Yoshimichi Hagiwara

Turbulence modification by compliant skin and strata-corneas desquamation of a swimming dolphin

Direct numerical simulation has been conducted for pulsating or non-pulsating turbulent water flow near the compliant wall with the separation of pieces of the surface with time to examine how much the turbulence is modified and how the drag reduction is performed in swimming dolphins. The surface consists of a model of beads, springs and dashpots to represent the stratum cornea of dolphin skin. The computational results show that the compliant wall contributes to the drag reduction by reducing the number density of hairpin vortices. The small separated pieces increase the wall shear stress, which is consistent with our experimental result for turbulent water flow over a wavy wall covered with small pieces of film. This is because the models in the linear sublayer accelerate the flow between the models and the wall. The Reynolds shear stress decreases due to the attenuation of the hairpin vortices and the Q4 events by many models transported into the buffer region. This paper was chosen from Selected Proc...

Turbulence modification by the clusters of settling particles in turbulent water flow in a horizontal duct

Abstract Simultaneous visualization is carried out for depositing fine copper particles and fluorescent tracer particles in turbulent water flow in a horizontal duct with a new technique by using the color video camera. Particle image velocimetry (PIV) and particle tracking velocimetry (PTV) techniques are applied to the images of the scattered light from the depositing particles and those of the fluorescence of the tracer particles, respectively. It is found that the increase in the streamwise and downward mean velocities and the decrease in the streamwise turbulence intensity are mainly caused by some copper-particle clusters, which modify near-wall turbulence structure, and some other clusters, which fall with higher velocity than the still-fluid settling velocity and are elongated in the vertical direction. The Stokes number based on the initial state for the group of particles introduced into the flow is found to be one of the criteria for particle clustering in the flow.

Turbulence modification in a homogeneous turbulent shear flow laden with small heavy particles

Abstract Numerical simulations have been conducted for a homogeneous turbulent shear flow laden with small heavy particles in order to investigate the modification of turbulence structures in shear flows due to particles. The effects on the turbulence modification of gravitational settling of particles in the sheared direction are examined for the particles whose response times are comparable to the Kolmogorov time-scale of turbulence. It is found that the growth rate of turbulence energy of the carrier fluid, which is reduced by the particles in zero gravity, is increased by the effect of weak gravity through the enhancement of Reynolds shear stress, but decreased by the effect of strong gravity through the increase in drag dissipation. It is shown in finite gravity that particle clusters are generated due to the accumulation of particles in two types of regions; downward flows sandwiched between counter rotating quasi-streamwise vortex tubes and regions just beneath the vortex layers with negative spanwise vorticity. The particle clusters between the pair vortex tubes intensify the downward flows between them to enhance the Reynolds shear stress, while the particle clusters beneath the vortex layers enhance the drag dissipation of fluid turbulence energy in the streamwise direction. The downward flows induced by particle clusters activate the tilting of the spanwise vorticity toward the vertical direction.

PTV measurement on interaction between two immiscible droplets and turbulent uniform shear flow of carrier fluid

Abstract A flow visualization and a particle-tracking-velocimetry measurement have been conducted for a turbulent water plane Couette flow with two oil droplets in the central region in order to elucidate the effect of secondary flow induced by the droplets on the modification of shear-dominant turbulence. The diameters of these droplets are about 80 times larger than the micro scale of turbulence. The tracer particles coated with fluorescence color and the optical filter for the video camera is found to be effective for diminishing the reflected light from the interface and the moving wall. The experimental results show that zero-velocity regions exist near the interface around the axis because the direction of the wall-normal secondary flow is opposite to that of water moving with the rotating droplets. It is also found that the turbulence intensities in the region between the two droplets and in the small regions for the confluence and stagnation are attenuated due to the weakening of the wall-normal secondary flow particularly in the case where the distance between the centers of the two droplets is twice as long as the droplet diameter.

