FVENS

This is a cell-centered finite volume solver for the two-dimensional compressible Euler and Navier-Stokes equations. Unstructured grids having both triangles and quadrangles are supported. It includes gradient computation using either Green-Gauss or weighted least-squares methods. WENO (weighted essentially non-oscillatory), MUSCL and linear reconstructions are availble with the Van Albada limiter for MUSCL reconstruction, and the Barth-Jespersen and Venkatakrishnan limiters for linear reconstruction. A number of numerical inviscid fluxes are available - local Lax-Friedrichs (Rusanov), Van Leer flux vector splitting, AUSM, HLL (Harten - Lax - Van Leer), HLLC and Roe-Pike. Modified average gradients are used for viscous fluxes.

For steady-state problems, both explicit and implicit pseudo-time stepping are avaible. Explicit time-stepping uses the forward Euler scheme while implicit time stepping uses the backward Euler scheme; both use local time-steps. 'Dimension independent code' - using the same source code for 2D and 3D problems with only recompilation needed - is a goal.

If you wish to contribute, please read the guidelines.

Features

• Modular, using C++ inheritence
• Almost everything implemented here is parallelized with OpenMP - this means multiple CPU cores can be utilized
• Supports 2D hybrid grids containing both triangles and quadrangles
• Several different inviscid numerical flux options available
• Two different gradient computation schemes
• Several reconstruction schemes using different limiters
• A matrix-free implicit solver implementation is available; however, it uses an approximate stored Jacobian for preconditioning

Limitations

• Limited automated testing as of now
• Two spatial dimensions only at present
• Not many types of boundary conditions are currently implemented - only the simplest or most common ones
• No support for distributed parallelism (no MPI)
• No adaptive mesh refinement
• Cell-centred discretization; this is sometimes a disadvantage with regard to reconstruction, especially in 3D
• Uses PETSc for implicit solution, so the linear solver itself is not thread-parallel as of now

Building

The following libraries and programs are required:

• Boost, present in the default package repositories of all GNU/Linux distributions
• The Eigen matrix library (preferably the development version, or version 3.3.4) - needs a variable called EIGEN3_ROOT to be set to the top-level Eigen directory
• PETSc version 3.8 for sparse linear solvers; needs PETSC_DIR and PETSC_ARCH set, unless PETSc has been installed in standard system locations.
• Gmsh is required for building some of the tests. It is available in most GNU/Linux distributions' official repositories.
• Optionally, BLASTed sparse linear algebra library - needs an environment variable called BLASTED_DIR to be set to the top level BLASTed source directory and BLASTED_BIN_DIR to be set to the BLASTed build directory.

The variables needed can either be passed as arguments to CMake during configuration (see below) or set as environment variables. OpenMP will be used if available (default builds of GCC on most GNU/Linux distributions have this, for instance).

To build go to the FVENS root directory and issue

	mkdir build && cd build
	cmake .. 

and for a release build

	cmake .. -DCMAKE_BUILD_TYPE=Release

-DNOOMP=1 should be appended if OpenMP is not available. -DAVX=1 etc. can be passed if your CPU supports the corresponding vector instructions. See the header of the top-level CMakeLists.txt for a list of all build options. Finally,

	make -j<N>

where '<N>' should be replaced by the number of threads to use for building. The build is known to work with recent versions of GCC C++ (5.4 and above) and Intel C++ (2017 and 2018) compilers on GNU/Linux systems.

To run the tests, run make test or ctest in the build directory.

To build the Doxygen documentation, please type the following command in the doc/ directory:

	doxygen fvens_doxygen.cfg

Of course, this requires you to have Doxygen installed. This will generate HTML documentation, which can be accessed through doc/html/index.htm.

Browsing the code

A tags file is built when the code is built (if ctags is available), which makes navigation in Vim convenient. Using tags in Vim or Spacemacs is easy.

Running

The executables should be called with the paths to a control file (required) and a PETSc options file as input. Set OMP_NUM_THREADS to the number of threads you want to use.

	export OMP_NUM_THREADS=4
	./fvens_steady /path/to/testcases/2dcylinder/implicit.control -options_file /path/to/testcases/2dcylinder/opts.petscrc

Make sure to set the paths of input and output files appropriately in the control file (see the next section below).

Control files

Examples are present (.ctrl files) in the various test cases' directories. Note that the locations of mesh files and output files should be relative to the directory from which the executable is called.

Command Line Options

• --mesh_file If given, this overrides the mesh file specified in the control file.

PETSc options for FVENS

• -mesh_reorder (string argument): If mentioned, the mesh cells will be reordered in the preprocessing stage, into one of the supported PETSc orderings.
• -matrix_free_jacobian (no argument): If mentioned, matrix-free finite-difference Jacobian will be used, but the first-order approximate Jacobian will still be stored for the preconditioner.
• -matrix_free_difference_step (float argument): The finite difference step length to use in case the matrix-free solver is requested; if not mentioned, this defaults to 1e-7.
• -fvens_log_file (string argument): Prefix (path + base file name) of the file into which to write timing logs (.tlog extension), and if requested, nonlinear residual histories (.conv extension). Note that this option, if specified, overrides the corresponding option in the control file.

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Copyright (C) 2016 - 2018, Aditya Kashi. See LICENSE.md for terms of redistribution with/without modification and those of linking.

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.