Second-order Approximation Scheme Research Articles

High order accuracy scheme for modeling the dynamics of predator and prey in heterogeneous environment

The aim of this work is to develop a compact finite-difference approach for modeling the dynamics of predator and prey based on reaction-diffusion-advection equations with variable coefficients. Methods. To discretize a spatially inhomogeneous problem with nonlinear terms of taxis and local interaction, the balance method is used. Species densities are determined on the main grid whereas fluxes are computed at the nodes of the staggered grid. Integration over time is carried out using the high-order Runge-Kutta method. Results. For the case of one-dimensional annular interval, the finite-difference scheme on the three-point stencil has been constructed that makes it possible to increase the order of accuracy compared to the standard second-order approximation scheme. The results of computational experiment are presented and comparison of schemes for stationary and non-stationary solutions is carried out. We conduct the calculation of accuracy order basing on the Aitken process for sequences of spatial grids. The calculated values of the effective order accuracy for the proposed scheme were greater than the standard two: for the diffusion problem, values of at least four were obtained. Decrease was obtained when directional migration was taken into account. This conclusion was also confirmed for non-stationary oscillatory regimes. Conclusion. The results demonstrate the effectiveness of the derived scheme for dynamics of predator and prey system in a heterogeneous environment.

Implementation of Flux Limiters in Simulation of External Aerodynamic Problems on Unstructured Meshes

The study is dedicated to the peculiarities of implementing the flux limiter of the flow quantity gradient when solving 3D aerodynamic problems using the system of Navier–Stokes equations on unstructured meshes. The paper describes discretisation of the system of Navier–Stokes equations on a finite-volume method and a mathematical model including Spalart–Allmaras turbulence model and the Advection Upstream Splitting Method (AUSM+) computational scheme for convective fluxes that use a second-order approximation scheme for reconstruction of the solution on a facet. A solution of problems with shock wave structures is considered, where, to prevent oscillations at discontinuous solutions, the order of accuracy is reduced due to the implementation of the limiter function of the gradient. In particular, the Venkatakrishnan limiter was chosen. The study analyses this limiter as it impacts the accuracy of the results and monotonicity of the solution. It is shown that, when the limiter is used in a classical formulation, when the operation threshold is based on the characteristic size of the cell of the mesh, it facilitates suppression of non-physical oscillations in the solution and the upgrade of its monotonicity. However, when computing on unstructured meshes, the Venkatakrishnan limiter in this setup can result in the occurrence of the areas of its accidental activation, and that influences the accuracy of the produced result. The Venkatakrishnan limiter is proposed for unstructured meshes, where the formulation of the operation threshold is proposed based on the gas dynamics parameters of the flow. The proposed option of the function is characterized by the absence of parasite regions of accidental activation and ensures its operation only in the region of high gradients. Monotonicity properties, as compared to the classical formulation, are preserved. Constants of operation thresholds are compared for both options using the example of numerical solution of the problem with shock wave processes on different meshes. Recommendations regarding optimum values of these quantities are provided. Problems with a supersonic flow in a channel with a wedge and transonic flow over NACA0012 airfoil were selected for the examination of the limiter functions applicability. The computation was carried out using unstructured meshes consisting of tetrahedrons, truncated hexahedrons, and polyhedrons. The region of accidental activation of the Venkatakrishnan limiter in a classical formulation, and the absence of such regions in case a modified option of the limiter function, is implemented. The analysis of the flow field around a NACA0012 indicates that the proposed improved implementation of the Venkatakrishnan limiter enables an increase in the accuracy of the solution.

An alternative full multigrid SIMPLEC approach for the incompressible Navier–Stokes equations

An alternative approach to solve the steady-state incompressible Navier–Stokes equations using the multigrid (MG) method is presented. The mathematical model is discretized using the finite volume method with second-order approximation schemes in a uniform collocated (nonstaggered) grid. MG is employed through a full approximation scheme-full MG algorithm based on V-cycles. Pressure-velocity coupling is ensured by means of a developed modified SIMPLEC algorithm which uses independent V-cycles for relaxing the pressure-correction and momentum equations. The coarser grids are used only internally in these cycles. All other original SIMPLEC steps can be performed only on the finest grid of the current full MG level. The model problem of the lid-driven flow in the unitary square cavity is used for the tests of the numerical model. Computational performance is measured through error and residual decays and execution times. Good performances were obtained for a wide range of Reynolds numbers, with speedups of orders as high as Linear relationships between execution times and grid sizes were observed for low and high Re values ( and 7,500). For intermediate Re values ( and 1,000), the linear trend was observed from more refined grids (512 onwards).

