F Florian Janoschek

Simplified particulate model for coarse-grained hemodynamics simulations.

Human blood flow is a multiscale problem: in first approximation, blood is a dense suspension of plasma and deformable red cells. Physiological vessel diameters range from about one to thousands of cell radii. Current computational models either involve a homogeneous fluid and cannot track particulate effects or describe a relatively small number of cells with high resolution but are incapable to reach relevant time and length scales. Our approach is to simplify much further than existing particulate models. We combine well-established methods from other areas of physics in order to find the essential ingredients for a minimalist description that still recovers hemorheology. These ingredients are a lattice Boltzmann method describing rigid particle suspensions to account for hydrodynamic long-range interactions and-in order to describe the more complex short-range behavior of cells-anisotropic model potentials known from molecular-dynamics simulations. Paying detailedness, we achieve an efficient and scalable implementation which is crucial for our ultimate goal: establishing a link between the collective behavior of millions of cells and the macroscopic properties of blood in realistic flow situations. In this paper we present our model and demonstrate its applicability to conditions typical for the microvasculature.

Lattice Boltzmann simulations of anisotropic particles at liquid interfaces

Complex colloidal fluids, such as emulsions stabilized by particles with complex shapes, play an important role in many industrial applications. However, understanding their physics requires a study at sufficiently large length scales while still resolving the microscopic structure of a large number of particles and of the local hydrodynamics. Due to its high degree of locality, the lattice Boltzmann method, when combined with a molecular dynamics solver and parallelized on modern supercomputers, provides a tool that allows such studies. Still, running simulations on hundreds of thousands of cores is not trivial. We report on our practical experiences when employing large fractions of an IBM Blue Gene/P system for our simulations. Then, we extend our model for spherical particles in multicomponent flows to anisotropic ellipsoidal objects rendering the shape of, e.g., clay particles. The model is applied to a number of test cases including the adsorption of single particles at fluid interfaces and the formation and stabilization of Pickering emulsions or bijels.

Rotational behaviour of red blood cells in suspension: a mesoscale simulation study

The nature of blood as a suspension of red blood cells makes computational haemodynamics a demanding task. Our coarse-grained blood model, which builds on a lattice Boltzmann method for soft particle suspensions, enables the study of the collective behaviour of the order of 106 cells in suspension. After demonstrating the viscosity measurement in Kolmogorov flow, we focus on the statistical analysis of the cell orientation and rotation in Couette flow. We quantify the average inclination with respect to the flow and the nematic order as a function of shear rate and haematocrit. We further record the distribution of rotation periods around the vorticity direction and find a pronounced peak in the vicinity of the theoretical value for free model cells, even though cell–cell interactions manifest themselves in a substantial width of the distribution.

Simulations of blood flow in plain cylindrical and constricted vessels with single cell resolution

Understanding the physics of blood is challenging due to its nature as a suspension of soft particles and the fact that typical problems involve different scales. This is valid also for numerical investigations. In fact, many computational studies either neglect the existence of discrete cells or resolve relatively few cells very accurately. The authors recently developed a simple and highly efficient yet still particulate model with the aim to bridge the gap between currently applied methods. The present work focuses on its applicability to confined flows in vessels of diameters up to 100 micrometres. For hematocrit values below 30 percent, a dependence of the apparent viscosity on the vessel diameter in agreement with experimental literature data is found.

Toward a continuum model for particle-induced velocity fluctuations in suspension flow through a stenosed geometry

Nonparticulate continuum descriptions allow for computationally efficient modeling of suspension flows at scales that are inaccessible to more detailed particulate approaches. It is well known that the presence of particles influences the effective viscosity of a suspension and that this effect has thus to be accounted for in macroscopic continuum models. The present paper aims at developing a nonparticulate model that reproduces not only the rheology but also the cell-induced velocity fluctuations, responsible for enhanced diffusivity. The results are obtained from a coarse-grained blood model based on the lattice Boltzmann (LB) method. The benchmark system comprises a flow between two parallel plates with one of them featuring a smooth obstacle imitating a stenosis. Appropriate boundary conditions are developed for the particulate model to generate equilibrated cell configurations mimicking an infinite channel in front of the stenosis. The averaged flow field in the bulk of the channel can be described well by a nonparticulate simulation with a matched viscosity. We show that our proposed phenomenological model is capable to reproduce many features of the velocity fluctuations.

Simplified models for coarse-grained hemodynamics simulations

Human blood can be approximated as a dense suspension of red blood cells in plasma. Here, we present two models we recently developed to investigate blood flow on different scales: in the first part of the paper we concentrate on describing individual cells or model systems such as vesicles with high resolution in order to understand the underlying fundamental properties of bulk hemodynamics. Here, we combine a lattice Boltzmann solver for the plasma with an immersed boundary algorithm to describe the cell or vesicle membranes. This method allows a detailed study of individual particles in complex hydrodynamic situations. Further, this model can be used to provide parameters for a more coarse-grained approach: in that second approach we simplify much further than existing particulate models. We find the essential ingredients for a minimalist description that still recovers hemorheology. These ingredients include again a lattice Boltzmann method describing hydrodynamic long range interactions mediated by the plasma between cells. The cells themselves are simplified as rigid ellipsoidal particles, where we describe the more complex short-range behavior by anisotropic model potentials. Recent results on the behaviour of single viscous red blood cells and vesicles in confined flow situations are shown alongside with results from the validation of our simplified model involving thousands or even millions of cells.