Bastien Chopard

Lattice Boltzmann method with regularized pre-collision distribution functions

An extended numerical scheme for the simulation of fluid flows by means of a lattice Boltzmann (LB) method is introduced. It is conceptually related to the lattice BGK scheme, which it enhances by a regularization step. The result is a numerical scheme that is both more accurate and more stable in the hydrodynamic regime.

Cellular Automata And Lattice Boltzmann Techniques: An Approach To Model And Simulate Complex Systems

We discuss the cellular automata approach and its extensions, the lattice Boltzmann and multiparticle methods. The potential of these techniques is demonstrated in the case of modeling complex systems. In particular, we consider applications taken from various fields of physics, such as reaction-diffusion systems, pattern formation phenomena, fluid flows, fracture processes and road traffic models.

Cellular automata model of car traffic in a two-dimensional street network

We propose a road traffic cellular automata model suitable for an urban environment. North, east, south and west car displacements are possible and road crossings are naturally implemented as rotary junctions. We consider the traffic in a Manhattan-like city and study the flow diagram and the car density profile along road segments. We observe that the length of the car queues obeys a complex dynamics and is not uniform across the network. The street length between two junctions and the turning strategies at rotaries are relevant parameters of the model. Our results are also confirmed by fully continuous traffic simulations.

The application of multiscale modelling to the process of development and prevention of stenosis in a stented coronary artery

The inherent complexity of biomedical systems is well recognized; they are multiscale, multiscience systems, bridging a wide range of temporal and spatial scales. While the importance of multiscale modelling in this context is increasingly recognized, there is little underpinning literature on the methodology and generic description of the process. The COAST (complex autonoma simulation technique) project aims to address this by developing a multiscale, multiscience framework, coined complex autonoma (CxA), based on a hierarchical aggregation of coupled cellular automata (CA) and agent-based models (ABMs). The key tenet of COAST is that a multiscale system can be decomposed into N single-scale CA or ABMs that mutually interact across the scales. Decomposition is facilitated by building a scale separation map on which each single-scale system is represented according to its spatial and temporal characteristics. Processes having well-separated scales are thus easily identified as the components of the multiscale model. This paper focuses on methodology, introduces the concept of the CxA and demonstrates its use in the generation of a multiscale model of the physical and biological processes implicated in a challenging and clinically relevant problem, namely coronary artery in-stent restenosis.

Treatment of Renal Artery Aneurysm With the Multilayer Stent

Purpose: To describe a new type of stent consisting of a 3-dimensional (3D) braided tube made of 2 interconnected layers without any covering to treat a renal artery aneurysm. Case Report: A 78-year-old hypertensive man with multiple comorbidities was incidentally found to have a large (28-×30 mm) saccular aneurysm in the main right renal artery involving the inferior renal artery. Via a percutaneous femoral approach, a 6–3×0-mm Multilayer stent was deployed easily in front of the aneurysm neck covering the inferior renal artery. Blood flow inside the sac was immediately and significantly reduced. All the renal artery branches remained patent. Blood pressure returned to normal after the procedure. At 6 months, angiography showed complete shrinkage of the aneurysm wall; all the inferior renal artery branches remained patent. Conclusion: The 3D multilayer fluid modulating stent concept appears to be a viable alternative for renal aneurysm exclusion. A larger study is underway to evaluate this new stent in other peripheral aneurysms.

Parallel genetic programming and its application to trading model induction

This paper presents a scalable parallel implementation of genetic programming on distributed memory machines. The system runs multiple master-slave instances each mapped on all the allocated nodes and multithreading is used to overlap message latencies with useful computation. Load balancing is achieved using a dynamic scheduling algorithm and comparison with a static algorithm is reported. To alleviate premature convergence, asynchronous migration of individuals is performed among processes. We show that nearly linear speedups can be obtained for problems of large enough size. The system has been applied to infer robust trading strategies which is a compute-intensive financial application.

Cellular Automata Model for the Diffusion Equation

We consider a new cellular automata rule for a synchronous random walk on a two-dimensional square lattice, subject to an exclusion principle. It is found that the macroscopic behavior of our model obeys the telegraphistss equation, with an adjustable diffusion constant. By construction, the dynamics of our model is exactly described by a linear discrete Boltzmann equation which is solved analytically for some boundary conditions. Consequently, the connection between the microscopic and the macroscopic descriptions is obtained exactly and the continuous limit studied rigorously. The typical system size for which a true diffusive behavior is observed may be deduced as a function of the parameters entering into the rule. It is shown that a suitable choice of these parameters allows us to consider quite small systems. In particular, our cellular automata model can simulate the Laplace equation to a precision of the order (λ/L)6, whereL is the size of the system andλ the lattice spacing. Implementation of this algorithm on special-purpose machines leads to the fastest way to simulate diffusion on a lattice.

Lattice Boltzmann computations and applications to physics

We discuss the lattice Boltzmann computing approach, its connection with cellular automata and present a new model for simulating wave propagation in complex environments. We illustrate the behavior of our model on several applications like radio wave propagation in a city, solid body motion and fracture phenomena.

A Complex Automata approach for in-stent restenosis: two-dimensional multiscale modelling and simulations

In-stent restenosis, the maladaptive response of a blood vessel to injury caused by the deployment of a stent, is a multiscale system involving a large number of biological and physical processes. We describe a Complex Automata model for in-stent restenosis, coupling bulk flow, drug diffusion, and smooth muscle cell models, all operating on different time scales. Details of the single scale models and of the coupling interfaces are described, together with first simulation results, obtained with a dedicated software environment for Complex Automata simulations. Preliminary results show that the model can reproduce growth trends observed in experimental studies and facilitate testing of hypotheses concerning the interaction of key factors.

Cellular automata and lattice Boltzmann methods: a new approach to computational fluid dynamics and particle transport

Cellular automata (CA) and lattice Boltzmann (LB) approaches are computational methods that offer flexibility, efficiency and outstanding amenability to parallelism when modeling complex phenomena. In this paper, the CA and LB approach are combined in the same model, in order to describe a system where point-particles are transported in a fluid flow. This model is used to simulate the snow transport, erosion and deposition by the wind.