This example introduces the following OpenFOAM features for the first time: generation of a mesh using blockMesh using full mesh grading capability; steady turbulent flow.

Initial conditions— required in OpenFOAM input files but not necessary for the solution since the problem is steady-state. Solver name simpleFoam : an implementation for steady incompressible flow. The solver has full access to all the turbulence models in the incompressibleTurbulenceModels library and the non-Newtonian models incompressibleTransportModels library of the standard OpenFOAM release.

In general, the regions of highest shear are particularly critical, requiring a finer mesh than in the regions of low shear. We can anticipate where high shear will occur by considering what the solution might be in advance of any calculation.

At the inlet we have strong uniform flow in the direction and, as it passes over the step, it generates shear on the fluid below, generating a vortex in the bottom half of the domain.

The regions of high shear will therefore be close to the centreline of the domain and close to the walls. The user can see that blocks 4,5 and 6 use the full list of 12 expansion ratios. The expansion ratios correspond to each edge of the block, the first 4 to the edges aligned in the local direction, the second 4 to the edges in the local direction and the last 4 to the edges in the local direction. In blocks 4, 5, and 6, the ratios are equal for all edges in the local and directions but not for the edges in the direction that corresponds in all blocks to the global.

If we consider the ratios used in relation to the block definition in section?? The purpose of this differential grading is to generate a fine mesh close to the most critical region of flow, the corner of the step, and allow it to expand into the rest of the domain. The mesh can be generated using blockMesh from the command line and viewed as described in previous examples.

The boundary conditions can be specified as: the upper and lower walls are set to Wallthe left patch to Inlet and the right patch to Outlet. These physical boundary conditions require us to specify a fixedValue at the inlet onand. We assume that the inlet turbulence is isotropic and estimate the fluctuations to be of at the inlet. We have 3.

Special attention should be paid to the solver settings of the fvSolution dictionary. Although the top level simpleFoam code contains only equations for andthe turbulence model solves equations forandand tolerance settings are required for all 5 equations. A tolerance of and relTol of 0.

Under-relaxation of the solution is required since the problem is steady. A relaxationFactor of 0. Finally, in the controlDict dictionary, the time step deltaT should be set to 1 since in steady state cases such as this is effectively an iteration counter. With benefit of hindsight we know that the solution requires iterations reach reasonable convergence, hence endTime is set to Ensure that the writeInterval is sufficiently high, e. Run the case and post-process the results.

After a few iterations, e.

### 2D Mesh Tutorial using GMSH

Terms of Use Privacy Policy. Design by The open source CFD toolbox. About us Contact Jobs Legal. Steady turbulent flow over a backward-facing step [ prev ] [ next ].This post should help point you in direction of the mesher you could use to accomplish your simulating goals! As this would become a novel. This is just my opinion on what mesher I use in a few typical cases. During my PhD we were very spoiled and used the software Pointwise to generate our meshes.

I still love Pointwise, and if I ever decide to move away from open source meshing that will be my choice. But today is not that day! If that were the case — needless to say — countless applications would be impossible.

You can mesh pretty much anything given the tools that are out there! Of these, each meshing software has its own advantages and disadvantages and have types of meshing where they perform the best.

**FreeCAD, Gmsh, OpenFoam - an opensource CFD case**

Of course, some people will disagree with my views as meshing somehow can be a deeply personal adventure. If your geometry is easily represented by a reasonable number of points and curves — blockMesh is a good option. You would be surprised what you can mesh in blockMesh — if you have the time… but who has the time! A major advantage of using the native OpenFOAM meshers is that they can interface seamlessly with whatever scripting language you prefer. I frequently use blockMesh to build a template.

I then leave blanks where my control variables will go and then fill them in at run-time using either bash or python scripting! This makes parametric studies a breeze. There are lots of limitations of using tet-dominant meshes in CFD.

There are many times when a tet-mesh is the most straight-forward to produce. Key to the effective generation of tetrahedral dominant meshes is the ability to generate layers at the wall. This can be a challenge in each of these programs.

So no matter what you use — prepare to be patient. I have found that the boundary layer generation in Salome is quite robust. However, it occasionally produces some very poor quality regions which you need to be careful of. I have other tools — and generally prefer hex-meshes anyway.

