Kazuyasu Sugiyama

Microbubble Transport in Turbulent Channel Flow

Microbubble transport in fully developed turbulent channel flow is investigated using an Eulerian-Lagrangian approach. The carrier-phase flow is computed using Direct Numerical Simulation (DNS) or Large Eddy Simulation (LES) of the incompressible Navier-Stokes equations. Lagrangian particle tracking is employed for a dispersed phase comprised of small, rigid spheres of negligible density compared to the carrier-phase flow and obeying an equation of motion in which the forces used to predict the motion of the bubble are drag, pressure gradient, and added mass. In general, DNS and LES yield similar predictions of the carrier phase flow and dispersed-phase properties. The bubble Stokes number is varied over a range for which the dispersed phase essentially follows the carrier flow to larger values for which strong segregation of the microbubbles into coherent vortical structures occurs. In general, simulation results show that microbubble response is not a monotonic function of the Stokes number. The most significant structure in the concentration field occurs for Stokes numbers close to the turbulence timescales in the buffer layer. More than 2/3 of the microbubble population in the buffer layer resides in coherent structures that occupy approximately 1/3 of the computational volume.Copyright

Continuum and stochastic approach for cell adhesion process based on Eulerian fluid-capsule coupling with Lagrangian markers

Abstract This paper presents a novel development for a full Eulerian coupling method between a fluid and capsule with hyperelastic membrane to address a cell adhesion process. The capsule motion in the fluid field and their mechanical interactions are expressed in the Eulerian framework, while solving the capsule adhesion process by stochastic events of ligand-receptor bindings in a discrete level, with using Lagrangian markers distributed on the implicit surface of the capsule. In addition to this, numerical improvements of the full Eulerian method for calculating an interface normal vector and dealing with multiple capsules by introducing the temporarily constructed level set function. A numerical validity of the present development is investigated through a comparison with available numerical data in a validation problem, and a capability for the capsule adhesion is shown in a single capsule adhesion and platelet adhesion in a suspension with blood cells.

Multiscale Simulations for Fluid Structure Interaction Problems with Biomedical Applications

A numerical method for massively parallel computing to solve fluid-structure interaction problems was developed and the method was employed for solving the multiscale problems in biomedical applications. As one of the examples, a platelet adhesion process to the vessel wall, which occurs at the initial stage of a thrombosis, was analyzed using the multiscale method of coupling continuum scale finite difference method with the molecular scale Monte Carlo method. The platelets adhesion to the injured vessel wall is caused by the protein-protein binding (GP1b-α on the platelet—VWF on the wall.). This protein-protein binding force is evaluated by Monte Carlo simulation, solving the stochastic process of each biding. Adhered platelets also feel the fluid mechanical force from blood flow and this force is affected by the presence of red blood cells, which causes the drastic change to the adhesion process. As another example of multiscale simulations, ultrasound therapy method using microbubbles are also explained.

Sub- and Super-Synchronous Self-Excited Vibrations of a Columnar Rotor Due to Axial Clearance Flow

Sub- and super-synchronous self-excited vibrations due to axial clearance flows were observed in a columnar rotor with an upstream seal in experiments. A smaller clearance on the downstream seal had a larger effect of stabilizing the rotor. In computations, it was found that the rotordynamic fluid force tangential to the whirling orbit, which is caused as a response to the vibrations (whirling motions), destabilizes the rotor in the case of the upstream seal and stabilizes the rotor in the case of the downstream seal. It was clarified in the 1-D flow model that the tangential rotordynamic fluid force is mainly caused by an inertia of the clearance flow.

Effects of the Encapsulating Membranes on the Translations of Pairs of Non-spherical Bubbles

In this paper, we perform a theoretical analysis on the translational motions ofgas and encapsulated bubbles undergoing deformation. We verify our theoretical model for the case of a gas bubble collapsing near a wall. By comparison, the encapsulated bubble near the wall is less unstable in shape. In the second case, the translations of two gas bubbles subjected to an oscillating driving pressure agree with Bjerknes theory, that is, the two bubbles repel each other when the driving frequency is between the two natural frequencies ω01 ω02. For encapsulated bubbles, the translational displacements are smaller than those of gas bubbles. The encapsulating membrane and the deformation affect the translational direction. The encapsulated bubbles are prone to experience repulsive motion at resonance.

A Full-Eulerian Approach for the Fluid–Structure Interaction Problem

A fixed-mesh method is developed for the fluid–structure interaction problem, based on a fully Eulerian formulation. A material phase for an elastic solid/membrane immersed in a fluid field is represented by a volume-fraction function (or phase function) without any material point. In addition, its material deformation at a current configuration is given by solving a transport equation for the left Cauchy-Green deformation tensor on a fixed Eulerian mesh. A set of partial differential equations in a mixture form is monolithically discretized by a finite difference/volume method which has been developed in the field of the multiphase flow analysis. The present fully Eulerian method does not require a mesh generation, mesh moving and remeshing (or reconnection) procedures, thus it straightforwardly addresses not only biological problems in which geometric data are given by a measurement image, but also suspension flows in which soft materials are largely moved and deformed in a fluid, e.g. blood flow with multiple red blood cells and platelets, without any numerical or technical modification. A numerical accuracy of the present method is well investigated in a grid refinement manner and also in comparisons with that of the existing Lagrangian methods.

Theoretical study on the shape instability of an encapsulated bubble in an ultrasound field

A theoretical study on the shape instability of a slightly deformed bubble encapsulated by a viscoelastic membrane in an ultrasound field is performed. The membrane effects of the inplane stress and the bending moment are incorporated into the traction jump condition at the bubble surface. The spherical motion of the bubble is numerically obtained by solving the Rayleigh-Plesset equation with the elastic stress. The deflection therefrom is linearized and expanded with respect to the Legendre polynomial. Two amplitudes for each shape mode are introduced because the membrane has mobility not only in the radial direction but also in the tangential direction. A simple expression for the natural frequency of shape mode is derived. Stability diagrams for the higherorder shape mode are mapped out in the phase space of driving amplitude versus driving frequency. The most unstable driving frequency is found to satisfy an integer multiple relationship with twice of the higher-order natural frequency. This finding is justified by a fact that the system with a boundary layer approximation is simplified into Mathieu’s equation. Liquid viscosity plays an important role in the shape stability due to the vorticity generation on the deformed membrane.A theoretical study on the shape instability of a slightly deformed bubble encapsulated by a viscoelastic membrane in an ultrasound field is performed. The membrane effects of the inplane stress and the bending moment are incorporated into the traction jump condition at the bubble surface. The spherical motion of the bubble is numerically obtained by solving the Rayleigh-Plesset equation with the elastic stress. The deflection therefrom is linearized and expanded with respect to the Legendre polynomial. Two amplitudes for each shape mode are introduced because the membrane has mobility not only in the radial direction but also in the tangential direction. A simple expression for the natural frequency of shape mode is derived. Stability diagrams for the higherorder shape mode are mapped out in the phase space of driving amplitude versus driving frequency. The most unstable driving frequency is found to satisfy an integer multiple relationship with twice of the higher-order natural frequency. This finding i...