A multiwave approximate Riemann solver for ideal MHD based on relaxation II: numerical implementation with 3 and 5 waves

In the first part of this work Bouchut et al. (J Comput Phys 108:7–41, 2007) we introduced an approximate Riemann solver for one-dimensional ideal MHD derived from a relaxation system. We gave sufficient conditions for the solver to satisfy discrete entropy inequalities, and to preserve positivity of density and internal energy. In this paper we consider the practical implementation, and derive explicit wave speed estimates satisfying the stability conditions of Bouchut et al. (J Comput Phys 108:7–41, 2007). We present a 3-wave solver that well resolves fast waves and material contacts, and a 5-wave solver that accurately resolves the cases when two eigenvalues coincide. A full 7-wave solver, which is highly accurate on all types of waves, will be described in a follow-up paper. We test the solvers on one-dimensional shock tube data and smooth shear waves.     

The homotopy analysis method to solve the modified equal width wave equation

In this article, to solve the modified equal width wave (MEW) equation, the homotopy analysis method (HAM) is proposed. The initial approximation can be freely chosen with possible unknown constant, which can be determined by using the boundary and initial conditions. The HAM contains the auxiliary parameter ?, which provides us to adjust and control the convergence region of solution series with a simple way. Three conservative quantities are reported. Numerical results show that this method is a promising and powerful tool to solve the MEW equation. © 2009 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 2010     

Revisiting acido-basicity of the MgO surface by periodic density functional theory calculations: role of surface topology and ion coordination on water dissociation

Low-coordinated (LC) ions at the MgO surface (noted Mg2+LC and O2-LC with L = 1-5), located on monatomic and diatomic steps, corners, step divacancies, and kinks, have been modeled thanks to periodic density functional theory (DFT) calculations (VASP). Ions of lowest coordination induce the strongest surface geometry relaxation and the highest surface energies. The hydration energies of these sites and thermodynamic stabilities of the resulting surfaces were studied. The factors controlling the interaction strength between water and the surface are the possibility for the hydroxyl group to adopt a bridging geometry between two Mg2+ cations in concave areas of the surface, such as the bottom of the monatomic step, and at second order the surface atomic coordination, and especially the presence of three-coordinated ions. The Lewis basicity and acidity of O2-LC and Mg2+LC, respectively, increase as their coordination number decreases, which implies the same trend for the Br?nsted basicity of the Mg2+-O2- pair toward water. However, this trend can be changed if pairs leading to the formation of bridging OH groups are involved, typically on monatomic steps or in step divacancies where O2C-H and O3C-H are obtained, respectively, instead of the expected O1C-H. Thanks to thermodynamic calculations, the state of the surface as a function of temperature can be determined at a given pressure, unraveling the roles of surface topology and ions coordination.     

Hybrid Lewis acid/hydrogen-bond donor receptor for fluoride

[reaction: see text] To verify if hydrogen-bond donor groups can assist fluoride binding at the boron center of triaryl boranes, o-(dimesitylboryl)trifluoroacetanilide has been synthesized. Reaction of this new borane with [n-Bu(4)N][F] in acetone affords the corresponding fluoroborate complex whose stability constant exceeds that of [Mes(3)BF](-) by at least 2 orders of magnitude. Presumably, the higher fluoride affinity of o-(dimesitylboryl)trifluoroacetanilide results from the cooperativity of the Lewis acidic boron center and the hydrogen-bond donor trifluoroacetamide group.     

Mechanically controlled DNA extrusion from a palindromic sequence by single molecule micromanipulation

A magnetic tweezers setup is used to control both the stretching force and the relative linking number DeltaLk of a palindromic DNA molecule. We show here, in absence of divalent ions, that twisting negatively the molecule while stretching it at approximately 1 pN induces the formation of a cruciform DNA structure. Furthermore, once the cruciform DNA structure is formed, the extrusion of several kilo-base pairs of palindromic DNA sequence is directly and reversibly controlled by varying DeltaLk. Indeed the branch point behaves as a nanomechanical gear that links rotation with translation, a feature related to the helicity of DNA. We obtain experimentally a very good linear relationship between the extension of the molecule and DeltaLk. We use then this experiment to obtain a precise measurement of the pitch of B-DNA in solution: 3.61 +/- 0.03 nm/turn.     

Geometric phase induced by a cyclically evolving squeezed vacuum reservoir

We propose a new way to generate an observable geometric phase by means of a completely incoherent phenomenon. We show how to imprint a geometric phase to a system by adiabatically manipulating the environment with which it interacts. As a specific scheme, we analyze a multilevel atom interacting with a broadband squeezed vacuum bosonic bath. As the squeezing parameters are smoothly changed in time along a closed loop, the ground state of the system acquires a geometric phase. We also propose a scheme to measure such a geometric phase by means of a suitable polarization detection.     

A time discretization scheme of a characteristics method for a fluid–rigid system with discontinuous density

We propose a new characteristics method for the time discretization of a fluid–rigid system in the case when the densities of the fluid and the solid are different. This method is based on a global weak formulation involving only terms defined on the whole fluid–rigid domain. The main idea is to construct a characteristic function which preserves the rigidity of the solid at the discrete time levels. A convergence result for this semi-discrete scheme is then given.     

Semigroups of transformations restricted by an equivalence

Suppose σ is an equivalence on a set X and let E(X, σ) denote the semigroup (under composition) of all α: X → X such that σ ⊆ α ∘ α ⁻¹. Here we characterise Green’s relations and ideals in E(X, σ). This is analogous to recent work by Sullivan on K(V, W), the semigroup (under composition) of all linear transformations β of a vector space V such that W ⊆ ker β, where W is a fixed subspace of V.     

Admission control for a multi-server queue with abandonment

In a M/M/N+M queue, when there are many customers waiting, it may be preferable to reject a new arrival rather than risk that arrival later abandoning without receiving service. On the other hand, rejecting new arrivals increases the percentage of time servers are idle, which also may not be desirable. We address these trade-offs by considering an admission control problem for a M/M/N+M queue when there are costs associated with customer abandonment, server idleness, and turning away customers. First, we formulate the relevant Markov decision process (MDP), show that the optimal policy is of threshold form, and provide a simple and efficient iterative algorithm that does not presuppose a bounded state space to compute the minimum infinite horizon expected average cost and associated threshold level. Under certain conditions we can guarantee that the algorithm provides an exact optimal solution when it stops; otherwise, the algorithm stops when a provided bound on the optimality gap is reached. Next, we solve the approximating diffusion control problem (DCP) that arises in the Halfin–Whitt many-server limit regime. This allows us to establish that the parameter space has a sharp division. Specifically, there is an optimal solution with a finite threshold level when the cost of an abandonment exceeds the cost of rejecting a customer; otherwise, there is an optimal solution that exercises no control. This analysis also yields a convenient analytic expression for the infinite horizon expected average cost as a function of the threshold level. Finally, we propose a policy for the original system that is based on the DCP solution, and show that this policy is asymptotically optimal. Our extensive numerical study shows that the control that arises from solving the DCP achieves a very similar cost to the control that arises from solving the MDP, even when the number of servers is small.