what is the meaning of boundary markers in unstructured grid.how we will use it. i am depending on downloaded unstructured grid codes(suggest a link also)for my compressible flow problem. i should give the boundary markers as input. in unstrucred grid, cell numbering and node numbeing in random fashion. so how we can pass the one cell information to other. please suggest me the related books.

I've marked the BC along cell faces. You can pass the information in many ways, for example using a node based approach where you save all cells/elements connected to a certain node or alternatively using a face-based approach where you save the two or three points of the face and the cells on the two sides of the face.

However, I would not use a downloaded grid unless you are able to modify it somehow. I would think that you would need to show grid convergence as a part of your studies. Can't do that with just one grid. I would think that you would be better off with a self-generated structured grid that you can modify as needed.

-- Jarmo