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Old   August 29, 2000, 07:36
Default Grid
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Hi there|

What is the difference between structured and unstructured grid?

Tanks in adavance
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Old   August 29, 2000, 09:04
Default Re: Grid
Rich E
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In a structured grid, the nodal conectivity (and hence cell connectivity) is implicit. In 2d, node (i,j) ALWAYS has neigbours (i+1,j),(i-1,j),(i,j+1), (i,j-1). The indicies stand for the dimensions through which we can move (e.g in turbomachinery often i=pitchwise, j=meridional, k=spanwise). This is great when writing computer programs as all the variables and geometry can be stored in 3 dimensional arrays and moving around the mesh from connected node to connected node just involves incrementing the array counters. A quick drawing will show you that 3d structured meshes always use hexahedral (6 sided cuboid) cells

In an unstructured grid nodal (and cell) connectivity must be explicity stored. The computer program must contain 'look up tables' to tell it which node/cell it is going to arrive at by moving in a certain direction. Unstructured meshes are more flexible when it comes to meshing complex geometries and you can have different shaped cells (tets,hexes,prisms) to suit the problem your looking at, but the larger computational overhead means they take longer to arrive at a solution.
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Old   August 29, 2000, 10:59
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John C. Chien
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(1). When you start writing the program, it will be much easier to understand. (2). There is a good example, using the street layout in a city. In some cities, (mostly in the western part of US), the city streets are just like the chess board or graph paper. You can give the street a numerical name, or names in alphabetical order, in both east-west, and north-south direction. This is structured mesh layout (ordered). (3). In most old cities, the streets are sort of randomly distributed. They don't follow a particular order. (for historical reasons, I guess. In some countries, the houses and streets are determined individually because of their culture.) So, if the first street is near the city hall, the second street is problably somewhere in the other part of the city. So, they are not next to each other. It is not ordered, and you can call it unstructured mesh.
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