What is the basic structure of Open Source CFD Solver?
By Dr. Sharad N. Pachpute
1. Introduction to OpenFOAM
OpenFOAM is an open source CFD software which has a C++ library for more than 80 applications of CFD modeling. This solver has a large number of solvers and utilities covering a broad range of problems related to fluid flow. Any equation as a function of field variable like scalar, vector and tensors can be coded there in the Open FOAM framework . Hence, this solver has been popular in academia and industries.
 OpenFOAM solve the Partial Differential Equations (PDEs) numerically using the finite volumes method (FVM)
 Customized Multiphysics CFD solver for complex flows
 Provides numerical solutions for 3D geometries
 Opensource software developed in C++ (objectoriented programming)
 Can be freely downloaded at www.openfoam.com
 Designed as a toolbox which is easily customizable
 Parallel computation implemented and users can use unlimited cores on HPC
 Can be installed on windows and Linux operating systems. It needs a C++compiler for running the simulation, hence Linux is preferred
OpenFOAM has pre and postprocessing environments. The interface to the pre and postprocessing are called ads utilities. These things ensure that all variables are in OpenFOAM environments. The overall structure of open source CFD solver, OpenFOAM is shown below.
Parts of CFD Simulations in OpenFOAM
 Preprocessing:
 Utilities: these are provided to create or read the mesh model and boundary conditions for simulations and manipulation of input data
 Meshing platforms: blockMesh, snappyHexMesh
 Mesh conversion utilities: ANSYS, Salome, ideas, CFX, StarCCM, Gmsh etc
 Example: blockMesh, CheckMesh, runTime
 Solvers:
 To numerically solve a specific problem based on continuum mechanics for fluid flows
 Different applications are coded in OpenFOAM for CFD simulations: incompressible or compressible flow , heat transfer, multiphase flow, combustion, electromagnetism, turbulence modelling (DNS, RANS, LES) and solid mechanics, etc.
 Example: icoFoam, rhoSonicFoam etc.
 Postprocessing
 After simulation, CFD results can be analzed in ParaView
 The data from OpenFoam can be converted to other data files for other postprocessing tools (FoamToTecplot)
1.2 Advantage of OpenFOAM
 No licensing fees
 Easy to use CFD solvers on any HPC with unlimited cores for complex problems (LES, DNS)
 Direct access to source codes to customize CFD solvers
 additional codetocode for benchmarks,
 Users get regular updates
 More than 80 different solvers and various tutorials
 Ease to implement and program for any equations in partial differential equations
 Active academic and professional communities by forum, conference, training schools
1.3 Disadvantage of OpenFOAM
 Require more time to learn initially
 Lack of complete user guide or direct CFD support
 No integrated graphical user interface (GUI) for preprocessing, solver setting and monitoring the simulations
 More suitable for Linux platform
 CFD users must know Unix command lines and C++ programming
2. OpenFOAM Directory
After installation of OpenFOAM, we can see many subdirectories like applications, source code (src), tutorials, dictionary files, other supporting installation and compilation file.
3. Case Set up in OpenFOAM
To set a case for CFD simulation, we have to create three subdirectories for numerical simulation
 0 :
 All initial boundary conditions are given with different files created for pressure (P), velocity(U), temperature (T), concentration (C) and turbulent viscosity (nut) as key variables implemented in CFD solver
 This directory is common for both steady and unsteady problems
 Constant
 Mesh files: files for boundary conditions, faces and nodes
 Transport properties /Material properties
 Selection of turbulence models like RANS/LES
 System
 conrtolDict: Files to control the simulation
 decomposeParDict: files for parallel simulation on HPC
 fvSchemes: discretization schemes for both temporal and spatial
 fvSolution: selection of tolerance, residual and pressurevelocity coupling
 Additional files for mesh and postprocessing
 After simulation different directories of iteration of time steps are created which have data of domain for velocity, pressure, temperature, TKE, Dissipation rate etc.
4.Discretization schemes in OpenFOAM
 In OpenFOAM case set up, there is a fvSchemes dictionary in the system directory to define different numerical schemes for discretization of different terms in governing equations
Different interpolation schemes are used for various terms in equations. However, the linear interpolation is more effective in most of cases
 OpenFOAM provides enough choices to CFD user for selection of interpolation schemes for different interpolation terms.
4.1 Schemes for Vector fields
For vector fields such as gradient of scalar, various numerical schemes are used in OpenFOAM
4.2 Unsteady Terms
For transient flows, there is an unsteady term (d/dt) governing equation. This is called is the first time derivative. In OpenFoam solver, it is presented as ddtSchemes. The discretisation schemes for each term can be selected from those listed below.
 Steady State: time derivative is set with zero value
 Euler Scheme: this time scheme is suitable for unsteady, 1^{st} order implicit and bounded solution
 Backward scheme: it is suitable for unsteady, 2^{nd} order implicit and potentially unbounded problems
 Crank Nicolson scheme: it is suitable for unsteady, 2^{nd} order implicit and bounded problems. This scheme requires to set a coefficient of 1 for Crank Nicolson and zero for Euler scheme. For most engineering problems it is set with 0.9 to get stable solutions
 In openfam it is written as
 A list of time schemes available in OpenFOAM is given below
4.3 Gradient schemes
a) Gauss linear
 finite volume method (FVM) discretisation of Gaussian integration which interpolate of values from cell centres to face centres
 Linear interpolation scheme or central differencing is used
 Its value can be specified with 1 for boundness and 0 for no boundness
 In some cases, cell Limited scheme used to limit the calculated gradient due to extrapolation from faces. It helps to improve boundedness and stability for velocity gradients for poor mesh quality
b) leastSquares: this is a a secondorder scheme and calculates the least squares distance using all neighbour cells.
c) Gauss Cubic: thirdorder scheme that is suitable for the direct numerical simulation (dnsFoam) on a regular mesh.