For hex-dominant meshing, your options are blockMesh if you can create your geometry easily enoughcfMesh, or snappyHexMesh. But compared to cfMesh I find it much less robust. The key area where this is true is in layer generation. There are ways to get consistent layer generation in snappyHexMesh i. I have found that in almost all cases, I can achieve consistent, high-quality, boundary layers using cfMesh.

Not only this, but I have found the workflow using cfMesh to be a breeze. Especially if you use the supplied Salome scripts to produce your geometry file separate post on this to follow.

Multi-region meshing is required for several different applications. The main ones I have encountered are rotating zones for machinery and turbinesand conjugate heat transfer for separate solid and fluid regions.

The primary reason for this is that is extremely simple to create in your workflow. When using cfMeshgmshor Salome you typically have to create both meshes, and then combine them using the mergeMeshes command. This is can be very tedious. H oweverin some cases it can be worth it if you can achieve better mesh quality with one of these programs.In this chapter we shall describe in detail the process of setup, simulation and post-processing for some OpenFOAM test cases, with the principal aim of introducing a user to the basic procedures of running OpenFOAM.

Before attempting to run the tutorials, the user must first make sure that OpenFOAM is installed correctly. The tutorials are organised into a set of directories according to the type of flow and then subdirectories according to solver. The user can copy cases from the tutorials directory into their local run directory as needed. Read More. See our Essential CFD and Applied CFD courses for details Essential CFD Introduction to meshes : meshing strategy, blockMesh quick start, boundary types, patch groups snappyHexMesh introduction : surface patching, castellated mesh, surface snapping snappyHexMesh enhancements : assessing mesh quality, layer insertion, cell refinement introduction snappyHexMesh refinement : tri-surface manipulation, surface refinement, region refinement, more on layers.

Applied CFD Meshing tools : anisotropic refinement, extruding a 2D mesh, patch manipulation, meshing workflow snappyHexMesh feature capturing : extracting feature, explicit feature capturing, adjusting features snappyHexMesh meshing baffles : baffle geometry, face zones, creating baffles Multi-region meshing : geometry for multi-regions, specifying regions, capturing region boundaries.The principle behind blockMesh is to decompose the domain geometry into a set of 1 or more three dimensional, hexahedral blocks.

Edges of the blocks can be straight lines, arcs or splines. The mesh is ostensibly specified as a number of cells in each direction of the block, sufficient information for blockMesh to generate the mesh data.

Each block of the geometry is defined by 8 vertices, one at each corner of a hexahedron. The edge connecting vertices 1 and 5 is curved to remind the reader that curved edges can be specified in blockMesh.

Each block has a local coordinate system that must be right-handed. A right-handed set of axes is defined such that to an observer looking down the axis, with nearest them, the arc from a point on the axis to a point on the axis is in a clockwise sense.

### OpenFOAM v7 User Guide: 5 Mesh generation and conversion

The local coordinate system is defined by the order in which the vertices are presented in the block definition according to: the axis origin is the first entry in the block definition, vertex 0 in our example; the direction is described by moving from vertex 0 to vertex 1; the direction is described by moving from vertex 1 to vertex 2; vertices 0, 1, 2, 3 define the plane ; vertex 4 is found by moving from vertex 0 in the direction; vertices 5,6 and 7 are similarly found by moving in the direction from vertices 1,2 and 3 respectively.

The convertToMeters keyword specifies a scaling factor by which all vertex coordinates in the mesh description are multiplied. However any edge may be specified to be curved by entries in a list named edges. The list is optional; if the geometry contains no curved edges, it may be omitted. The keyword is then followed by the labels of the 2 vertices that the edge connects. Following that, interpolation points must be specified through which the edge passes.

For a arca single interpolation point is required, which the circular arc will intersect. For splinepolyLine and BSplinea list of interpolation points is required. The line edge is directly equivalent to the option executed by default, and requires no interpolation points. Note that there is no need to use the line edge but it is included for completeness. The shape is always hex since the blocks are always hexahedra. Number of cells The second entry gives the number of cells in each of the and directions for that block.

