Hello everybody

What's the difference between structured mesh and unstructred mesh? Thanks.

Andrew

Andrew:

Typically a structured mesh is comprised of hex (brick)elements (quads in 2D) that follow a unifrom pattern. An unstructured mesh does not follow a unifrom pattern, usually comprised of tet elements (tris in 2D).

derrek

Hi Andrew,

A single block structured mesh may comprise of square elements (2D) or hexahedral elements (3D) which are orthogonal in i, j space (2D) or i, j, k space (3D). Although is is also possible to have wedges (3D), triangles (2D) and pyramids (3D) in a structured mesh.

Looking at a 2D example, for simplicity, every node in a 2D structured mesh has a corresponding integer i and j index value which is unique. The physical locations of the nodes are stored in a table or are functionally related to the mesh space (ie (x,y)= f(i,j)). It is also implied then that the neigbours of node (i,j) are (i-1,j), (i+1,j), (i,j-1), (i,j+1), (i-1,j-1), and (i+1,j+1). If you are writing a CFD code, a structured mesh makes it very easy to loop through neighbours and can be efficient with memory.

A structured mesh has many coding advantages, but it may be difficult to conform a single block to a complicated shape. Code developers have got around this by allowing multiple blocks (multiblock unstructured), but this can make the internal memory strucutres more inefficient. Another way to make the mesh generation simpler, and improve code performance in other ways is to throw away the block structure and replace indices with node numbers and a connectivity table. This is known as an unstructured mesh, because it lacks the i,j,k structure.

A common misperception is that a hex mesh is strucutred and a tet mesh is unstructured. It is accurate to say that a tet mesh is unstructured, however a hex mesh can also be unstructred. The difference between a structured hex mesh and an ustructured hex mesh is simply in how the data is stored.

Regards, Robin

Robin:

Thanks for the thorough explanation. I should have used the word "typically" a little more forcefully. I didn't mean to imply a definition of structured & unstructured.

derrek

[Jaiganesh S]

1.What is the main different between in structured hexa and unstructured hexa?

[ghorrocks]

Structured hex meshes are in a grid, so an element can be identified by and ijk coordinate. Unstructured hex meshes are, well, unstructured.

[heidar]

hey guys
In fact a mesh is said to be structured if one can found a primitive motif which is regular in space. This implies that for instance node connectivities has a fixed pattern. But the geometry may be not constant, for instance your mesh can be deformed from place to place (for instance to match a specific boundary). The only thing you know for sure when the mesh is regular is that the node (or elements) connectivty is always the same. This may be useful for algorithm design since you always have the same pattern.
However unstructured mesh are generally more flexible, since the connectivity pattern is not fixed, at the cost of having to store the connectivity. There is no generic answer to your last question, since it depends strongly upon the application you have in mind.