiment, and it is described (at a low-Reynolds number≪1). The observations of the shape and the phase velocity of waves show that these wave characteristics are modified by their interactions. There are three modes of interaction of periodic wave trains: (1) in-phase slow periodic wave train mode; (2) in-phase fast periodic wave train mode and (3) out-of-phase fast periodic wave train mode. The geometry of the periodic wave trains and the normalized amplitudes of the wave train, A, control the mode. Each mode has a characteristic spatial and temporal distribution of waves as a result of the interaction of wave trains. These spatial and temporal variations generated by the interaction of wave trains may be observed in various magmatic activities, for instance, mid-ocean ridges, island arcs, and hotspots.

Thermal Convection of a Phase-Changing Fluid

To investigate the effect of phase transitions of the fluid on thermal convection, we are conducting experiments of thermal convection of the mixture of a thermosensitive gel and water. The gel absorbs water and swells below a certain critical temperature, while it discharges water and contracts above the critical temperature. The swelling ratio, as well as the critical temperature, can be controlled, which enables us to investigate the effect of phase transition in a series of experiments, where the critical temperature is set between the top and bottom boundary temperatures and the swelling ratio is systematically changed. In flow visualization, we have observed temporally stable and spatially fixed low-velocity regions which are surrounded by distinct high-velocity regions. Such a phenomenon is not observed in thermal convection of a single-phase fluid like water or air, and peculiar to the phase-changing fluid. Also, we have measured the rheological properties of the fluid and found, for example, the shear-thinning behavior, with which we try to explain the observed convection behaviors.

Flow of a Viscoelastic Fluid Around a Falling Sphere

Velocity vector fields around a falling sphere in a 1.0 wt % polyacrylamide (PAA) solution are obtained on a vertical cross section by particle image velocimetry (PIV). PAA solution is known as non-Newtonian fluid, which has shear thinning and viscoelastic property. Strain rate tensor fields and deformation fields are calculated from the velocity vector fields in order to visualize the dynamic behavior of the fluid quantitatively. In velocity vector field, two typical flow regions are observed in the wake of the sphere: approaching flow to the sphere, rising flow called “negative wake” [1]. Results show that the strain rate tensor field gives fluid strain at the approaching flow region and the edge of the negative wake. Furthermore deformation history of one portion of the fluid shows that fluid is strained in the approaching flow region, and the strain rate at the edge of the negative wake represents their recovery to the original status of the fluid in the moving frame.Copyright

Dynamics of Cell Pattern Formation in Internally Heated Convection Viewed From Local to Global Particle Image Thermometry

Internally heated convection is a fundamental phenomenon, largely governing the dynamics of natural systems such as the atmosphere and Earth’s mantle. It also plays an important role in industrial applications. Here we have investigated the separation of the top thermal boundary layer in order to understand the cell enlargement and the dynamics of the cell pattern formation. To observe the development of the thermal boundary layer non-invasively, the temperature distribution of the vertical plane in a convective cell was visualized by particle image thermometry (PIT). Micro-encapsulated thermo-chromic liquid crystals (TLCs) were seeded in the test fluid and illuminated by a white light sheet, and scattering light was taken by a digital camera. For quantitative temperature measurement, we have calibrated the temperature changes with the variation of the hue color component. The development of the thermal boundary layer with respect to the Rayleigh number has been investigated. The results show the local Rayleigh number determined from the thickness of the thermal boundary layer, which increases towards a critical local Rayleigh number ∼600.Copyright