Questions tagged [multigrid]
An approach to solving systems of equations by projecting the problem from a fine scale representation onto a coarser one. A coarse representation generally has fewer unknowns, making it faster to solve than the original problem. The coarse solution can then be projected back onto the finer problem as an initial guess of the solution to the finer problem.
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How do multigrid approaches deal with Gibbs phenomenon?
I know (from https://scicomp.stackexchange.com/a/31339/20545, among others) that I need a certain mesh density in FEM, else I might get non-physical oscillations in my solution.
How do multigrid ...
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Null space for smoothed aggregation algebraic multigrid
I do not really get the point of null space usage for creating the prolongation operator for smoothed aggregation algebraic multigrid. I know what the null space is per definition and I know that the ...
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Algebraic multigrid for coupled equations
As far as I understand is algebraic multigrid(AMG) a method that was intentionally developed to solve linear systems where every grid point or node has a single DOF. When AMG should now be used for ...
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Algebraic multigrid for complex valued matrices
Assume one uses the classical AMG with Ruge-Stuben coarsening and direct interpolation for solving real valued problems. How can this approach be recycled to also solve complex valued problems like ...
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103 views
Multigrid preconditioner for conjugate gradient methods
When solving $A*x=b$ using preconditioned conjugate gradient methods one has to solve $z=K^{-1}*r$ for the preconditioning where $K$ is the preconditioner of $A$ and $r$ is the residual vector.
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34 views
Converting mass density to point mass approximation on a grid
In an nbody gravity simulation, instead of doing exact(all-pair brute force) solution, I added masses of each body into cells of a 3D grid(each cell is just a float value having a mass value). Then ...
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335 views
Neumann-Neumann boundary intersection
I am trying to solve a Vertex Centered, Finite Difference, Poisson equation $-\nabla^{2}u=f$ with Dirichlet boundaries on the left and bottom and Neumann boundaries on the top and right using ...
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1answer
256 views
Algebraic multigrid in PETSc
Consider a potential/poisson equation on a very large, complicated geometry. Currently, an self-written FEM and linear solvers from NumPy are used. Performance is, of course, not good enough for ...
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233 views
For which problems Krylov subspace methods are preferred over multigrid methods?
As multigrid methods are known to have grid independent convergence rates with $O(N)$ computational cost, then why would one be interested in using Krylov subspace methods at all, for which ...
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230 views
V-cycle Multigrid for 2D transient heat transfer on a square plate using finite difference
I'm currently developing a program to solve 2D transient state heat conduction on a square plate using the V-cycle multigrid. Althought my program is able to reach the steady state solution, it's ...
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1answer
181 views
What is the default smoother for the “PCMG” preconditioner in PETSc?
For a large parallel sparse matrix (mpiaij type matrix) in my code, I was experimenting with various preconditioners to see which one would do best with GMRES/BiCGSTAB. I tried the PCMG ...
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126 views
Can I use multigrid to solve linear algebra problems that do not arise from a differential equation?
Can the Multigrid method be applied to solve a linear system of equations in the form $Ax=b$, that is not necessarily related to differential equations?
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Geometric Multigrid for Conform and Non–″ Elements: Restriction Operators
First of all, let me set up some notations.
Suppose we have a hierarchy of meshes $\mathcal{T}_0 \subset \mathcal{T}_1 \subset \dots \subset \mathcal{T}_n$. For simplicity I restrict myself here to $...
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84 views
What does it take to prove that a multigrid algorithm scales linearly with system size?
It is my understanding that the multigrid solution techniques are generally the preferable method to solve large Poisson problems. Now assume I have written a multigrid solver that is tailored to my ...
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147 views
Multigrid for Robin boundary conditions
I have a question regarding the treatment of Robin boundary conditions in a multigrid solver. I am solving the Poisson equation in $\Omega=(0,1)^2$ with Robin boundary conditions on the boundary,
$$- \...
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172 views
Algorithm for Adaptive Mesh Refinement
I am trying to implement Adaptive Mesh Refinement. I am not a Mathematics/Computational Science person so I will try to write the algorithm in a simpler way. I will be grateful if experts can comment ...
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2answers
480 views
Python environments for AMG and Gauss Seidel as solvers instead of preconditioners
I am working on block preconditioning and seemingly it is common to write customised Krylov solvers for them. Within each solver, the individual block linear system with preconditioners are ...
