Create some nodes into an existing mesh and accessing to those nodes
 

Consider a simple rectangular inlet/outlet flow.(flow coming from one side and going out from the other side)
I want to create some nodes into the existing geometry in which their coordinates represents the shape of a circle.
Then, i need to access to these nodes to interpolate the velocity of them with the nearest cells of the existing mesh.
I have an idea, e.g to make an array and put the coordinates in a way to represent the shape of a circle, but i don't know how to access to the nearest cell of these coordinates.
I would appreciate any suggestion, or suggestion for any other approach rather than creating an array...