Stochastic perturbation method for optimal control problem governed by parabolic PDEs with small uncertainties

In this paper, we investigate the first-order and second-order perturbation approximation schemes for an optimal control problem governed by parabolic PDEs with small uncertainties. First, we propose the finite dimensional noise assumption for the random coefficient, and expand the state and co-state variables up to a certain order with respect to a small parameter by using the perturbation technique. Then, we insert all the expansions into the known deterministic parametric optimality system, and obtain the first-order and second-order optimality systems. These two systems are discretized by the finite element method in space and the backward Euler method in time to establish the first-order and second-order approximate schemes. Further, a priori error estimates are derived for the state, co-state and control variables of the first-order and second-order approximation, respectively. Finally, some numerical experiments are presented to verify the theoretical results.

Numerical Solotion of a Problem of Fluid Filtration in a Viscoelastic Porous Medium

The paper considers a model for filtering a viscous incompressible fluid in a deformable porous medium. The filtration process can be described by a system consisting of mass conservation equations for liquid and solid phases, Darcy's law, rheological relation for a porous medium, and the law of conservation of balance of forces. This paper assumes that the poroelastic medium has both viscous and elastic properties. In the one-dimensional case, the transition to Lagrange variables allows us to reduce the initial system of governing equations to a system of two equations for effective pressure and porosity, respectively. The aim of the work is a numerical study of the emerging initial-boundary value problem. Paragraph 1 gives the statement of the problem and a brief review of the literature on works close to this topic. In paragraph 2, the initial system of equations is transformed, as a result of which a second-order equation for effective pressure and the first-order equation for porosity arise. Paragraph 3 proposes an algorithm to solve the initial-boundary value problem numerically. A difference scheme for the heat equation with the righthand side and a Runge–Kutta second-order approximation scheme are used for numerical implementation.

A Finite-Difference Scheme for the One-Dimensional Maxwell Equations

This paper deals with a difference scheme of second-order approximation using Laquerre transform for the one-dimensional Maxwell equations. Supplementary parameters are introduced into this difference scheme. These parameters are obtained by minimizing the error of a difference approximation for a Helmholtz equation. The optimal parameters do not depend on the step size and the number of nodes in the difference scheme. It is shown that the use of the Laguerre decomposition method allows obtaining higher accuracy of approximation of the equations in comparison with similar difference schemes when using the Fourier decomposition method. The second-order finite difference scheme with the parameters is compared to a fourth-order difference scheme in two cases: The use of the optimal difference scheme when solving a problem of electromagnetic pulse propagation in an inhomogeneous medium yields a solution accuracy comparable to that obtained with the fourth-order difference scheme. When solving an inverse problem the second-order difference scheme allows obtaining higher solution accuracy as compared to the fourth-order difference scheme. In these problems the second-order difference scheme with the supplementary parameters has decreased the calculation time by 20–25% as compared to the fourth-order difference scheme.

Performance of Geometric Multigrid Method forTwo-Dimensional Burgers’ Equations with Non-Orthogonal, Structured Curvilinear Grids

This paper seeks to develop an efficient multigrid algorithm for solving the Burgers problem with the use of non-orthogonal structured curvilinear grids in L-shaped geometry. For this, the differential equations were discretized by Finite Volume Method (FVM) with second-order approximation scheme and deferred correction. Moreover, the algebraic method and the differential method were used to generate the non-orthogonal structured curvilinear grids. Furthermore, the influence of some parameters of geometric multigrid method, as well as lexicographical Gauss–Seidel (Lex-GS), η-line Gauss–Seidel (η-line-GS), Modified Strongly Implicit (MSI) and modified incomplete LU decomposition (MILU) solvers on the Central Processing Unit (CPU) time was investigated. Therefore, several parameters of multigrid method and solvers were tested for the problem, with the use of nonorthogonal structured curvilinear grids and multigrid method, resulting in an algorithm with the combination that achieved the best results and CPU time. The geometric multigrid method with Full Approximation Scheme (FAS), V-cycle and standard coarsening ratio for this problem were utilized. This article shows how to calculate the coordinates transformation metrics in the coarser grids. Results show that the MSI and MILU solvers are the most efficient. Moreover, the MSI solver is faster than MILU for both grids generators; and the solutions are more accurate for the Burgers problem with grids generated using elliptic equations.

