Monday, March 31, 2014

Von Karman street part 4, meshing

This post is the fourth part of the Von Karman Vortex Street tutorial, precedent article in this link .
To follow this post you should recover the step file from the last tutorial part, it is useful to have at hand the domain drawings. In this post we will mesh the simulation domain, the mesh procedure is based on Salome and the mesh is prepared for Elmer.
The general approach is to divide the surface that represents the air in CFD domain in sub-faces that can be meshed with a quadrilateral structured mesh; every face should have exactly four sides. To mesh the sub-meshes the four edges are manually divided into the desired number of segments.
All the proposed procedures are reported into the tutorial video; simply repeat the procedures for all the sub-meshes. A brief text description is reported here for reference. To ease the video shooting and mesh procedures showing the grid shown in the video is coarser and may be different from those of the here downloadable file.

Video 1 Tutorial of meshing procedures

Launch Salome and go to the geometry module, import menu and load the step geometry file. Now go to the imported object and explode it in edges. Select all the edges that compose the free perimeter of the whole air domain and build a face. That face should be divided using the internal edges of our geometry, issue a partition command on the face object and select all the internal edges as partition geometry.
Now explode the partition geometry faces and edges. The faces that you have just parted are the faces that should be sub-meshed to cover the entire domain. Repeat for every face.
At this point is necessary to group the entities that represent the simulation boundaries. Referring to the precedent post figure P35.2
Figure P35.2 Simulation domain

is necessary to aggregate the edges to form A, B, C, D and W boundaries. That task is accomplished by the group command launched after partition selection.
Switch to the mesh module.
Define the main mesh, with create mesh command. Now proceed, in a systematic way; create a sub-mesh for every edge that compose a face and a sub-mesh for every face.
For a real CFD study the mesh should be redone many times, it is important to proof that solution is not dependent on a particular mesh grid or grid size. Following the tutorial video you will attain a mesh that has different dimensions in function of the domain part.  In those regions where it is know that the streamlines flow along x axis the resolution on y axis is coarse. In the zones just downstream of the body the grid is dense to capture the vortex dynamics. All the other parts of the domain have a loose grid.  Note also that some regions are constrained to have a grid similar to those of near regions that is to avoid sudden changes in grid dimensions.  There is no close form solution to the problem of the grid size picking; an important issue to bear in mind is the problem physics. If we expect a phenomenon with a spatial size of one mm our grid should be finer that that size, a special and common case is the boundary layer. Predict the boundary layer thickness in complex cases is not practical, anyway we can estimate the expected magnitude of the thickness using the formulation from other simplex cases.

Figure P40.1 Bounmdary layer mesh detail

It's honest to expect an upstream boundary layer with a thickness in the order of millimeter. So the grid next to the body walls is finer than one millimeter; in the example mesh near the wall the mesh grid have a dimension under 1/20 mm.

Next post will cover again meshing and introduce the simulation phase.
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