nickbobolea/flow-header

Introduction

Purpose

This guide describes a workflow for setting up and executing OpenFOAM studies and uses the simulation of water flow through a header to illustrate the approach. The workflow is comprised of a number of steps discussed individually. The purpose of the study is to evaluate the pressure, the velocity and the temperature patterns developing in the fluid domain for different conditions.

The study consists of two cases symmetric-velocity-temperature and asymmetric-velocity-temperature. The cases use the same Geometry and Mesh, but employ different Initial Conditions (IC) and Boundary Conditions (BC) for the inlet patches, as presented in the Model section.

Platform

This study is executed on a desktop workstation with the following configuration:

CPU (lscpu): AMD Ryzen 7 1800X Eight-Core Processor
OS (uname -a): Linux nick-AX370-Gaming 5.4.0-26-generic #30-Ubuntu SMP Mon Apr 20 16:58:30 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
OpenFOAM: Version 8, Build 8-1c9b5879390b
Gnuplot (gnuplot -V): gnuplot 5.2 patchlevel 8

Geometry

The geometry generation must create a smooth, clean and watertight geometry. A watertight geometry means a close body with no holes or overlapping surfaces. The mesh quality and, hence the solution quality, depend on the geometry. The geometry should adequately represent the fluid domain. Geometry defeaturing is used to simplify the geometry to retain only the features important for the fluid phenomena under investigation. The solid modeling application used for the geometry generation is Onshape.

The fluid domain is comprised of two inlet pipes, a common header and an outlet pipe. Each inlet pipe is comprised of a cylindrical vertical section and a cylindrical horizontal section. The horizontal section of an inlet pipe is connected to the header at a 90 degrees angle. The header consists of a horizontal pipe section. The outlet pipe consists of a horizontal section connected to the middle of the header at a 90 degrees angle.

The fluid domain geometry is created as a single watertight solid by extruding and sweeping geometry sketches. To facilitate meshing and model development, the solid geometry faces representing patches subsequently used to define fluid boundary conditions (e.g., inlet, outlet) are deleted. After the faces are deleted, the solid geometry becomes a surface geometry. The inlet and outlet patches are recreated as individual surfaces by revolving geometry sketches.

The final fluid domain geometry is comprised of 4 surface objects. The surface objects are exported from Onshape as individual stl files (text format) using the high resolution option. The 4 surface objects, inlet1.stl, inlet2.stl, outlet.stl and shell.stl, are saved to the constant/triSurface folder. The fluid domain geometry, in meters, is shown in Figure 1.

Figure 1 - Fluid Domain Geometry and Dimensions

Mesh

The mesh generation uses blockMesh and snappyHexMesh.

blockMesh

blockMesh is used to generate the background mesh for snappyHexMesh. A quality background mesh is generated when the following criteria are met:

The background mesh must consist purely of hexahedral cells,
The cell aspect ratio (i.e. the ratio of the longest to the shortest side of a cell) should be close to 1, at least near the stl surface,
The cell size shall be sufficiently small to adequately resolve the smallest geometry feature,
There must be at least one intersection of a cell edge with the stl surface.

blockMesh uses the system/blockMeshDict dictionary file to generate the mesh. The mesh size is set by variables xmin, xmax, ymin, ymax, zmin and zmax which define a block enclosing the fluid domain geometry. The number of cells along X, Y and Z dimensions is set by variables xcells, ycells and zcells selected such that the criteria above are met. The boundary of the mesh, given in the list named boundary, consists of 5 patches frontandback, inlet, outlet, lowerwall and, upperwall. These patches will be discarded during the mesh generation with snappyHexMesh and replaced by the geometry information defined in the system/snappyHexMeshDict dictionary file.

In addition to the system/blockMeshDict dictionary file, blockMesh also requires the system/controlDict dictionary file. blockMesh is executed in the case folder using:

blockMesh | tee log.01.blockMesh

blockMesh execution generates the background hexahedral mesh in constant/polyMesh folder and the log file log.blockMesh in the case folder. The mesh information is contained in the constant/polyMesh/boundary, constant/polyMesh/faces, constant/polyMesh/neighbour, constant/polyMesh/owner and constant/polyMesh/points files.