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Does algebraic multigrid reuse its coarse grids?
Admittedly, I'm new to the subject, so this is probably a really simple question.
Let's assume I want to solve the (large sparse) linear system Ax = b multiple times with algebraic multigrid and b ...
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1answer
235 views
Robust smoothers for geometric multigrid
I'm searching for robust smoothers for geometric multigrids.
By robust I mean:
Effective for high order approximations (say spectral element, spectral Discontinuous Galerkin),
Parallel (suitable for ...
2
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1answer
393 views
How to define residual in multigrid approach?
I wish to solve the two-dimensional Navier Stokes equations using multigrid method on a collocated grid using the Predictor-Corrector method mentioned below. But first, let me elaborate on what I had ...
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2answers
384 views
Algebraic Multigrid: Why does the product of interpolation and restriction not result in something with norm 1?
I'm currently working with "A Multigrid Tutorial" by Briggs et al, Chapter 8.
The construction of the interpolation operator is given as:
Then construction of restriction operator and fine grid ...
2
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1answer
72 views
Solving new linear system that comes from an $p$ enrichment
Let's say I am solving a simple Poisson problem using a Mixed (DG) finite element method. If we use orthogonal polynomials as basis functions we can write the finite-dimensional linear system as
$$
...
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804 views
Quadtree type Grid
I would like to code for a quadtree type meshing but don't know how to do. If anyone can help or can share any starting code?
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1answer
126 views
Solve FEM matrix from coupled system
I'm developing an FEM solver for a coupled system. I have diffusion and potential equations which result in positive definite matrices for each equation, but the coupling makes the overall system ...
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1answer
328 views
Increasing V-cycles for constant Coarsest Grid Size and increasing Fine Grid size
Problem statement
I implemented geometric multigrid for $-\nabla^{2}=f$ where $f=\frac{3\pi^{2}}{4}sin \frac{\pi x}{2} sin \frac{\pi y}{2} sin \frac{\pi z}{2}$ on $\Omega \in [0,1]$ on a unit cube. ...
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Is it usual to have no convergence checking in Multigrid?
I just read Chapter 3 in "A Multigrid Tutorial" by Briggs/Henson/McCormick, link.
The text is about Multigrid cycles such as V-cycle, mu-cycle, FMG. What caught my eye: In most iterative procedures ...
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228 views
Multigrid on “not perfectly rectangular” grid
Multigrid introductions normally use a rectangular grid. Interpolation of values is then straight forward: Just interpolate linearly on the edge between two adjacent nodes of the coarse grid to find ...
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207 views
FENICS subdomains - restriction/ prolongation operators
I am trying to implement my own multigrid method in fenics. Is there any "smart/ fenics" way how to assemble subdomains and obtain restriction/ prolongation operators ?
Thanks!
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1answer
148 views
stabilizing advection-diffusion with multi-grid?
If one chooses to discetize the advection-diffusion (AD) equation using the standard Galerkin finite element method, stability issues may arise in cases of high Peclet number (i.e., high advection to ...
3
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1answer
152 views
Challenges in implementing Algebraic Multigrid on millions of processors
I just implemented an Algebraic Multigrid solver for a Mixed Dirichlet-Neumann Boundary Value problem and was surprised to see the speed-up as compared to a simple iterative solver for a large problem ...
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1answer
110 views
A doubt in Multigrid V-cycle
Assume I have 3 levels of grids. Finest Grid = level 2, Coarser Grid = level 1, Coarsest Grid = level 0.
Relax $u$ on $Au = b$ at level 2 for 3 times.
Find residual $r2$ at level 2, then restrict to ...
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2answers
150 views
Specific questions for 2-D Multigrid
I am simulating $\nabla^{2}u=0$ with mixed Dirichlet-Neumann boundary conditions on 2-D using 2-Grid method. Dirichlet ...
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1answer
771 views
How is Krylov-accelerated Multigrid (using MG as a preconditioner) motivated?
Multigrid (MG) may be used to solve a linear system $Ax=b$ by constructing an initial guess $x_0$ and repeating the following for $i=0,1..$ until convergence:
Compute the residual $r_i = b-Ax_i$
...