4.4 Divergence schemes for Advection and Diffusion
In governing equation such as momentum, energy or species transport there are advection and diffusion terms. These terms are treated as divSchemes in Open FOAM. The nonadvective terms is generally interpolated with the Gauss integration considering a linear variation
 Upwind: This is a firstorder bounded scheme
 Linear: this scheme is based on second order, unbounded.
 Linear Upwind: second order, upwindbiased, unbounded (but much less so than linear). For this discretization, velocity gradients need to be specified
 LUST (Linear Upwind Stabilised Transport) : This scheme is combination of both linear (75%) and linear Upwind (25%). For this discretization, velocity gradients need to be specified. This scheme is used for large eddy simulation (LES).
 Limited Linear: it is a modified linear scheme which limits towards the upwind schemes in the region of a sudden change in gradients. It needs to define with a coefficient of one for upwind and zero for linear.
5. Application source code in OpenFOAM
 The main code application is included in the *.C file.
 In OpenFOAM, depending on flow application, the source code is developed in several other files
 This file is given at the beginning of any piece of code using the class, including the class declaration code itself
 Any piece of code in ( *.C file) can have a number of classes and which are declared in *.H files
 The classes in these files may have other classes
 In OpenFOAM Header files are included in the application code using # include statements: # include “otherHeader.H”
 It is given for compiling to suspend reading from the current file to read the file specified
 Any selfcontained piece of code or custome code is put into a header file and included at the relevant location in the main code in order to improve code readability
 For example, in most OpenFOAM applications , the code for creating or reading field of input data is given in a file createFields.H
Structure of Application in OpenFOAM
We scan consider the sonicFoam application as an example of application directory This application can be found at:
The top level source file takes the application name with the .C extension. For any application, classes are defined on *.H files and governing equations are generally coded in *.C files
5.2 Compilation of Application using the wmake command
For the compilation of a new application, the directory should have a Make subdirectory which contains 2 files: options and files. The wmake command is used to compile a modifies application.
There are several files (*.C and *.H) are given in a particular directory of application. Do required changes as per OpenFOAM framework without any error in compilation
The OpenFoam compiler checks for the included header files in the following order. It is specified with the I option in wmake:
 lnInclude is added as a local directory for incompressible solver (icoFoam): $WM_PROJECT DIR/src/icoFoam/lnInclude
 Platform dependent paths are set in files : $WM_PROJECT_DIR/wmake/rules/$WM_ARCH/
 The full paths of directory to locate header files given in “ Make/options” file using the following syntax:
EXE_INC = \ I$(LIB_SRC)/finiteVolume/lnInclude
6. Implementation of Governing Equations in OpenFOAM
6.1 Mass, Momentum and Enrgy Equation in OpenFOAM
 The conservation of mass, momentum and energy equations are presented in OpenFOAM as follows
 The turbulent kinetic energy equation is given as below
 In open Foam, the above governing equations are implemented as equation representation
Here, the first term represents of the left side the unsteady term, the second term is the divergence f mass flux (phi) and K. The four term is diffusion term.
6.2 Implementation of compressible flow solver (rhoSonicFoam)
 The conservation of equations for compressible flow in OpenFOAM are presented as
7. Pressure and Velocity Coupling in OpenFOAM
a) SemiImplicit Method for PressureLinked Equations (SIMPLE)

 The details of SIMPLE algorithm it is useful for most simple steady flows
 You can find on open foam wiki website:
 The SIMPLE is used to couple the mass and moemtum (NavierStokes) equations during iterative procedure, which is given as below
 set the domain with boundary conditions
 Solve numerically the discretized momentum equation to calculate the intermediate velocities
 Calculate the mass fluxes at the cells faces.
 Solve the pressure corrected equation and apply underrelaxation factors
 Correct the mass fluxes for all the cell faces.
 Correct the velocity fields based on new pressure fields
 Update the boundary conditions.
 Repeat above steps till convergence
b) PISO: PressureImplicit with Splitting of Operators
 This algorithm is more suitable to solve the NavierStokes equations in unsteady (transient) problems of CFD modeling
 It can support worst meshes
c) PIMPLE: combination of SIMPLE and PISO
 This is more suitable to use adaptive steps with a fixed courant number in large eddy simulations (LES)
The PISO loop in OpenFOAM
8.Boundary Conditions in OpenFOAM
 Most of boundary conditions are defined based on fixed value or fixed gradients
 It is important to understand these values before CFD modelling
 For details of boundary conditions: \src\finiteVolume\fields\fvPatchFields\
 There major types of boundary conditions:
 Basic: calculated, fixed value, fixed gradient, zero gradient
 Constraint: symmetry, wedge, cyclic, jump, cyclic AMI
 Derived: slip, no slip, fan pressure, inlet outlet, outlet inlet, total pressure
Reference
 Open FOAM downloads and details of solvers: www.openfoam.org
 OpenFOAM discussion forum: www.cfdonline.com/Forums/openfoam/)
 communitydriven wiki: www.openfoamwiki.net
 Open Foam Blog: Fumiya Nozaki
 Customization of CFD solvers: Chalmer University tutorials on OpenFOAM