Coarse-grained numerical bifurcation analysis of lattice Boltzmann models

In this paper we study the earlier proposed coarse-grained bifurcation analysis approach. We extend the results obtained then for a one-dimensional FitzHugh–Nagumo lattice Boltzmann (LB) model in several ways. First, we extend the coarse-grained time stepper concept to enable the computation of periodic solutions and we use the more versatile Newton–Picard method rather than the Recursive Projection Method (RPM) for the numerical bifurcation analysis. Second, we compare the obtained bifurcation diagram with the bifurcation diagrams of the corresponding macroscopic PDE and of the lattice Boltzmann model. Most importantly, we perform an extensive study of the influence of the lifting or reconstruction step on the minimal successful time step of the coarse-grained time stepper and the accuracy of the results. It is shown experimentally that this time step must often be much larger than the time it takes for the higher-order moments to become slaved by the lowest-order moment, which somewhat contradicts earlier claims.

Design considerations for engineering 3D models to study vascular pathologies in vitro

Many cardiovascular diseases (CVD) are driven by pathological remodelling of blood vessels, which can lead to aneurysms, myocardial infarction, ischaemia and strokes. Aberrant remodelling is driven by changes in vascular cell behaviours combined with degradation, modification, or abnormal deposition of extracellular matrix (ECM) proteins. The underlying mechanisms that drive the pathological remodelling of blood vessels are multifaceted and disease specific; however, unravelling them may be key to developing therapies. Reductionist models of blood vessels created in vitro that combine cells with biomaterial scaffolds may serve as useful analogues to study vascular disease progression in a controlled environment. This review presents the main considerations for developing such in vitro models. We discuss how the design of blood vessel models impacts experimental readouts, with a particular focus on the maintenance of normal cellular phenotypes, strategies that mimic normal cell-ECM interactions, and approaches that foster intercellular communication between vascular cell types. We also highlight how choice of biomaterials, cellular arrangements and the inclusion of mechanical stimulation using fluidic devices together impact the ability of blood vessel models to mimic in vivo conditions. In the future, by combining advances in materials science, cell biology, fluidics and modelling, it may be possible to create blood vessel models that are patient-specific and can be used to develop and test therapies

Governance in Service Delivery in the Middle East and North Africa. World Development Report Background Paper

This paper examines the clientelistic equilibrium that remains prevalent in much of the Middle East and North Africa (MENA) region during the post-independence period, undermining service delivery and creating inequality in access. Political institutions and social practices that shape incentives for policymakers, service providers, and citizens create what can be called a potentially tenuous, “clientelistic equilibrium.” Service delivery is influenced by political institutions that allow for the capture of public jobs and service networks, and by social institutions that call upon individuals to respond more readily to members of their social networks than to others. The result is poor quality service delivery (e.g., absenteeism, insufficient effort), difficulties in access (e.g., need for bribes, connections), and inequalities in the provision of services

Topologically massive non-Abelian theory: superfield formalism

We apply the well-established techniques of geometrical superfield approach to Becchi-Rouet-Stora-Tyutin (BRST) formalism in the context of four (3 + 1)-dimensional (4D) dynamical non-Abelian 2-form gauge theory by exploiting its inherent "scalar" and "vector" gauge symmetry transformations and derive the corresponding off-shell nilpotent and absolutely anticommuting BRST and anti-BRST symmetry transformations. Our approach leads to the derivation of three (anti-) BRST invariant Curci-Ferrari (CF)-type restrictions that are found to be responsible for the absolute anticommutativity of the BRST and anti-BRST symmetry transformations. We derive the coupled Lagrangian densities that respect the (anti-) BRST symmetry transformations corresponding to the "vector" gauge transformations. We also capture the (anti-) BRST invariance of the CF-type restrictions and coupled Lagrangian densities within the framework of our superfield approach. We obtain, furthermore, the off-shell nilpotent (anti-) BRST symmetry transformations when the (anti-) BRST symmetry transformations corresponding to the "scalar" and "vector" gauge symmetries are merged together. These off-shell nilpotent "merged" (anti-) BRST symmetries are, however, found to be not absolutely anticommuting in nature.Comment: LaTeX2e file, 33 pages, journal versio

Repair of Scour Holes and Levees After the 1993 Flood

The record high water during the summer of 1993 significantly impacted the flood control levee structures in the U.S. Army Corps of Engineers, Kansas City District. Scour holes in the levees and their foundations reached bedrock, up to 75 feet deep in some places, and extended up to 2,000 feet landward of the landside toe on lengths reaching 2,100 feet along selected levee embankments. Different methods used by the Corps of Engineers to repair the scoured levee embankment and foundation soils, their hydraulic impact on river stages, and the efficiency of different methods are presented. The methods discussed consist of: (1) backfill of the riverside scour holes; (2) backfill of the scour hole and reconstruction of the levee embankment to the original centerline; (3) realignment of levees landward of the scour boles; and (4) a grouted cut-off wall in a rockfill embankment and construction of a ring levee around the landside scour hole. The efficiency of different methods was evaluated by observation of the levee system during subsequent flood events

Accelerated Expansion of the Universe in Gauss-Bonnet Gravity

We show that in Gauss-Bonnet gravity with negative Gauss-Bonnet coefficient and without a cosmological constant, one can explain the acceleration of the expanding Universe. We first introduce a solution of the Gauss-Bonnet gravity with negative Gauss-Bonnet coefficient and no cosmological constant term in an empty $(n+1)$-dimensional bulk. This solution can generate a de Sitter spacetime with curvature $n(n+1)/\{(n-2)(n-3)|\alpha|\}$. We show that an $(n-1)$-dimensional brane embedded in this bulk can have an expanding feature with acceleration. We also considered a 4-dimensional brane world in a 5-dimensional empty space with zero cosmological constant and obtain the modified Friedmann equations. The solution of these modified equations in matter-dominated era presents an expanding Universe with negative deceleration and positive jerk which is consistent with the recent cosmological data. We also find that for this solution, the $"n"th$ derivative of the scale factor with respect to time can be expressed only in terms of Hubble and deceleration parameters.Comment: 12 pages, no figure, references added, typos corrected, Section 4 ammended, an appndix added, version to be appeared in Phys. Rev.

Asymptotically (anti)-de Sitter solutions in Gauss-Bonnet gravity without a cosmological constant

In this paper we show that one can have asymptotically de Sitter (dS), anti-de Sitter (AdS) and flat solutions in Gauss-Bonnet gravity without any need to a cosmological constant term in field equations. First, we introduce static solutions whose 3-surfaces at fixed $r$ and $t$ have constant positive ($k=1$), negative ($k=-1$), or zero ($k=0$) curvature. We show that for $k=\pm1$, one can have asymptotically dS, AdS and flat spacetimes, while for the case of $k=0$, one has only asymptotically AdS solutions. Some of these solutions present naked singularities, while some others are black hole or topological black hole solutions. We also find that the geometrical mass of these 5-dimensional spacetimes is $m+2\alpha | k|$, which is different from the geometrical mass, $m$, of the solutions of Einstein gravity. This feature occurs only for the 5-dimensional solutions, and is not repeated for the solutions of Gauss-Bonnet gravity in higher dimensions. We also add angular momentum to the static solutions with $k=0$, and introduce the asymptotically AdS charged rotating solutions of Gauss-Bonnet gravity. Finally, we introduce a class of solutions which yields an asymptotically AdS spacetime with a longitudinal magnetic field which presents a naked singularity, and generalize it to the case of magnetic rotating solutions with two rotation parameters.Comment: 13 pages, no figur