hi all i have a question that how far the outer boundary then it is safe to assume a free stream values . regrads salem

Depends on the problem and the solver you use (ie. how the far-field conditions are applied).

For aerodynamics start at 5 characteristic lengths away from the body and move those further away. When the solution does not change, you are "far enough".

Unsteady flow requires much larger domain than steady, some flow over a cylinder computation have the downstream boundary 50 or more diameters behind the cylinder.

This is just guidelines based on my own experiences, someone else may have different opinion.

-- Jarmo

thanks jarmo monttinen for kind and useful answer.