The blockMesh mesh quality is assessed using:

checkMesh -allGeometry -allTopology | tee log.02.checkMesh.block

snappyHexMesh

The fluid domain mesh generation with snappyHexMesh requires system/fvSchemes and system/fvSolution dictionary files, which are discussed in the Solver section. The mesh generation process is comprised of a number of steps, described below.

Geometry Edge Extraction

The surfaceFeatures utility is used to extract the geometry edges and allow for better meshing with snappyHexMesh on these edges. The system/surfaceFeaturesDict dictionary file lists the geometry surface (stl) files in the surfaces list. The edges marked for extraction are those whose adjacent surfaces normal are at an angle less than the angle specified by includedAngle in the system/surfaceFeaturesDict dictionary file. The surfaceFeatures utility is executed in the case folder using:

surfaceFeatures | tee log.03.surfaceFeatures

The execution of surfaceFeatures utility creates the following files for each stl file:

A .eMesh file in the constant/triSurface folder,
A .extendedFeatureEdgeMesh and a _edgeMesh.obj in the constant/extendedFeatureEdgeMesh folder.

Mesh Parameters

The system/snappyHexMeshDict dictionary file contains the snappyHexMesh parameters. The execution of snappyHexMesh consists of three steps castellating, snapping and layering. These steps can be enabled or disabled as needed.

The geometry dictionary lists the geometry surfaces (stl) files along with their type and name (user defined). The name is used in the Model section for IC and BC. A refinement region, refinementBox, is also defined.

The castellatedMeshControls dictionary controls the parameters for mesh refining in the castellating step.

The features dictionary is used for edge refinement of the *.eMesh edges to the desired refinement level.
The refinementSurfaces dictionary is used for surface based refinement. For each surface, two refinement levels are defined. The first level is the minimum level that every cell intersecting the surface gets refined up to. The second level is the maximum level of refinement. The patchInfo dictionary sets the patch type for each surface. The surface patch type must correspond to the associated BC type defined in the IC and BC dictionary files from the Model section.
The resolveFeatureAngle parameter allows edges, whose adjacent surfaces normal are at an angle higher than the value set, to be resolved. A lower value for resolveFeatureAngle results in a better resolution at sharp edges.
The refinementRegions dictionary contains the volume based refinement parameters for the shell region defined in the geometry dictionary. The first number of the levels parameter represents the distance from the geometry within which all cells are refined while the second number represents the refinement level.
The locationInMesh parameter identifies a location in the final mesh (inside the fluid domain) from which snappyHexMesh will mark and keep all connected cells.

The snapControls dictionary controls the parameters for refining the mesh in the snapping step. This step adapts the castellated mesh to the geometry.

The addLayersControls dictionary controls the parameters inserting prismatic cell layers on shell surface. The number of layers for the shell surface is set by nSurfaceLayers to 3. Because the relativeSizes is set to false, the thickness of the first layer is set by firstLayerThickness to 0.004 m. The minimum thickness of any layer is set by minThickness to 0.004 m. The increase in size from one layer to the next is set by expansionRatio to 1.2.

Mesh Quality Parameters

The mesh quality for snappyHexMesh is controlled by the entries in the meshQualityControls dictionary in the system/snappyHexMeshDict dictionary file. The meshQualityControls dictionary uses an include statement to include the mesh quality parameters defined in the system/meshQualityDict dictionary file.

Mesh Decomposition

The decomposePar utility is used to decompose the mesh into sub-domains, allocated to separate processors, to allow for parallel mesh generation or solver execution. The system/decomposeParDict dictionary file contains the decomposePar utility parameters. The mesh is decomposed into 8 sub-domains by setting numberOfSubdomains to 8. The decomposition method is set to simple using the method parameter. For the simple method, the n parameter in the simpleCoeffs dictionary decomposes the mesh into 2 sub-domains along the x, y and z directions, respectively. The decomposePar utility is executed in the case folder using:

decomposePar | tee log.04.decomposePar

The execution of decomposePar utility creates a processorX/constant/polyMesh folder for each processor. The folder contains the sub-domain blockMesh mesh assigned to that processor.