Numerical simulation of the interactions of highly entangled polymers with coherent structure in a turbulent channel flow

Abstract Direct numerical simulations have been conducted with many cluster models of beads and connecting springs in a turbulent channel flow in order to investigate how the low-speed streaks and the small-scale flow related to the streaks are affected by the cluster models. The cluster model represents highly entangled polymers observed in the quiescent and flowing aqueous solution. The cluster models were introduced in the buffer region to simulate the experiments of Tiederman et al. (Tiederman, W.G., Luchik, T.S., Bogard, D.G., 1985. Wall-layer structure and drag reduction. J. Fluid Mech. 156, 419–437) in which the polymer solution was injected from a slot in the channel wall. An experiment has been carried out for visual observation of highly entangled polymers in a turbulent channel flow of the same Reynolds number in order to determine a parameter of the cluster model. The computational results show that the minor streaks and the small-scale eruptive flows associated with the streamwise vortices are attenuated selectively by the cluster models. The length scale and time scale of these structures are comparable to those of the cluster model. On the other hand, the dynamics of large-scale streaks are found to be basically unchanged. These results are consistent with the measurement of a suppression of low threshold Reynolds-stress producing motion by Harder and Tiederman (Harder, K.J., Tiederman, W.G., 1991. Drag reduction and turbulent structure in two-dimensional channel flows. Philos. Trans. R. Soc. Lond. 336, 19–34).

Experimental study on the interaction between large scale vortices and particles in liquid–solid two-phase flow

Abstract We have carried out an experiment on the interaction between concentrated particles (hereafter called the Cluster) and Karman vortices in solid–liquid two-phase flow. The Karman vortices have been obtained by moving a cylinder in a shallow towing tank. The density of the particle is nearly the same as that of liquid. We have adopted High-resolution PIV to distinguish the Cluster velocity from the vector field of the surrounding flow. From the obtained data, we conclude that the Clusters enhance the steady rigid body rotation of the Karman vortices and the flow along the vortices. Furthermore, the Clusters keep their rotational motion and do not follow the flow along the vortices completely.

Effects of ions on winter flounder antifreeze protein and water molecules near an ice/water interface

We have carried out molecular dynamics analysis on an aqueous solution of winter flounder antifreeze protein (AFP), sodium ions and chloride ions near ice surfaces to understand the effect of ions on water and protein. It is found from the statistical quantities that the translational motion and freely rotational motion of water molecules near the ions are restricted. The interaction between the AFP and water molecules, which are hydrated with the ions, is attenuated. When the ion concentration near a part of the protein and the ice surface is higher than the average, the part of the protein approaches the ice surface.

Interaction between a twelve-residue segment of antifreeze protein type I, or its mutants, and water molecules

A molecular dynamics simulation has been carried out for water molecules with a rigid segment of antifreeze protein type I. The segment consists of nine alanine residues, two threonine residues and one asparagine residue. Mutant segments, in which the threonine residues are replaced with valine residues, or serine residues, are also used. It is predicted that the hydrogen site of asparagine residue, and that of threonine residue, play an important role in the hydrogen bond of water molecules in these sites. This hydrogen bond is not noticeable between water molecules and the valine residue, or serine residue. The existence of four hydrophilic sites enhances the mobility of water molecules close to the serine residue of the mutant segment. The difference in the zenith-angle fluctuations of the original segment and the valine-mutant segment is less noticeable in the case of 230 K. This is because the gathering of water molecules due to the hydrophobic hydration is predominant near the alanine residues of the segments at this temperature.

Inhibition of Ice Nucleus Growth in Water by Alanine Dipeptide

A molecular dynamics simulation has been carried out for the mixture of an ice nucleus, supercooled water and a molecule of alanine dipeptide (AD). The dipeptide molecule has been allocated near the nucleus surface which corresponds to the prism plane of ice crystal. The molecule is found to approach the ice surface so that the two hydrophilic sites on one side of the molecule (Oc2 and Hn1) are closest to the surface. The hydrogen bond between Hn1 site and the oxygen atom on the prism plane of the ice nucleus is expected. The perturbations of two hydrophilic sites (Oc1 and Hn2), which are surrounded by hydrophobic sites and are pointing away from the surface, attenuate the approach of water molecules to these sites. Thus, these water molecules diffuse. The hydrogen bond between the oxygen atoms on the prism plane and the hydrogen atoms of water molecules is attenuated by the diffusion.

Molecular Dynamics Simulation for the Mixture of Water and an Ice Nucleus

We have carried out a molecular dynamics simulation for the melting and solidification of ice nucleus. An ice crystal was formed by controlling the displacement and velocity of each molecule. Part of the ice was melted by supplying energy to it in order to produce a mixture of an ice nucleus and surrounding water at approximately freezing point. By using this mixture, a mixture of an ice nucleus and supercooled water was made. It was found that the time scale for the correlation between the potential energy and the rotational component of the kinetic energy was different from that of the correlation between the potential energy and the translational component of the kinetic energy. Also, it was predicted that the ice nucleus received forces whose directions were not exactly the same as those of the growth of ice crystal.