Cell expansion ratios The third entry gives the cell expansion ratios for each direction in the block. The expansion ratio enables the mesh to be gradedor refined, in specified directions. Each of the following keywords specify one of two types of grading specification available in blockMesh. In some cases, it reduces complexity and effort to be able to control grading within separate divisions of a single block, rather than have to define several blocks with one grading per block.

For example, to mesh a channel with two opposing walls and grade the mesh towards the walls requires three regions: two with grading to the wall with one in the middle without grading. OpenFOAM v2. This multi-grading is specified by replacing any single value expansion ratio in the grading specification of the block, e. They can be specified as percentages, fractions, absolute lengths, etc.

The example above can be specified using percentages, e. The boundary is broken into patches regionswhere each patch in the list has its name as the keyword, which is the choice of the user, although we recommend something that conveniently identifies the patch, e.

This means that for a 2 dimensional geometry, the user has the option to omit block faces lying in the 2D plane, knowing that they will be collected into an empty patch as required.Meshes can be created interactively using a GUI or by writing a. For easy modification of the geometry, it is useful to start with a definition of the relevant parameters:.

Next, we specify the grid points that define the geometry, based on the parameters above. The expression inside the parentheses is the point's ID number; the first three columns inside the braces are the x, y, z coordinates, and the 4th column denotes the prescribed mesh element size near that point.

Between points, the mesh element size will interpolate, so grading could be introduced by placing points at the centre of the channels. Points are joined together by lines, with the numbers in braces specifying the two points you wish to connect:. Arcs and splines can also be easily created by specifying a centre of rotation and a number of control points, respectively. Plane surfaces are created using a line loop. For the line loop, the numbers in braces specify, in order, the lines which constitute the perimeter of the surface.

The line loop must be closed i. Strictly speaking, lines 2 and 3 aren't needed, and surfaces 1, 2, and 3 could be combined into a single surface. However, by default, Gmsh uses an unstructured mesh and it is often a good idea to partition the geometry to constrain the mesh generation.

As the channel sections are straight, it is convenient to use a structured mesh for them; for this, we use the extrude command to translate a line:. The extrude command automatically creates all the necessary points, lines, and surface between the chosen line s and its translated counterpart. The first set of braces gives the x, y, z components of the extrusion; the second set of braces specifies a the line s you wish to extrude, b how many cells you wish to divide the extruded surface into, and c an optional command to recombine the triangular cells into rectangles.

OpenFOAM requires meshes to be 3D, so we next have to extrude the entire surface by one layer of arbitrary thickness in the z-direction. This is achieved using the same extrude command, but specifying surfaces as an argument rather than lines:. Similarly to the line extrusions, the variable zV contains all the extruded surfaces and volumes.

Finally, we create "physical surfaces" that OpenFOAM will recognise as patches for boundary conditions:. Note, all physical surfaces appear in the "boundary" file as type "patch", so you will need to change "topAndBottom" to type "empty". Naming this file "bifurcation2d.

If the mesh is of poor quality, the flag -optimize is useful for optimising the mesh element quality. The flags -clmin float and -clmax float are also useful for constraining the minimum and maximum element sizes, respectively. This will create the file "bifurcating2d. If the. Below, shows the resulting mesh in Paraview, and the solution for the steady-state velocity profile using the icoFoam solver.

We can easily change the geometry of the network by altering the inputs.Note that two ways of geometry generation are shown. The other one directly creates the geometry file. Meshing might be done with or without GUI in both ways. In a single plane 2Dcreate the geometry by first creating all pointsthen combining the points into linesand then the lines into a surface.

Select Surface, then click the surface in the viewer. This will extend the surface into 3D space. The distance and direction of the extension is defined under the Contextual Geometry Definitions window that appears, under the Translate tab.

Now that the shape is 3D, the boundaries can be defined. For example, if the shape is a square, select the bottom, left, top, right, front, and back faces and individually add them as a Physical Group If your boundary consists of multiple faces, you can select them together and then add the Physical Group.

The volume of the geometry must also be named. The shape should be saved as a. Then open the. You can also assign internal surfaces to physical surfaces to make internal walls in the geometry. For example the numbers may be different :. In the. This ensures that the mesh blocks are created only in 2D and then extended to 3D, and not divided in the 3rd axis. For example:. Save the. There should be no errors produced by GMSH.