Mesh Execution

The snappyHexMesh is executed in parallel in the case folder using:

mpirun -np 8 snappyHexMesh -overwrite -parallel | tee log.05.snappyHexMesh

Mesh Quality Evaluation

The snappyHexMesh mesh quality is assessed using:

mpirun -np 8 checkMesh -latestTime -allGeometry -allTopology -parallel | tee log.06.checkMesh.snappy

Mesh Reconstruction

The reconstructParMesh utility reads the sub-domain (processor) snappyHexMesh mesh and updates the mesh in the constant/polyMesh folder. The reconstructParMesh utility is executed in the case folder using:

reconstructParMesh -latestTime -constant | tee log.07.reconstructParMesh

After the snappyHexMesh mesh is reconstructed, the sub-domain (processor) mesh can be removed using:

rm -rf processor* > /dev/null 2>&1

Mesh Optimization

The renumberMesh utility is used to reduce bandwidth and speed up computation on the generated mesh. The renumberMesh utility is executed in the case folder using:

renumberMesh -overwrite | tee log.08.renumberMesh

Mesh Visualization

The snappyHexMesh mesh can be visualized with Paraview using:

paraFoam

Model

The study simulates incompressible, nonisothermal, buoyant, turbulent flow of water through a header using the buoyantPimpleFoam OpenFOAM solver.

Initial Conditions (ICs) and Boundary Conditions (BCs)

The Initial Conditions (ICs) and Boundary Conditions (BCs) are contained in the 0.orig folder as individual dictionary files for each field.

Velocity

The ICs and BCs for the velocity field, [m/s], are contained in the 0.orig/U dictionary file. The fluid domain initial velocity is set by the internalField keyword to a uniform value of 0 m/s.

inlet1 and inlet2 are defined as patch type patch in the refinementSurfaces sub-dictionary of system/snappyHexMeshDict dictionary file. The boundary condition type is set to fixedValue which defines a constant inlet uniform velocity as presented in Table 1.

Table 1 - Velocity Boundary Conditions

Case	`inlet1` Velocity [m/s]	`inlet2` Velocity [m/s]
symmetric-velocity-temperature	1.5	1.5
asymmetric-velocity-temperature	1.5	2.0

outlet is defined as patch type patch in the refinementSurfaces sub-dictionary of system/snappyHexMeshDict dictionary file. The boundary condition type is set to pressureInletOutletVelocity which defines a zero-gradient condition for flow out the fluid domain.

shell is defined as patch type wall in the refinementSurfaces sub-dictionary of system/snappyHexMeshDict dictionary file. The boundary condition type is set to fixedValue of 0 m/s.

Pressure

The ICs and BCs for the pressure field, [Pa], are contained in the 0.orig/p dictionary file. The fluid domain initial pressure is set by the internalField keyword to a uniform value of 0 Pa.

The boundary condition type for inlet1, inlet2 and shell is set to zeroGradient. The boundary condition type for outlet is set to totalPressure and the pressure magnitude is set by the p0 keyword.

Pseudo Hydrostatic Pressure

The ICs and BCs for the pseudo hydrostatic pressure field, [Pa], are contained in the 0.orig/p_rgh dictionary file. The fluid domain initial pseudo hydrostatic pressure is set by the internalField keyword to a uniform value of 0 Pa.

The boundary condition type for inlet1, inlet2 and shell is set to zeroGradient. The boundary condition type for outlet is set to prghTotalPressure and the pseudo hydrostatic pressure magnitude is set by the p0 keyword.

Temperature

The ICs and BCs for the temperature field, [K], are contained in the 0.orig/T dictionary file. The fluid domain initial temperature is set by the internalField keyword to a uniform value of 300 K.

The boundary condition type for outlet and shell is set to zeroGradient. The boundary condition type for inlet1 and inlet2 is set to fixedValue which defines a constant inlet temperature as presented in Table 2.

