arXiv subjects

Shigeo Wada

Publications and source records attributed to Shigeo Wada.

5 recordsLinked to original sources

arXiv2025

Diffuse interface approach to oxygen transport and metabolism under blood flow dynamics in microcirculations

The relationship between the spatiotemporal distribution of oxygen transport and blood flow dynamics, accounting for the motion and deformation of individual red blood cells (RBCs), is of fundamental importance for understanding microcirculation systems. Three-dimensional (3D) modeling is indispensable for addressing complex oxygen transport and cellular behaviors in capillary networks; however, the computational approach is formidable for enforcing interface (or jump) conditions on largely moving and deforming interfaces. In this paper, we propose a diffuse interface approach for the oxygen transport using a mixture formulation. We formulate oxygen transport using an advection-diffusion-reaction equation and rewrite all governing equations in mixture forms using phase indicator functions, where all the interface conditions are included in the governing equations. This innovation avoids the complexity associated with discontinuities for largely moving interfaces in highly dense RBC conditions. We model cellular flow as a fluid-membrane interaction problem using the immersed boundary method (IBM). The method allows the seamless calculation of coupling problems for cellular flows and oxygen transports in the cytoplasm (internal fluid) of the RBC, plasma (external fluid), and tissue regions using a fixed Cartesian coordinate mesh. The proposed method accurately captures the analytical solution for spherically symmetric diffusion, and successfully demonstrates oxygen transport in both straight capillaries and their networks. These rigorous analyses suggest that RBCs can autonomously regulate the oxygen supply to tissues in response to the local tissue oxygenation level, resulting in the establishment of homogeneous tissue oxygenation.

physics.flu-dyn↗

arXiv2024

Phase changes of the flow rate in the vertebral artery caused by debranching thoracic endovascular aortic repair: effects of flow path and local vessel stiffness on vertebral arterial pulsation

Despite numerous studies on cerebral arterial blood flow, there has not yet been a comprehensive description of hemodynamics in patients undergoing debranching thoracic endovascular aortic repair (dTEVAR), a promising surgical option for aortic arch aneurysms. A phase delay of the flow rate in the left vertebral artery (LVA) in patients after dTEVAR compared to those before was experimentally observed, while the phase in the right vertebral artery (RVA) remained almost the same before and after surgery. Since this surgical intervention included stent graft implantation and extra-anatomical bypass, it was expected that the intracranial hemodynamic changes due to dTEVAR were coupled with fluid flow and pulse waves in cerebral arteries. To clarify this issue, A one-dimensional model (1D) was used to numerically investigate the relative contribution (i.e., local vessel stiffness and flow path changes) of the VA flow rate to the phase difference. The numerical results demonstrated a phase delay of flow rate in the LVA but not the RVA in postoperative patients undergoing dTEVAR relative to preoperative patients. The results further showed that the primary factor affecting the phase delay of the flow rate in the LVA after surgery compared to that before was the bypass, i.e., alteration of flow path, rather than stent grafting, i.e., the change in local vessel stiffness. The numerical results provide insights into hemodynamics in postoperative patients undergoing dTEVAR, as well as knowledge about therapeutic decisions.

physics.bio-ph↗

arXiv2022

Numerical analysis of viscoelasticity of two-dimensional fluid membranes under oscillatory loadings

Biomembranes consisting of two opposing phospholipid monolayers, which comprise the so-called lipid bilayer, are largely responsible for the dual solid-fluid behavior of individual cells and viruses. Quantifying the mechanical characteristics of biomembrane, including the dynamics of their in-plane fluidity, can provide insight not only into active or passive cell behaviors but also into vesicle design for drug delivery systems. Despite numerous studies on the mechanics of biomembranes, their dynamical viscoelastic properties have not yet been fully described. We thus quantify their viscoelasticity based on a two-dimensional (2D) fluid membrane model, and investigate this viscoelasticity under small amplitude oscillatory loadings in micron-scale membrane area. We use hydrodynamic equations of bilayer membranes, obtained by Onsager's variational principle, wherein the fluid membrane is assumed to be an almost planar bilayer membrane. Simulations are performed for a wide range of oscillatory frequencies $f$ and membrane tensions. Our numerical results show that as frequencies increase, membrane characteristics shift from an elastic-dominant to viscous-dominant state. Furthermore, such state transitions obtained with a 1-$\mu$m-wide loading profile appear with frequencies between $O(f) = 10^1-10^2$ Hz, and almost independently of surface tensions. We discuss the formation mechanism of the viscous- or elastic-dominant transition based on relaxation rates that correspond to the eigenvalues of the dynamical matrix in the governing equations.

physics.bio-ph↗

arXiv2022

Inertial migration of red blood cells under a Newtonian fluid in a circular channel

We present a numerical analysis of the lateral movement and equilibrium radial positions of red blood cells (RBCs) with major diameter of 8 $μ$m under a Newtonian fluid in a circular channel with 50-$μ$m diameter. Each RBC, modelled as a biconcave capsule whose membrane satisfies strain-hardening characteristics, is simulated for different Reynolds numbers $Re$ and capillary numbers $Ca$, the latter of which indicate the ratio of the fluid viscous force to the membrane elastic force. The effects of initial orientation angles and positions on the equilibrium radial position of an RBC centroid are also investigated. The numerical results show that depending on their initial orientations, RBCs have bistable flow modes, so-called rolling and tumbling motions. Most RBCs have a rolling motion. These stable modes are accompanied by different equilibrium radial positions, where tumbling RBCs are further away from the channel axis than rolling ones. The inertial migration of RBCs is achieved by alternating orientation angles, which are primarily affected by the initial orientation angles. Then the RBCs assume the aforementioned bistable modes during the migration, followed by further migration to the equilibrium radial position at much longer time periods. The power (or energy dissipation) associated with membrane deformations is introduced to quantify the state of membrane loads. The energy expenditures rely on stable flow modes, the equilibrium radial position of RBC centroids, and the viscosity ratio between the internal and external fluids.

physics.flu-dyn↗

arXiv2019

Hemorheology in dilute, semi-dilute and dense suspensions of red blood cells

We present a numerical analysis of the rheology of a suspension of red blood cells (RBCs) in a wall-bounded shear flow. The flow is assumed as almost inertialess. The suspension of RBCs, modeled as biconcave capsules whose membrane follows the Skalak constitutive law, is simulated for a wide range of viscosity ratios between the cytoplasm and plasma: $λ$ = 0.1-10, for volume fractions up to $ϕ$ = 0.41 and for different capillary numbers ($Ca$). Our numerical results show that an RBC at low $Ca$ tends to orient to the shear plane and exhibits the so-called rolling motion, a stable mode with higher intrinsic viscosity than the so-called tumbling motion. As $Ca$ increases, the mode shifts from the rolling to the swinging motion. Hydrodynamic interactions (higher volume fraction) also allows RBCs to exhibit both tumbling or swinging motions resulting in a drop of the intrinsic viscosity for dilute and semi-dilute suspensions. Because of this mode change, conventional ways of modeling the relative viscosity as a polynomial function of $ϕ$ cannot be simply applied in suspensions of RBCs at low volume fractions. The relative viscosity for high volume fractions, however, can be well described as a function of an effective volume fraction, defined by the volume of spheres of radius equal to the semi-middle axis of the deformed RBC. We find that the relative viscosity successfully collapses on a single non-linear curve independently of $λ$ except for the case with $Ca \geq$ 0.4, where the fit works only in the case of low/moderate volume fraction, and fails in the case of a fully dense suspension.

physics.flu-dyn↗