If everything is set up correctly via the Extrude modificationsthe sides of the 3D mesh should only have straight lines connecting the front face to the back face. OpenFOAM will produce an error if the mesh does not meet this requirement. The smaller the number, the smaller the mesh elements. For the changes in element size factor to take place, just go back to Geometry, Reload, then back to Mesh and click 3D.

This will create a file using the same name as the. Be aware that the Line Loop function needs sorted input. The end- and start-points of adjacent lines have to match and the direction e.

The direction of lines may be reversed with a minus. There are several ways to obtain a structured mesh in GMSH. One easy way for simple geometries is adding. Copy the. If there are any errors, ensure that a proper case directory is set up refer to OpenFOAM documentation, or use a pre-shipped case directory such as icoFoam or simpleFoam.

If the errors indicate no 3D elements exist, or there is another mesh problem, make sure all steps were followed in the GMSH section of this tutorial. Each boundary is defined here. You can also use the Contrib modifyPatches to automate that.

Ensure that all other boundaries as defined in the GMSH.

## Tutorial Guide

You could also name all your boundaries of the same kind with a suffix e.The snappyHexMesh utility generates 3-dimensional meshes containing hexahedra hex and split-hexahedra split-hex automatically from triangulated surface geometries, or tri-surfaces, in Stereolithography STL or Wavefront Object OBJ format.

The mesh approximately conforms to the surface by iteratively refining a starting mesh and morphing the resulting split-hex mesh to the surface. An optional phase will shrink back the resulting mesh and insert cell layers. The specification of mesh refinement level is very flexible and the surface handling is robust with a pre-specified final mesh quality. It runs in parallel with a load balancing step every iteration. The objective is to mesh a rectangular shaped region shaded grey in the figure surrounding an object described by a tri-surface, e.

Note that the schematic is 2-dimensional to make it easier to understand, even though the snappyHexMesh is a 3D meshing tool. The snappyHexMeshDict dictionary includes: switches at the top level that control the various stages of the meshing process; and, individual sub-directories for each process.

The entries are listed below. All the geometry used by snappyHexMesh is specified in a geometry sub-dictionary in the snappyHexMeshDict dictionary. The following criteria must be observed when creating the background mesh: the mesh must consist purely of hexes; the cell aspect ratio should be approximately 1, at least near surfaces at which the subsequent snapping procedure is applied, otherwise the convergence of the snapping procedure is slow, possibly to the point of failure; there must be at least one intersection of a cell edge with the tri-surface, i.

The entries for castellatedMeshControls are presented below. The features list in the castellatedMeshControls sub-dictionary permits dictionary entries containing a name of an edgeMesh file and the level of refinement, e.

The minimum level is applied generally across the surface; the maximum level is applied to cells that can see intersections that form an angle in excess of that specified by resolveFeatureAngle. The refinement can optionally be overridden on one or more specific region of an STL surface. The region entries are collected in a regions sub-dictionary. The keyword for each region entry is the name of the region itself and the refinement level is contained within a further sub-dictionary.

Cell removal requires one or more regions enclosed entirely by a bounding surface within the domain. The region in which cells are retained are simply identified by a location vector within that region, specified by the locationInMesh keyword in castellatedMeshControls.

The refinementRegions sub-dictionary in castellatedMeshControls contains entries for refinement of the volume regions specified in the geometry sub-dictionary. A refinement mode is applied to each region which can be: inside refines inside the volume region; outside refines outside the volume region distance refines according to distance to the surface; and can accommodate different levels at multiple distances with the levels keyword.

The process is: displace the vertices in the castellated boundary onto the STL surface; solve for relaxation of the internal mesh with the latest displaced boundary vertices; find the vertices that cause mesh quality parameters to be violated; reduce the displacement of those vertices from their initial value at 1 and repeat from 2 until mesh quality is satisfied.

The method uses the settings in the snapControls sub-dictionary in snappyHexMeshDictlisted below. The layer addition procedure uses the settings in the addLayersControls sub-dictionary in snappyHexMeshDict ; entries are listed below.