Uzawa Method Research Articles

The algorithmic resolution of spectral-element discretization for the time-dependent Stokes problem

We consider two algorithms for the resolution of the time-dependent Stokes problem with nonstandard boundary conditions by the domain-decomposition spectral-element method. The first algorithm (Elimination method) is based on the Uzawa method by decoupling the linear system, while the second algorithm (Global inversion) is based on the overall resolution of the system by the GMRES method. A detailed implementation is proposed and some numerical tests are carried out in two and three dimensions and where the domain is multiply connected.

Optimized parameterized Uzawa methods for solving complex Helmholtz equations

As we know that, the parameterized Uzawa (PU) method can be very efficient when used to solve the standard saddle point problem, especially, when we have good and accurate estimation of preconditioned Schur complement matrix. In this paper, by taking full use of the special structure of coefficient matrix arising from complex Helmholtz equations, two types of optimized PU (OPU) methods are discussed theoretically and experimentally. Specifically, the convergence factors of these two OPU methods are less than 0.172, and the optimal result of first OPU method can reach 0.0396 with σ1≥σ2>0, which is currently the best theoretical result in the literatures. Moreover, the second OPU method has better computational advantages compared with the first OPU method as it avoids the inverse computation of W at each step, indicating the less CPU time will be costed by the second OPU method. In addition, the optimal parameters involved in our algorithms consist of constants, reducing the computational complexity associated with parameter selection. Finally, numerical results are given, not only show the effectiveness of OPU methods, but also confirm the rationality of theoretical analysis.

Unconditionally stable fully‐discrete finite element numerical scheme for active fluid model

AbstractIn this paper, we propose a linear, decoupled, unconditionally stable fully‐discrete finite element scheme for the active fluid model, which is derived from the gradient flow approach for an effective non‐equilibrium free energy. The developed scheme is employed by an implicit‐explicit treatment of the nonlinear terms and a second‐order Gauge–Uzawa method for the decoupling of computations for the velocity and pressure. We rigorously prove the unique solvability and unconditional stability of the proposed scheme. Several numerical tests are presented to verify the accuracy, stability, and efficiency of the proposed scheme. We also simulate the self‐organized motion under the various external body forces in 2D and 3D cases, including the motion direction of active fluid from disorder to order. Numerical results show that the scheme has a good performance in accurately capturing and handling the complex dynamics of active fluid motion.

An improved preconditioned inexact Uzawa method for elliptic optimal control problems

An improved preconditioned inexact Uzawa method for elliptic optimal control problems

The Gauge–Uzawa methods for incompressible fluids with microstructure

In this paper, the Gauge–Uzawa method for the time‐dependent incompressible micropolar fluid problem is proposed. Firstly, the first‐order and second‐order Euler semi‐implicit Gauge–Uzawa methods for the micropolar fluid are constructed, and the unconditionally stability analysis and error estimate of the corresponding first‐order scheme are deduced. Secondly, the fully discrete scheme with finite element approximation in space is presented, and the stability result of the corresponding first‐order scheme is derived. Finally, some numerical results are given, which test the theoretical analysis results.

Gauge–Uzawa-based, highly efficient decoupled schemes for the diffuse interface model of two-phase magnetohydrodynamic

In this paper, we design two totally decoupled, linear and unconditionally energy stable schemes with first-order time accuracy for the multi-physics diffuse interface model of two-phase magnetohydrodynamic (MHD) problem. These schemes combine the semi-implicit stabilization method/invariant energy quadratization (IEQ) method for the phase-field equations, Gauge–Uzawa method for the MHD equations, with some subtle implicit–explicit treatments for nonlinear coupled terms. Then this strong nonlinear and coupled complex system is split into a series of small linear elliptic equations, which makes the calculation more efficient. Additionally, compared to the standard projection method, the given schemes can overcome the selection of the initial value about pressure p and the treatment of artificial boundary condition on p which can lead to boundary layers and lose accuracy. Finally, rigorous proof of unconditional energy stability and several simulations of numerical experiment are conducted to demonstrate the validity of the decoupled schemes.

