Magnetic coupling properties of Mn-doped AlN nanowires: First-principles calculations

Based on first-principles within the framework of the density functional theory, we have studied the magnetic coupling properties of Mn-doped AlN nanowires. By analyzing the results of different Mn-doped AlN nanowires, we found that for the passivated nanowire, ferromagnetic state is more stable, while for the unpassivated nanowire, the favorable state transits into anti-ferromagnetic state, which can be well explained by the band coupling model. The results indicate that the degree of surface passivation of dangling bonds is an important factor in the magnetic properties of doped nanowires.     

Self-Supported Graphite/Graphene/NiFe-LDH Electrodes for High Performance Oxygen Evolution Reaction

Reducing energy consumption and improving energy utilization efficiency has become the focus of research in the 21^st century. Electrocatalytic water splitting is one of the promising strategies for producing hydrogen energy. In this study, the non-noble nickel-iron layered double hydroxide (NiFe-LDH) catalyst is deposited on the electrochemically intercalated graphite/graphene (G/GE) substrate and directly used as the self-supported and binder-free electrode for electrocatalytic water oxidation. The Ni₂Fe₁-LDH@G/GE catalyst shows a low overpotential of 194 mV at a current density of 10 mA cm^–2, which is better than the noble metal catalyst IrO₂ (314 mV) and RuO₂ (330 mV) and many other related works. This research provides a facile way to directly prepare the catalyst electrode with high performance and low cost.     

Parallel large eddy simulation of turbulent flow around MIRA model using linear equal‐order finite element method

A parallel large eddy simulation code that adopts domain decomposition method has been developed for large‐scale computation of turbulent flows around an arbitrarily shaped body. For the temporal integration of the unsteady incompressible Navier–Stokes equation, fractional 4‐step splitting algorithm is adopted, and for the modelling of small eddies in turbulent flows, the Smagorinsky model is used. For the parallelization of the code, METIS and Message Passing Interface Libraries are used, respectively, to partition the computational domain and to communicate data between processors. To validate the parallel architecture and to estimate its performance, a three‐dimensional laminar driven cavity flow inside a cubical enclosure has been solved. To validate the turbulence calculation, the turbulent channel flows at Re_τ = 180 and 1050 are simulated and compared with previous results. Then, a backward facing step flow is solved and compared with a DNS result for overall code validation. Finally, the turbulent flow around MIRA model at Re = 2.6 × 10⁶ is simulated by using approximately 6.7 million nodes. Scalability curve obtained from this simulation shows that scalable results are obtained. The calculated drag coefficient agrees better with the experimental result than those previously obtained by using two‐equation turbulence models. Copyright © 2007 John Wiley & Sons, Ltd.     

Electronic Structure Engineering of Carbon Nitride Materials by Using Polycyclic Aromatic Hydrocarbons

The design of charge separation sites under illumination in semiconductors is a standing challenge for their utilization as photo(electro)catalysts. Here, the synthesis of modified carbon nitride materials (CNs) with donor–acceptor (D–A) domains, with altering electronic structure, is reported. To do so, new monomers based on polycyclic aromatic hydrocarbons (PAH)-substituted 1,3,5-triazine were designed, which were then embedded within cyanuric acid–melamine supramolecular assemblies to form CN precursors. The conjugation degree of PAHs was systematically changed, from single benzene ring up to pyrene unit, elucidating the role of the conjugation degree on the morphology, structure and electronic properties as well as photo(electro)catalytic activity. The careful design of the D–A sites results in excellent photocatalytic activity as well as long-term stability for the hydrogen evolution reaction. Moreover, PAH–CNs films exhibit enhanced charge separation, optical absorption, electrochemical surface area and electronic conductivity, leading to an outstanding photoelectrochemical (PEC) activity compared to pristine CN.     

Accurate particle splitting for smoothed particle hydrodynamics in shallow water with shock capturing

The solution for the shallow water equations using smoothed particle hydrodynamics is attractive, being a mesh‐free, automatically adaptive method without special treatment for wet–dry interfaces. However, the relatively new method is limited by the variable kernel size or smoothing length being inversely proportional to water depth causing poor resolution at small depths. Boundary conditions at solid walls have also not been well resolved. To solve the resolution problem in small depths, a particle splitting procedure was developed (conveniently into seven particles), which conserves mass and momentum by varying the smoothing length, velocity and acceleration of each refined particle. This improves predictions in the shallowest depths where the error associated with splitting is reduced by one order of magnitude in comparison to other published works. To provide good shock capturing behaviour, particle interactions are treated as a Riemann problem with Monotone Upstream‐centred Scheme for Conservation Laws (MUSCL) reconstruction providing stability. For solid boundaries, the recent modified virtual boundary particle method was developed further to enable the zeroth moment to be accurately conserved where the smoothing length of particles is changing rapidly during particle splitting. The resulting method is applied to the one‐dimensional and the two‐dimensional axisymmetric wet‐bed dam break problems showing close agreement with analytical solutions, demonstrating the need for particle splitting. To demonstrate wetting and drying in a more complex case, the scheme is applied to oscillating water in a two‐dimensional parabolic basin and produces good agreement with the analytical solution. The method is finally applied to the European Concerted Action on DAm break Modelling dam‐break test case representative of realistic conditions and good predictions are made of experimental measurements with a 40% reduction in the computational time when particle splitting is employed. The overall method has thus become quite sophisticated but its generality and versatility will be attractive for various shallow water problems. Copyright © 2011 John Wiley & Sons, Ltd.     

Two‐objective optimization strategies using the adjoint method and game theory in inverse natural convection problems

This paper considers the problem of estimating the strengths of two time‐varying heat sources simultaneously, from measurements of the temperature inside the square domain in a porous medium, when prior knowledge of the source functions is not available. This problem is an inverse natural convection problem. In order to circumvent this problem, we define several optimization criteria (objective functionals) that measure discrepancies between model and measured data, where objective functionals depend on two heat sources and use multi‐criteria optimization to identify Nash equilibria, which are solutions to the non‐cooperative game according to game theory. Two non‐cooperative game strategies are considered: competitive (Nash) game and hierarchical (modified Stackelberg) game. The methodology that we employ relies on a combination of mixed finite element space approximations, finite difference time discretizations, adjoint equation and sensitivity equation techniques, and nonlinear conjugate gradient algorithms for the solutions of estimating two heat sources. Applying the Sobolev gradient for the noise removal is investigated. The performance of the present technique of inverse analysis is evaluated, by means of several numerical experiments, and is found to be very accurate as well as efficient. Copyright © 2012 John Wiley & Sons, Ltd.     

Non-existence of generalized splitting methods with positive coefficients of order higher than four

We prove that generalized exponential splitting methods making explicit use of commutators of the vector fields are limited to order four when only real coefficients are admitted. This generalizes the restriction to order two for classical splitting methods with only positive coefficients.