On the theory of a pulse laser with microwave pumping

The construction of a laser with a microwave pumping based on the light-pumping cell that uses radiation of sulfur vapor exposed to electromagnetic waves of the microwave band is discussed. The advantages of this device are high efficiency of the optical pumping cell, simple structure, the possibility of cooling the pumping element, and easiness of modifying the emission spectrum of the optical pumping by changing the admixtures in the light-emitting cell. A mathematical tool for the simulation of the short-pulse operation of this laser is proposed basing on the solving a generalized wave equation. The microwave waveguide with the sulfur vapors is interpreted as a regular transmission line with essentially nonlinear dispersion characteristic and substantial dissipation. Possible techniques for the numerical solving the generalized wave equation are described. The continuous approximation of the regular dispersive line is used. In the Fourier approach, the electric field in the system is calculated as a series in the longitudinal wavenumber. As an alternative, the D’Alembert approach may be used, with evaluating the electric field as a series in the frequency. Three-layer explicit and implicit second-order approximation schemes for the solving generalized wave equation in the Fourier approach are constructed. The modeling of radio pulse propagation in the “cold” regular electrodynamic system was performed. Stability of the algorithm and qualitative agreement between the numerical and analytic results confirm the correctness of the base equations as well as their solutions based at the finite-difference schemes.

Численное решение одномерной задачи фильтрации несжимаемой жидкости в вязкой пористой среде

Процесс фильтрации жидкости в деформируемой пористой среде описывается системой, состоящей из уравнений сохранения массы для жидкой и твердой фаз, закона Дарси, реологического соотношения типа Максвела и закона сохранения баланса сил. Предполагается, что пороупругая среда обладает преимущественно вязкими свойствами и плотности фаз являются постоянными. В случае одной пространственной переменной переход к переменным Лагранжа позволяет свести исходную систему определяющих уравнений к одному уравнению для искомой пористости. Целью работы является численное исследование возникающей начально-краевой задачи. В пункте 1 дается постановка задачи и краткий обзор литературы по близким к данной теме работам. В пункте 2 проводится преобразование системы уравнений, в результате которого для пористости возникает нелинейное уравнение третьего порядка. В пункте 3 предложен алгоритм численного решения одномерной начально-краевой задачи. Для численной реализации используется однородная разностная схема для уравнения второго порядка с переменными коэффициентами и схема Рунге — Кутта второго порядка аппроксимации. Полученное решение удовлетворяет физическому принципу максимума. В пункте 4 рассматривается более общий случай сведения исходной системы к одному уравнению.DOI 10.14258/izvasu(2018)4-11

Numerical methods for solving the first-kind boundary value problem for a linear second-order differential equation with a deviating argument

Abstract This work is devoted to the numerical methods for solving the first-kind boundary value problem for a linear second-order differential equation with a deviating argument in minor terms. The sufficient conditions of the one-valued solvability are established, and the a priori estimate of the solution is obtained. For the numerical solution, the problem studied is reduced to the equivalent boundary value problem for an ordinary linear differential equation of fourth order, for which the finite-difference scheme of second-order approximation was built. The convergence of this scheme to the exact solution is shown under certain conditions of the solvability of the initial problem. To solve the finite-difference problem, the method of five-point marching of schemes is used.

Parameter-free numerical method for modeling thermal convection in square cavities in a wide range of Rayleigh numbers

