| 1. |
- Andersson, Arne
(författare)
-
Structural Dynamics of a V2O5/SnO2 Catalyst in the Ammoxidation of 3-Picoline
- 1981
-
Ingår i: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 69:2, s. 465-474
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of the reaction parameters on the conversion of 3-picoline and the yield of nicotinonitrile in the ammoxidation of 3-picoline on a View the MathML source catalyst was studied. The highest yield of nicotinonitrile was 82%. This was obtained at 400 °C, with a space velocity of 4000 h−1 and the following mole ratios: ammonia/3-picoline = 12, air/3-picoline = 210, and water vapour/3-picoline = 40. It was found that the composition of the surface of the catalyst depended strongly on the values of the reaction parameters. The dynamic structure of the catalyst surface was studied by determining the phases present by X-ray diffraction analysis and the average oxidation number of vanadium by titrimetric methods. These results were correlated, showing that catalysts with both V2O5 and V6O13 were especially active and selective. The composition of the SnO2 phase was analyzed and was found to be V0.03Sn0.97O2, and did not depend on the reaction parameters. Infrared investigations showed that the role of SnO2 was to weaken the short VO bonds. This can probably be seen as a consequence of incorporation of Sn4+ in the V2O5 lattice.
|
|
| 2. |
- Engquist, Björn, et al.
(författare)
-
Numerical radiation boundary conditions for unsteady transonic flow
- 1981
-
Ingår i: Journal of Computational Physics. - : Elsevier BV. - 0021-9991 .- 1090-2716. ; 40:1, s. 91-103
-
Tidskriftsartikel (refereegranskat)abstract
- A family of numerical boundary conditions for far-field-computational boundaries in calculations involving unsteady transonic flow is devised. These boundary conditions are developed in a systematic fashion from general principles. Both numerical and analytic comparisons with other currently used methods are given.
|
|
| 3. |
- Engquist, Björn, et al.
(författare)
-
One-sided difference approximations for nonlinear conservation laws
- 1981
-
Ingår i: Journal of Computational Physics. - 0021-9991 .- 1090-2716. ; 36, s. 321-351
-
Tidskriftsartikel (refereegranskat)abstract
- We analyze one-sided or upwind finite difference approximations to hyperbolic partial differential equations and, in particular, nonlinear conservation laws. Second order schemes are designed for which we prove both nonlinear stability and that the entropy condition is satisfied for limit solutions. We show that no such stable approximation of order higher than two is possible. These one-sided schemes have desirable properties for shock calculations. We show that the proper switch used to change the direction in the upwind differencing across a shock is of great importance. New and simple schemes are developed for which we prove qualitative properties such as sharp monotone shock profiles, existence, uniqueness, and stability of discrete shocks. Numerical examples are given.
|
|
| 4. |
|
|
| 5. |
|
|
| 6. |
|
|
| 7. |
- Araujo-Cabarcas, Juan Carlos, 1981-, et al.
(författare)
-
Shape optimization for the strong routing of light in periodic diffraction gratings
- 2023
-
Ingår i: Journal of Computational Physics. - : Elsevier. - 0021-9991 .- 1090-2716. ; 472
-
Tidskriftsartikel (refereegranskat)abstract
- In the quest for the development of faster and more reliable technologies, the ability to control the propagation, confinement, and emission of light has become crucial. The design of guide mode resonators and perfect absorbers has proven to be of fundamental importance. In this project, we consider the shape optimization of a periodic dielectric slab aiming at efficient directional routing of light to reproduce similar features of a guide mode resonator. For this, the design objective is to maximize the routing efficiency of an incoming wave. That is, the goal is to promote wave propagation along the periodic slab. A Helmholtz problem with a piecewise constant and periodic refractive index medium models the wave propagation, and an accurate Robin-to-Robin map models an exterior domain. We propose an optimal design strategy that consists of representing the dielectric interface by a finite Fourier formula and using its coefficients as the design variables. Moreover, we use a high order finite element (FE) discretization combined with a bilinear Transfinite Interpolation formula. This setting admits explicit differentiation with respect to the design variables, from where an exact discrete adjoint method computes the sensitivities. We show in detail how the sensitivities are obtained in the quasi-periodic discrete setting. The design strategy employs gradient-based numerical optimization, which consists of a BFGS quasi-Newton method with backtracking line search. As a test case example, we present results for the optimization of a so-called single port perfect absorber. We test our strategy for a variety of incoming wave angles and different polarizations. In all cases, we efficiently reach designs featuring high routing efficiencies that satisfy the required criteria.
|
|
| 8. |
|
|
| 9. |
- Almanasreh, Hasan, 1981
(författare)
-
hp-Cloud approximation of the Dirac eigenvalue problem: The way of stability
- 2014
-
Ingår i: Journal of Computational Physics. - : Elsevier BV. - 0021-9991 .- 1090-2716. ; 272, s. 487-506
-
Tidskriftsartikel (refereegranskat)abstract
- We apply hp-cloud method to the radial Dirac eigenvalue problem. The difficulty of occurrence of spurious eigenvalues among the genuine ones in the computation is resolved. The method of treatment is based on assuming hp-cloud Petrov-Galerkin scheme to construct the weak formulation of the problem which adds a consistent diffusivity to the variational formulation. The size of the artificially added diffusion term is controlled by a stability parameter (tau). The derivation of tau assumes the limit behavior of the eigenvalues at infinity. The parameter tau is applicable for generic basis functions. This is combined with the choice of appropriate intrinsic enrichments in the construction of the cloud shape functions. (C) 2014 Elsevier Inc. All rights reserved.
|
|
| 10. |
- Almanasreh, Hasan, 1981, et al.
(författare)
-
Stabilized finite element method for the radial Dirac equation
- 2013
-
Ingår i: Journal of Computational Physics. - : Elsevier BV. - 0021-9991 .- 1090-2716. ; 236, s. 426-442
-
Tidskriftsartikel (refereegranskat)abstract
- A challenging difficulty in solving the radial Dirac eigenvalue problem numerically is the presence of spurious (unphysical) eigenvalues, among the genuine ones, that are neither related to mathematical interpretations nor to physical explanations. Many attempts have been made and several numerical methods have been applied to solve the problem using the finite element method (FEM), the finite difference method, or other numerical schemes. Unfortunately most of these attempts failed to overcome the difficulty. As a FEM approach, this work can be regarded as a first promising scheme to solve the spuriosity problem com- pletely. Our approach is based on an appropriate choice of trial and test function spaces. We develop a Streamline Upwind Petrov–Galerkin method to the equation and derive an explicit stability parameter.
|
|