Table 2 - Temperature Boundary Conditions

Case	`inlet1` Temperature [K]	`inlet2` Temperature [K]
symmetric-velocity-temperature	300	300
asymmetric-velocity-temperature	300	330

Turbulent Thermal Diffusivity

The ICs and BCs for the turbulent thermal diffusivity field, [kg/(m-s)], are contained in the 0.orig/alphat dictionary file. The fluid domain initial turbulent thermal diffusivity is set by the internalField keyword to a uniform value of 0 [kg/(m-s)].

The boundary condition type for inlet1, inlet2 and outlet is set to calculated. The boundary condition type for shell is set to compressible::alphatJayatillekeWallFunction which provides a thermal wall function for turbulent thermal diffusivity based on the Jayatilleke model.

Turbulent Kinematic Viscosity

The ICs and BCs for the turbulent kinematic viscosity field, [m2/s], are contained in the 0.orig/nut dictionary file. The fluid domain initial turbulent kinematic viscosity is set by the internalField keyword to a uniform value of 0 [m2/s].

The boundary condition type for inlet1, inlet2 and outlet is set to calculated. The boundary condition type for shell is set to nutkWallFunction which provides a turbulent kinematic viscosity wall function, based on turbulence kinetic energy.

Turbulent Kinetic Energy

The ICs and BCs for the turbulent kinetic energy field, [m2/s2], are contained in the 0.orig/k dictionary file. The fluid domain initial turbulent kinetic energy is set by the internalField keyword to a uniform value of 1 [m2/s2].

The boundary condition type for inlet1 and inlet2 is set to turbulentIntensityKineticEnergyInlet and the turbulence intensity is set by the intensity keyword to 0.05 (5% turbulence).

The boundary condition type for outlet is set to inletOutlet. The boundary condition type for shell is set to kqRWallFunction which provides a zero gradient condition for the turbulent kinetic energy at the wall.

Turbulent Kinetic Energy Dissipation Rate

The ICs and BCs for the turbulent kinetic energy dissipation rate field, [m2/s3], are contained in the 0.orig/epsilon dictionary file. The fluid domain initial turbulent kinetic energy dissipation rate is set by the internalField keyword to a uniform value of 1 [m2/s3].

The boundary condition type for inlet1 and inlet2 is set to turbulentMixingLengthDissipationRateInlet and the turbulent dissipation rate is based on a specified mixing length set by the mixingLength keyword to 0.2. The turbulent dissipation rate length scale, mixingLength, is equal to the diameter for inlet1 and inlet2 patches.

The boundary condition type for outlet is set to inletOutlet. The boundary condition type for shell is set to epsilonWallFunction which provides a turbulent dissipation rate wall function condition for turbulent flow cases.

Turbulence Model

The constant/momentumTransport dictionary file uses turbulence modelling based on the Reynolds-averaged stress (RAS) by setting simulationType to RAS. The RAS dictionary is used to select and enable the k-epsilon turbulence model by setting model to kEpsilon and turbulence to on, respectively. The k-epsilon model coefficients are printed to the terminal at simulation start by setting printCoeffs to on.

Gravitational Acceleration

The constant/g dictionary file defines the magnitude and the direction of gravitational acceleration vector, [m/s2].

Thermophysical Models and Data

The constant/thermophysicalProperties dictionary file contains the thermophysical model, the thermophysical submodels and the thermophysical properties data.

Thermophysical Models

The thermophysical model is contained in the thermoType dictionary. The thermophysical model is defined by setting the type to heRhoThermo. The heRhoThermo thermophysical model calculations are based on enthalpy and density. The thermophysical submodels are contained in the thermoType dictionary.

The mixture submodel, mixture, is set to pureMixture because the fluid is a single specie (i.e. water).

The transport submodel, transport, is set to polynomial to allow:

The calculation of dynamic viscosity, [kg/(m-s)], as function of temperature, [K], based on a polynomial fit,
The calculation of thermal conductivity, [W/(m-K)], as function of temperature, [K], based on a polynomial fit.

The thermodynamics submodel, thermo, is set to hPolynomial to allow the calculation of specific heat capacity at constant pressure, [J/(kg-K)], as function of temperature, [K], based on a polynomial fit.