Some numerical results for the two- and three-dimensional de Vahl Davis benchmark are presented. This benchmark describes thermal convection in a square (cubic) cavity with vertical heated walls in a wide range of Rayleigh numbers (104 to 1014), which covers both laminar and highly turbulent f lows. Turbulent f lows are usually described using a turbulence model with parameters that depend on the Rayleigh number and require adjustment. An alternative is Direct Numerical Simulation (DNS) methods, but they demand extremely large computational grids. Recently, there has been an increasing interest in DNS methods with an incomplete resolution, which, in some cases, are able to provide acceptable results without resolving Kolmogorov scales. On the basis of this approach, the so-called parameter-free computational techniques have been developed. These methods cover a wide range of Rayleigh numbers and allow computing various integral properties of heat transport on relatively coarse computational grids. In this paper, a new numerical method based on the CABARET scheme is proposed for solving the Navier–Stokes equations in the Boussinesq approximation. This technique does not involve a turbulence model or any tuning parameters and has a second-order approximation scheme in time and space on uniform and nonuniform grids with a minimal computational stencil. Testing the technique on the de Vahl Davis benchmark and a sequence of refined grids shows that the method yields integral heat f luxes with a high degree of accuracy for both laminar and highly turbulent f lows. For Rayleigh numbers up to 1014, a several percent accuracy is achieved on an extremely coarse grid consisting of 20 × 20 cells refined toward the boundary. No definite or comprehensive explanation of this computational phenomenon has been given. Cautious optimism is expressed regarding the perspectives of using the new method for thermal convection computations at low Prandtl numbers typical of liquid metals.

On The Stability of the Disturbance Algorithm for a Semi-Discrete Scheme of Solution of an Evolutionary Equation in the Banach Space

A purely implicit three-layer semi-discrete scheme of second-order approximation is considered in the Banach space. The three-layer semi-discrete scheme is reduced by means of the disturbance algorithm to two two-layer schemes. Using the solutions of these schemes, we construct an approximate solution of the initial problem. The stability of the constructed scheme is proved.

Second-order approximation scheme combined with [formula omitted]-Galerkin MFE method for nonlinear time fractional convection–diffusion equation

In this article, a second-order approximation scheme combined with an H1-Galerkin mixed finite element (MFE) method for solving nonlinear convection–diffusion equation with time fractional derivative is proposed and analyzed. By introducing an auxiliary variable, a coupled system is formulated, then the spatial direction is approximated by H1-Galerkin MFE method and the temporal fractional derivative and integer derivative are discretized by second-order weighted and shifted Grünwald difference (WSGD) formula and linearized second-order difference scheme, respectively. The optimal priori error estimates in L2 and H1-norm for the unknown function and the auxiliary variable with second-order convergent rate in time are obtained. Compared to the commonly used L1-approximation with (2−α)th-order convergence rate, our method can arrive at the order 2 in time. What is more, compared with the standard finite element method, our method can well approximate the auxiliary variable. Finally, the detailed computational process of the studied numerical algorithm is shown and a nonlinear numerical example with calculated data and some figures is provided to verify our theoretical analysis.

Parameter-free numerical method for modeling thermal convection in square cavities in a wide range of Rayleigh numbers

Some computational results for the two and three-dimensional Davis benchmark are presented. This benchmark represents thermal convection in a square (cubical) cavity with vertical active walls in a wide range of Rayleigh numbers (104to 1014), which covers both laminar and highly turbulent flows. A turbulence model with parameters that depend on a Rayleigh number and require adjustment is usually used to describe turbulent flows. An alternative is Direct Numerical Simulation (DNS) methods, but they demand extremely large computational grids. Recently there has been an increasing interest in DNS methods with incomplete resolution, which are able to provide in some cases acceptable results without resolving Kolmogorov scales. On the basis of such an approach the so-called parameter-free computational techniques have been developed. These methods cover wide range of Rayleigh numbers and allow computing various integral properties of heat transport on relatively coarse computational grids. In this paper, a new numerical method based on the CABARET scheme is proposed for solving Navier-Stokes equations with Boussinesq approximation. This turbulent model-free technique includes no additional parameters and has a second-order approximation scheme in time and space on uniform and non-uniform computational grids with minimal computational stencil. Testing of the technique on the Davis benchmark and the sequence of refined grids shows that the method allows one to compute integral heat fluxes with a high degree of accuracy both for laminar and highly turbulent flows. For the Rayleigh numbers up to 1014, a several percent accuracy has been achieved on an extremely coarse grid consisting of 20×20 cells refined toward boundary. There is no a definite and comprehensive explanation of this computational phenomenon. Cautious optimism exists regarding the perspectives of using the new method of thermal convection computations for low Prandtl numbers typical of liquid metals.