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Optimization of nonlocal stencil-like operator on subset of regular grid
I am trying to optimize the execution time for this particular piece of fortran code.
Details:
i_gc is a (ngpts, 3) array of containing (i,j,k) indices for each grid point. This is a subset of the ...
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2answers
222 views
Can F-cycle substitue FMG for update of existent solution?
I have a nicely working multigrid solver, which I use for solving the Poisson equation from an electrostatic problem. I solve this equation first without any charges, and then many times with a slowly ...
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2answers
100 views
Infinite Function Value on Dirichlet Boundary
I have been working on a multigrid solution to a non-homogeneous Dirichlet boundary value problem. However, the function goes to infinity on the boundary. This causes numerical overflow errors to be ...
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2answers
108 views
How to determine the number of c points in algebraic multi grid
I am trying to write an algebraic multi-grid solver (in c++). At a given level I determine which nodes are c-points and which nodes are f-points (where the total number of c and f points equals the ...
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Is the Thomas algorithm the fastest way to solve a symmetric diagonally dominant sparse tridiagonal linear system
I am wondering if the Thomas algorithm is the fastest way (provably?) to solve a symmetric diagonally dominate sparse tridiagonal system in terms of algorithmic complexity (not looking for ...
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1answer
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Multigrid stops converging when more grid levels are used
I'm having a problem with multigrid code I wrote. If I solve Laplace's equation in 2D and use more than 5 grid levels, the V-cycles stop converging after a few cycles (see below, convergence factor > ...
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1answer
195 views
What libraries provide an implementation of multigrid?
I am working on numerical method of Multigrid. What's the available implementation(solver) (actually used in scientific computation) of multigrid method?
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1answer
324 views
Linear Algebra / Numerical Solution Of Matrix With Nullspace
I have a question relating to linear algebra.
We have a fluid solver that solves the poisson equation for pressures. When there are areas of the domain that are entirely enclosed by Neumann ...
3
votes
2answers
377 views
Full Multigrid Performance for Poisson's equation using Higher Order Compact scheme as a Gauss Seidel smoother
I have a question regarding the FMG (Full Multi Grid) performance while computing Poisson's equation using Higher Order Compact discretization. I am using a sixth order compact scheme to discretize ...
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1answer
343 views
Full Multigrid Prolongation Operator
I am looking into full multigrid, FMG, and several sources, including these slides, that a lot of people are referring to, state that the prolongation operator used in FMG the first time you visit a ...
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3answers
439 views
FAS-multigrid slower than linear defect correction?
I have implemented a V-Cycle multigrid solver using both a linear defect correction (LDC) and full approximation scheme (FAS).
My problem is the following: Using LDC the residual is reduced by a ...
6
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1answer
170 views
How to perform multigrid technique when relaxation methods don't converge?
It is well known that, when a system of linear equations is obtained from discretization of partial differential equation, the solution process can be accelerate significantly by multigrid technique. ...
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1answer
117 views
explicitly forming coarse matrices with polynomial smoothing AMG
I've been reading about the algebraic multigrid algorithm and came across polynomial smoothers in this paper. It's my understanding that usually the coarse-level matrices
$A_H = I_h^HA_hI_H^h$
are ...
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47 views
Clarification on interpolation equalities given by Briggs
Briggs, "A Multigrid Tutorial" (pg. 35) has the following expressed as 2-D interpolation:
\begin{align*}
v^h_{2i,2j} &= v_{i,j}^{2h}\\
v^h_{2i+1,2j} &= 0.5\cdot(v_{i,j}^{2h} + v_{i+1,j}^{2h})\\...
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1answer
803 views
Prolongation/Restriction Operator in Multigrid
In Multigrid, using Poisson's equation, does the equality below always hold regardless of what type of boundary conditions you use?
$$
R= c\cdot I^T, \text{ for some constant }c
$$
where $R$ and $I$ ...
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1answer
275 views
Periodic BC for Multigrid in MD
I know that this question might be very specific and maybe nobody will know the answer, but this is probably the only community where I could find an answer:
So, as part of my master's project, I am ...
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2answers
268 views
I'm having trouble debugging multigrid. What to do?
I've spent far too much time coding and debugging multigrid. While I clearly can't post all of my code as it would be silly to ask someone to go through all that code, is there anything I should pay ...