The equation of state submodel, equationOfState, is set to icoPolynomial (incompressible polynomial equation of state) to allow the calculation of density, [kg/m3], as function of temperature, [K], based on a polynomial fit.

The specie submodel, specie, is set to specie.

The energy submodel, energy, is set to sensibleInternalEnergy as the energy formulation used in the solver solution.

Thermophysical Properties Data

The thermophysical properties data for the thermophysical submodels is contained in the specie dictionary named mixture.

The specie sub-dictionary contains the molecular composition for each mixture constituent. As the mixture submodel is set to pureMixture, the specie dictionary contains only the water molecular weight, molWeight, [g/mol].

The transport sub-dictionary contains:

The cubic polynomial fit coefficients for dynamic viscosity, muCoeffs, as function of temperature,
The cubic polynomial fit polynomial fit coefficients for thermal conductivity, kappaCoeffs, as function of temperature.

The thermodynamics sub-dictionary contains:

The cubic polynomial fit coefficients for specific heat capacity at constant pressure, CpCoeffs, as function of temperature,
The standard entropy, Sf, [J/(kg-K)],
The heat of formation, Hf, [J/kg].

The equationOfState sub-dictionary contains the cubic polynomial fit coefficients for density, rhoCoeffs, as function of temperature.

The dynamic viscosity, thermal conductivity, specific heat capacity and density as function of temperature are calculated based on the water property data at atmospheric pressure of 101325 Pa and temperature between 0 and 95 degrees C (273.15 and 368.15 degrees K) from IAPWS R7-97(2012) contained in the water property data file.

A Gnuplot script is used to calculate the polynomial fit coefficients for dynamic viscosity, thermal conductivity, specific heat capacity and density as function of temperature. The Gnuplot script is executed in the water-properties folder using:

gnuplot water-properties.plt

The Gnuplot script execution generates the water property polynomial functions presented in Table 3.

Table 3 - Water Property Data and Polynomial Functions

Dynamic Viscosity and Specific Heat Capacity	Thermal Conductivity and Density

Solver

Runtime Control

The system/controlDict dictionary file defines the solver, the time control, the data I/O and the run-time functions. The solver is selected using the application keyword which is set to buoyantPimpleFoam.

The time control uses startTime and endTime keywords to set the simulation start time and the simulation end time. The simulation time step, deltaT, is controlled by run-time function deltaT_control. The automatic time step adjustment, which limits the maximum Courant number, maxCo, to a specified value, is disabled by setting adjustTimeStep to no.

The data writing options allow the simulation data to be written at every writeInterval seconds of simulated time using the format set by writeFormat with no data compression, writeCompression set to off. The graph data format is set to gnuplot using the graphFormat keyword. The data reading option is enabled by setting runTimeModifiable to true to allow dictionaries, e.g. controlDict, to be re-read at the beginning of each time step.

The following run-time functions are defined:

residuals: Writes the solver residuals for U, p_rgh, e, k and epsilon fields at each time step to postProcessing/residuals/0/residuals.dat file,
minmaxdomain: Writes the fluid domain minimum and maximum values for p, p_rgh, U, k, epsilon and T at each time step to postProcessing/minmaxdomain/0/fieldMinMax.dat file,
yPlus: Writes the fluid domain y+ values at every writeInterval seconds of simulated time to postProcessing/yplus/0/yPlus.dat file,
inlet1_massflow: Writes the mass flow rate, phi, [kg/s], for inlet1 at each time step to postProcessing/inlet1_massflow/0/surfaceFieldValue.dat file,
inlet2_massflow: Writes the mass flow rate, phi, [kg/s], for inlet2 at each time step to postProcessing/inlet2_massflow/0/surfaceFieldValue.dat file,
outlet_massflow: Writes the mass flow rate, phi, [kg/s], for outlet at each time step to postProcessing/outlet_massflow/0/surfaceFieldValue.dat file,
probes_online: Writes the U, p, T and p_rgh values at selected locations in the fluid domain at each time step to postProcessing/probes_online/0/U, p, T and p_rgh files,
pressure_drop: Writes the difference between the inlet1 average pressure and the outlet average pressure at each time step. This function generates the following information:
- The inlet1 average pressure, [Pa], written to postProcessing/pressure_drop.region1/0/surfaceFieldValue.dat file,
- The outlet average pressure, [Pa], written to postProcessing/pressure_drop.region2/0/surfaceFieldValue.dat file,
- The difference between the inlet1 average pressure and the outlet average pressure, [Pa],written to postProcessing/pressure_drop/0/fieldValueDelta.dat file.
outlet_temperature: Writes the average temperature, T, [K], for outlet at each time step to postProcessing/outlet_temperature/0/surfaceFieldValue.dat file,
deltaT_control: Controls the simulation time step, deltaT, by using the small time step, defined in the system/controlDict-small-deltaT dictionary file, at the start of the simulation and a large time step, defined in the system/controlDict-large-deltaT dictionary file, for the reminder of the simulation. This approach avoids solver failures at the simulation start and reduces the simulation execution time while keeping the Courant number within a resonable range.