Algebraic-diagrammatic construction polarization propagator approach to indirect nuclear spin–spin coupling constants

A new polarization propagator approach to indirect nuclear spin-spin coupling constantans is formulated within the framework of the algebraic-diagrammatic construction (ADC) approximation and implemented at the level of the strict second-order approximation scheme, ADC(2). The ADC approach possesses transparent computational procedure operating with Hermitian matrix quantities defined with respect to physical excitations. It is size-consistent and easily extendable to higher orders via the hierarchy of available ADC approximation schemes. The ADC(2) method is tested in the first applications to HF, N(2), CO, H(2)O, HCN, NH(3), CH(4), C(2)H(2), PH(3), SiH(4), CH(3)F, and C(2)H(4). The calculated indirect nuclear spin-spin coupling constants are in good agreement with the experimental data and results of the second-order polarization propagator approximation method. The computational effort of the ADC(2) scheme scales as n(5) with respect to the number of molecular orbitals n, which makes this method promising for applications to larger molecules.

Method of adaptive artificial viscosity

A new finite-difference method for the numerical solution of gas dynamics equations is proposed. This method is a uniform monotonous finite-difference scheme of second-order approximation on time and space outside of domains of shock and compression waves. This method is based on inputting adaptive artificial viscosity (AAV) into gas dynamics equations. In this paper, this method is analyzed for 2D geometry. The testing computations of the movement of contact discontinuities and shock waves and the breakup of discontinuities are demonstrated.

Nonlinear monotonization of the Babenko scheme 1

Abstract The goal of the paper is to present and test the nonlinear monotonization of the Babenko scheme for solving 2D linear advection equation with alternating‐sign velocities. The numerical method of monotonization is based on the idea of limited artificial diffusion. There are some approaches for constructing quasi‐monotonic second order approximation schemes for solving hyperbolic systems and equations of gas dynamics: flux correction methods, the Godunov method, TVD methods and others. In particular, many authors developed the idea of TVD method. We try to use this idea to get a new quasi‐monotonic high order accuracy scheme based on the well‐known non‐monotonic Babenko scheme. The algorithm is presented for 1D problem. For testing 2D problem we use the splitting algorithm. The proposed monotonized scheme has shown the best results among all considered in the paper schemes especially for non‐smooth initial profile.

TVD scheme of second-order approximation on a nonstationary adaptive grid for hyperbolic systems

-In this paper we propose a finite difference scheme of second-order approximation, which is a generalization of the familiar Harten scheme to the case of nonstationary adaptive grids. We obtain the conditions under which the scheme satisfies the TVD condition. We test the constructed scheme, using the systems of shallow water equations and gas dynamics equations as an example. In the framework of the nonlinear dispersive Zheleznyak-Pelinovskii model we simulate the process of interaction of an undular bore with a wall.

A method for structural optimization which combines secondorder approximations and dual techniques

A structural optimization problem is usually solved iteratively as a sequence of approximate design problems. Traditionally, a variety of approximation concepts are used, but lately second-order approximation strategies have received most interest since high quality approximations can be obtained in this way. Furthermore, difficulties in choosing tuning parameters such as step-size restrictions may be avoided in these approaches. Methods that utilize second-order approximations can be divided into two groups; in the first, a Quadratic Programming (QP) subproblem including all available second-order information is stated, after which it is solved with a standard QP method, whereas the second approach uses only an approximate QP subproblem whose underlying structure can be efficiently exploited. In the latter case, only the diagonal terms of the second-order information are used, which makes it possible to adopt dual methods that require separability. An advantage of the first group of methods is that all available second-order information is used when stating the approximate problem, but a disadvantage is that a rather difficult QP subproblem must be solved in each iteration. The second group of methods benefits from the possibility of using efficient dual methods, but lacks in not using all available information. In this paper, we propose an efficient approach to solve the QP problems, based on the creation of a sequence of fully separable subproblems, each of which is efficiently solvable by dual methods. This procedure makes it possible to combine the advantages of each of the two former approaches. The numerical results show that the proposed solution procedure is a valid approach to solve the QP subproblems arising in second-order approximation schemes.

Approximation scheme for the three-particle propagator.

The three-particle (ppp) propagator, which is that component of the three-particle Green's function describing triple ionization, is investigated. In particular, a second-order approximation scheme for this propagator is presented, suitable for the calculation of triply charged electronic states. Two different methods are used to derive the explicit working equations. The first method is purely algebraic and is based on the connection of propagators and effective Hamiltonians. The secondis the well-established algebraic-diagrammatic-construction method, which makes use of Feynman diagrams