The Simulation of the Nonlinear Consolidation of Porous Media

In this paper, the general formulation of the problem of coupled deformation of a porous deformable medium with a fluid flowing through the pores is formulated, mathematically investigated and numerically implemented within the framework of physical and geometric nonlinearity. We present the formulation of the problem in velocities of solid phase displacements and the rate of pore pressure change in differential and variational forms. A phenomenological approach was used to formulate the mechanical model. The equations of the coupled consolidation model were derived from the general conservation laws of continuum mechanics using spatial averaging over a representative volume element. The consolidation model took into account the change in the porosity and permeability of the medium during deformation. The equations of filtration and porosity change, originally presented in Euler approach, were reformulated in Lagrangian coordinates of the solid phase using the relative fluid velocity according to ALE (Arbitrary Lagrangian - Eulerian) approach. The Gâteaux differentiation technique was used to linearize the variational equilibrium equations. For spatial discretization of the saddle system of equations, the finite element method (FEM) was used: quadratic serendipity elements for approximating the equilibrium equations and Brick type elements for approximating the filtration equation. To solve the system of equilibrium and filtration equations, a generalization of the implicit scheme with internal iterations at each time step by the Uzawa method was used. The results of numerical simulation of elastoplastic deformation of a water-saturated soil under load with fluid outflow are presented. To simulate the constitutive relations of elastoplastic deformation of soil under short-term loads, a generalization of S.S. Grigoryan's model to large deformations is proposed. The calculations were carried out in our own program code. The developed consolidation model can be used to simulate the formation of tracking ruts and unevenness of natural roads, as well as to calculate the uneven settlement of engineering structures.

Сравнение эффективности методов решения систем линейных уравнений в задаче моделирования контактного взаимодействия элементов твэла в термоупругой постановке

Various algorithms for solving systems of linear algebraic equations that arise when modeling problems of contact interaction of thermoelastic bodies with a changing configuration of the contact surface are considered. The implementations of modified iterative methods, the Uzawa method, as well as algorithms based on the use of the Gauss method and the generalized method of minimal residuals (GMRES) with two iterative refinement options allowing for the exit from contact of individual sections of the contact boundary are presented. The efficiency of the presented algorithms is compared in relation to a problem simulating thermomechanical processes in a section of a fuel element that includes up to 100 fuel pellets.

Enhanced algorithms for solving the spectral discretization of the vorticity\u2013velocity\u2013pressure formulation of the Navier\u2013Stokes problem

The objective of the article is to improve the algorithms for the resolution of the spectral discretization of the vorticity–velocity–pressure formulation of the Navier–Stokes problem in two and three domains. Two algorithms are proposed. The first one is based on the Uzawa method. In the second one we consider a modified global resolution. The two algorithms are implemented and their results are compared.

The modified Uzawa methods for solving singular linear systems

For nonsingular linear systems, Yun in 2013 studied three variants of the Uzawa method; see Yun (2013) [18]. These methods contain the Uzawa-AOR method and the Uzawa-SAOR method as special cases. On the basis of the Uzawa-AOR method and the Uzawa-SAOR method, Li in 2017 proposed two modified Uzawa methods by constructing the coefficient matrix of AOR method into a new form of the product of lower triangular matrix and upper triangular matrix; see Li (2017) [26]. In this paper, we present two modified Uzawa methods for solving singular linear systems. The semi-convergence of these methods is analyzed by using the techniques of singular value decomposition and Moore-Penrose inverse. The numerical results are used to verify the theoretical results.

A general Uzawa-type method for a class of 2 × 2 block structure linear system

Numerical methods for Navier-Stokes equation and elasticity problem lead to a class of 2×2 block structure linear system. Based on a general splitting for the (1, 1) block of coefficient matrix and the Uzawa method, we consider a general Uzawa-type method for solving this 2×2 block structure linear system in this paper. With different choices of splitting matrix, the proposed Uzawa-type method includes some existing Uzawa-type methods as its special cases. The convergence and semi-convergence of the proposed method for solving nonsingular and singular problems are studied, respectively. When the splitting matrix is used as a preconditioner, we analyze the spectral properties of the preconditioned matrix. Numerical results are presented to verify the efficiency of the proposed method.

Induced Charge Distribution Using Accelerated Uzawa Method

분자동역학에서의 원자들의 유도전하를 계산하기 위해서는 유도전하를 미지수로 하는 선형방정식을 풀어야 하는데 원자들의 위치가 변화할 때마다 필요한 계산이므로 상당한 계산비용이 요구된다. 따라서 효율적인 유도전하 계산 방법은 다양한 시스템을 해석하기 위해서 필수적이다. 본 연구에서는 constraints가 존재하는 Lagrange 방정식의 해에 대한 선형 시스템, 즉 saddle point를 가지는 문제를 해결하기 위해서 Uzawa method를 도입하였다. Uzawa 매개변수가 수렴 속도에 영향을 미치는 단점을 극복하고 행렬 연산의 효율성을 위해서 Schur complement와 preconditioned conjugate gradient (PCG) 방법을 통해 계산의 효율성을 극대화하는 가속 Uzawa algorithm을 적용한다. 두 금속 나노입자가 전기장에 놓여진 분자동역학 수치모델을 통해서 제시된 방법이 유도전하계산의 수렴성, 효율성 측면에서 모두 향상된 결과를 도출함을 확인하였다. 특히 기존의 가우스 소거법에 의한 계산보다 약 1/10으로 계산비용이 절감되었고, 기본 Uzawa method에 비하여 conjugate gradient (CG)의 높은 수렴성이 입증되었다.