Numerical Schemes

The system/fvSchemes dictionary file sets the numerical schemes for equation terms that are calculated during a simulation. The set of terms, for which numerical schemes must be specified, are subdivided within the system/fvSchemes dictionary file into the sub-dictionaries described below.

ddtSchemes: The ddtSchemes sub-dictionary specifies the time scheme for the first time derivative. The CrankNicolson discretization scheme is selected and the blending coefficient is used to balance scheme accuracy and stability. A blending coefficient of 0 is equivalent to running a pure Euler scheme (robust, but first order accurate), while a blending coefficient of 1 uses use a pure Crank-Nicolson scheme (second order accurate, but oscillatory).
gradSchemes: The gradSchemes sub-dictionary specifies the Gauss option for treatment of the standard finite volume discretization of Gaussian integration. The option requires the use of linear interpolation of values from cell centers to face centers. To guarantee boundedness and improve stability, the cellLimited gradient limiter scheme and a limiting coefficient are used to limit the gradient such that, when cell values are extrapolated to faces using the calculated gradient, the face values do not fall outside the bounds of values in surrounding cells.
divSchemes: The divSchemes sub-dictionary defines the treatment of advective divergence terms. The Gauss integration scheme using the flux phi is employed. The interpolation of the advected field to the cell faces uses the linearUpwindV, linearUpwind and linear schemes.
laplacianSchemes: The laplacianSchemes sub-dictionary uses the Gauss discretization scheme. The Gauss discretization scheme requires the selection of:
- An interpolation scheme for the diffusion coefficients. The selected interpolation scheme is linear.
- A surface normal gradient scheme. The selected surface normal gradient scheme is limited. The limited scheme is second order accurate, bounded (depending on the quality of the mesh), with non-orthogonal corrections. Since the maximum mesh non-orthogonality reported in the snappyHexMesh mesh quality log file is less than 70, the blending factor for the limited scheme is set to 1.
interpolationSchemes: The interpolationSchemes sub-dictionary sets the linear method to be used for interpolation of values from cell centers to face centers.
snGradSchemes: The snGradSchemes sub-dictionary contains surface normal gradient terms. It uses the same method as the one chosen for the Laplacian terms in the laplacianSchemes sub-dictionary, the limited scheme with the blending factor set to 1.

Solution and Algorithm Control

The system/fvSolution dictionary file defines the equation solvers, tolerances and algorithms and it is structured into the sub-dictionaries discussed below.

