Thursday, March 6, 2014

P36 Von Karman street part 3

This post is the third part of the Von Karman Vortex Street tutorial, precedent article in this link.
Within this post practical procedures for the domain drawing are introduced. Along the article the used softwares are DraftSight and Freecad.  All the necessary files are linked along the article.

In the precedent post  have been defined the layout and dimension of simulation domain. For numerical simulation is needed to divide the domain in a mesh, or grid. There are many different ways to mesh the domain and there is also the fascinating alternative to use an adaptive mesh, that last option uses a self-modifying mesh to achieve the best results. Right now is important to stress on the fact that we should consider the mesh type during the domain drawing phase, an accurate drawing phase will ease the meshing.

Among the different type of meshes is chosen the Quadrilateral. Quadrilateral mesh is a structured collection of four sided polygons or cells. The mesh is structured so it's possible to set exactly how the mesh will look like. Of course an unstructured mesh will do the same job; in fact for the preliminary simulation a triangular unstructured automatic generated mesh was used, indeed I should say a fast solution. First impact of the use of quadrilateral mesh is that we need to divide our domain in subdomains with exactly four sides. If a region have not exactly four sides cannot be meshed with quadrilateral cells.
F36.1 General layout and domain partition. Color indicates different flow conditions
Light blue indicates an almost unidirectional flow along y axis
Green is a zone near the stagnation point
Red is a region where flow have an increase in speed, in the order of two times the free stream velocity
Yellow identify the wake region, here the flow have high vorticity
For best results the mesh polygons should be aligned with the flow, that condition cannot be satisfied all along our transient simulation, in particular in the wake region. That is not a problem as long as the cell size is small enough. More details on the topic in the post about meshing, the important thing to focus on is to verify that different cells sizes are needed along the domain in function of flow condition.

Let's proceed to the domain drawing. We use Salome as mesher a domain file format compatible with this software is necessary, we chose step file format.   We use two software in sequence because we generate a DXF with DraftSight and then we convert it to Step format with Freecad.

DraftSight operations are basic, launch the software and draw line by line the partitioned domain shown on figure F36.1. Draw all your segments using as length the dimension in millimeters, in such a way, with default setup for the software tool chain,  the mesh imported into Elmer CSC will have the correct dimensions in meters.  Have a look to the following video for a complete description of the drawing and conversion phase.

V36.1 Video on the drawing procedure, including conversion to step file format

Right now you should have one DXF file and one STP file really similar to these files downloadable here.

Next post will cover meshing and exporting to an Elmer compatible format.
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