1710 FVDiscr Example

igor-konshin edited this page Mar 21, 2017 · 6 revisions

Parallel Finite Volume Discretization

The code for this example is located in Examples/FVDiscr

Brief

This example uses simple two-point FVM scheme to solve Poisson's equation in unit cube domain. The following classes are used: Mesh, Partitioner, Solver.

Description

This examples is used to solve the problem div(K grad U) = f with Dirichlet boundary conditions, where K is unit tensor and the right-hand side f is computed from the exact solution: U = sin(PI·x)·sin(PI·y)·sin(PI·z).

This example may run in both serial and parallel modes with NP processes.

The code loads the mesh for unit cube domain. If INMOST is built with USE_PARTITIONER=ON and input mesh is a serial mesh, then the Inner_RCM partitioner is used to partition the mesh.

One layer of ghost cells is created and exchanged. The simplest two-point FVM scheme is used to assemble local matrices. Using ghost cells effectively links local matrices in global matrix. Two-point FVM scheme is only valid when cell faces are orthogonal to segments connecting centers of neighboring cells.

Optionally the code saves the generated matrix and right-hand side in user provided files. The distributed matrix is solved using INNER_ILU2 solver. The solution is compared with known exact solution and C and L₂ norms are computed. The result mesh is saved either in result.vtk, or result.pvtk depending on number of NP.

Arguments

Usage: ./FVDiscr mesh_file [A.mtx b.rhs]
  • First parameter is the mesh file.
  • Two optional parameters – output file names for generated matrix and right-hand side.

Running example

If you compiled INMOST with USE_PARTITIONER=OFF you should provide the prepartitioned mesh, otherwise you can provide either serial mesh, or prepartitioned mesh.

You can generate meshes using GridGen generator. The following line uses /tmp/grid-32-32-32.pvtk mesh from GridGen example.

$ cd Examples/FVDiscr
$ mpirun -np 4 ./FVDiscr /tmp/grid-32-32-32.pvtk /tmp/A.mtx /tmp/b.rhs                                                                   
./FVDiscr
Processors: 4
Load(MPI_File): 0.274381
Assign id: 0.00758195
Exchange ghost: 0.0783911
Matrix assemble: 0.242945
Save matrix "/tmp/A.mtx" and RHS "/tmp/b.rhs": 0.0802951
Solve system: 0.2625042e-07 | 1e-05
err_C  = 0.000801001
err_L2 = 0.000283771
Compute true residual: 0.184962
Retrieve data: 0.000687122
Exchange phi: 0.000224113
Save "result.pvtk": 0.137682

If you have ParaView installed, you can open the result mesh file:

$ paraview --data=result.pvtk

You can view the following tags:

  • Solution – the solution to the problem
  • K – tensor K (constant equal to 1 in this example)
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