Krylov improvements of the Uzawa method for Stokes type operator matrices

The paper is devoted to Krylov type modifications of the Uzawa method on the operator level for the Stokes problem in order to accelerate convergence. First block preconditioners and their effect on convergence are studied. Then it is shown that a Krylov–Uzawa iteration produces superlinear convergence on smooth domains, and estimation is given on its speed.

An Optimal Finite Element Method with Uzawa Iteration for Stokes Equations including Corner Singularities

The Uzawa method is an iterative approach to find approximated solutions to the Stokes equations. This method solves velocity variables involving augmented Lagrangian operator and then updates pressure variable by Richardson update. In this paper, we construct a new version of the Uzawa method to find optimal numerical solutions of the Stokes equations including corner singularities. The proposed method is based on the dual singular function method which was developed for elliptic boundary value problems. We estimate the solvability of the proposed formulation and special orthogonality form for two singular functions. Numerical convergence tests are presented to verify our assertion.

Сравнение методов решения систем линейных уравнений, возникающих в задачах с изменяющейся конфигурацией контактной поверхности

The article discusses iterative algorithms for solving systems of linear algebraic equations arising when solving problems of contact interaction of thermoelastic bodies with a changing configuration of the contact surface. The implementations of the modified symmetric successive upper relaxation method (MSSOR), modified successive upper relaxation method (MSOR), modified Jacobi method (MJOR) and Uzawa method as well as algorithms based on the Gauss method with two options for iterative refinement, allowing to consider the exit from the contact of the contact border sections, are presented. The results of the application of these algorithms to the demo problem that simulates thermomechanical processes in the section of the fuel element, which includes two tablets and a fragment of the shell, are presented.

A GPIU method for fractional diffusion equations

The fractional diffusion equations can be discretized by applying the implicit finite difference scheme and the unconditionally stable shifted Grünwald formula. Hence, the generating linear system has a real Toeplitz structure when the two diffusion coefficients are non-negative constants. Through a similarity transformation, the Toeplitz linear system can be converted to a generalized saddle point problem. We use the generalization of a parameterized inexact Uzawa (GPIU) method to solve such a kind of saddle point problem and give a new algorithm based on the GPIU method. Numerical results show the effectiveness and accuracy for the new algorithm.

Convergence analysis for modified PAHSS-PU method with new parameter setting

Based on the modified preconditioned accelerated Hermitian and skew-Hermitian splitting (MPAHSS) and triangular splitting iterative (TSI) methods, this paper presents a new parameter setting to overcome the drawbacks of the MPAHSS-parameterized Uzawa (MPAHSS-PU) method proposed by Huang et al. (J Comput Appl Math 332:1–12, 2018) for sthe saddle point problems. A sufficient condition is provided to ensure the convergence of MPAHSS-PU method with the new parameter setting, and a selection strategy for its parameters is also given. The new parameter setting not only lessens the parameter limitation of the MPAHSS-PU method, but also improves its performance. The validity of the obtained results and the performance of MPAHSS-PU method with the new parameter setting are demonstrated by numerical examples.

An Uzawa-type algorithm for the coupled Stokes equations

An Uzawa-type algorithm is designed for the coupled Stokes equations discretized by the mixed finite element method. The velocity solved by the presented algorithm is weakly divergence-free, which is different from the one solved by the common Uzawa method. Besides, an optimal relaxation parameter of the presented algorithm is provided.

A fast constrained image segmentation algorithm

Normalized cut or Ncut has been one of the most widely used models for image processing. A constraint can also be included in the framework of Ncut to represent a priori information for an effective image segmentation. This results in the so-called constrained Ncut problem. In this paper we present an observation that the constrained Ncut problem can be formulated as an indefinite system of equations under a mild condition on targeted segmentation results. We then show that the indefinite system can be effectively handled by the Augmented Lagrangian Uzawa iterative method together with a classical Algebraic Multigrid Method. Both mathematically and numerically, we demonstrate that the Augmented Lagrangian Uzawa method achieves a solution in one iteration. We show how the proposed method can be efficiently applied for the newly tested recursive two-way Ncut with constraints, i.e., a new constrained sequential segmentation as well. A number of numerical experiments are presented to confirm our theory and to show the superiority of the proposed method. In particular, numerical experiments show that the speed of our algorithm can be orders of magnitude faster than the previously proposed Projected Power Method, a significant improvement of conventional image segmentation algorithms.

Variable parameter Uzawa method for solving a class of block three-by-three saddle point problems

In this work, we consider numerical methods for solving a class of block three-by-three saddle point problems, which arise from finite element methods for solving time-dependent Maxwell equations and a class of quadratic programs. We present a variant of Uzawa method with two variable parameters for the saddle point problems. These two parameters can be updated easily in each iteration, similar to the evaluation of the two iteration parameters in the conjugate gradient method. We show that the new iterative method converges to the unique solution of the saddle point problems under a reasonable condition. Numerical experiments highlighting the performance of the proposed method for problems are presented.