solvers: The solvers sub-dictionary specifies each linear-solver (the method of number-crunching to solve a matrix equation) that is used for each discretized equation. The GAMG (generalised geometric-algebraic multi-grid) solver and the GaussSeidel smoother are used for all fields. The solver tolerance represent the level at which the field equation residual (the measure of solution error) is small enough that the solution can be deemed sufficiently accurate. The solver stops if any one of the following conditions are reached:
- The residual falls below the solver tolerance tolerance,
- The ratio of current to initial residuals (at the current time step) falls below the solver relative tolerance relTol,
- The number of iterations exceeds a maximum number of iterations maxIter. The maximum number of iterations is not set explicitly and uses the solver default value.
PIMPLE: The PIMPLE sub-dictionary defines the parameters used by the iterative algorithm for coupling the solution of mass, momentum and energy conservation equations. The PIMPLE algorithm is a combination of the PISO (pressure-implicit split-operator) and the SIMPLE (semi-implicit method for pressure-linked equations) algorithms and it is used for transient problems. The PIMPLE parameters are described below.
- consistent: The consistent formulation of the SIMPLE algorithm, SIMPLEC, is used by setting the consistent keyword to yes. The SIMPLEC formulation for the pressure-velocity coupling method needs only a small amount of under-relaxation for velocity and other transport equations and does not need to use any relaxation on pressure. This results typically in more robust solution and faster convergence.
- momentumPredictor: The momentumPredictor is set to yes to enable the momentum predictor step which helps in stabilizing the solution as better approximations for the velocity field are computed. Enabling this parameter is recommended for highly convective flows and requires the definition of linear solvers for Final variables.
- nOuterCorrectors: The nOuterCorrectors enables the looping over the entire system of equations within on time step and represents the total number of times the system is solved. If the nOuterCorrectors is set to 1, the PISO algorithm is used. Using more outer correctors results in better stability, especially when using large time-steps (Courant number higher than 1), but at a higher computational cost.
- nCorrectors: The nCorrectors sets the number of times the algorithm solves the pressure equation and momentum corrector in each time step.
- nNonOrthogonalCorrectors: The nNonOrthogonalCorrectors sets the number of repeated solutions of the pressure equation, used to update the explicit non-orthogonal correction. Increasing the number of nNonOrthogonalCorrectors corrections adds more stability but at a higher computational cost.
- residualControl: The residualControl sets the the convergence controls based on residuals of fields from one time step to the next.

Execution

The Linux cp command is executed in the case folder to copy the 0.orig folder (ICs and BCs) to the to 0 folder prior to solver execution using:

cp -r 0.orig 0

Domain Decomposition

The decomposePar utility is used to decompose the fluid domain into sub-domains, allocated to separate processors, to allow for parallel mesh generation or solver execution. The system/decomposeParDict dictionary file contains the decomposePar utility parameters. The mesh is decomposed into 8 sub-domains by setting numberOfSubdomains to 8. The decomposition method is set to simple using the method parameter. For the simple method, the n parameter in the simpleCoeffs dictionary decomposes the mesh into 2 sub-domains along the x, y and z directions, respectively. The decomposePar utility is executed in the case folder using:

decomposePar | tee log.09.decomposeParSolver

The execution of decomposePar utility creates a processorX/ folder for each processor. The folder contains the sub-domain mesh and initial field values assigned to that processor.

Solver Execution

The buoyantPimpleFoam is executed in parallel in the case folder using:

mpirun -np 8 buoyantPimpleFoam -parallel | tee log.10.buoyantPimpleFoam

Solver Solution Reconstruction

The reconstructParMesh utility reads the sub-domain (processor) buoyantPimpleFoam solution and creates the time-dependent solution folders in the case folder. The reconstructParMesh utility is executed in the case folder using:

reconstructPar | tee log.11.reconstructPar

Case Execution Scripts

The execution commands presented so far have been consolidated into the following shell scripts:

./clean.sh: The clean.sh script contains the commands to delete the mesh information, the mesh and solver partition folders, the solver time step folders, the post-processing folder and the log files.
./mesh.sh: The mesh.sh script contains the commands to generate, check, reconstruct and renumber the mesh generated by blockMesh and snappyHexMesh.
./solve.sh: The solve.sh script contains the commands to decompose, execute in parallel and reconstruct the solver solution.

Results

The case simulation data, written to the postProcessing folder using the run-time functions defined in Runtime Control section, is processed using the flow-header.ipynb JupyterLab notebook located in the study folder visualizations. The notebook generates interactive parameter plots using Plotly and can be used to monitor the solver execution. The notebook and the interactive parameter plots are available in html format.