why this type of bc are there?
 

I have query that, why for comp flow we are using mass flow at inlet and for external flow we can use velo at inlet? pls reply Thanks

Re: why this type of bc are there?
 

Maybe because in a steady flow computation you eventually want to have mass in = mass out for the internal case, and for an external flow case we don't have that as a hard and fast restriction (i.e. mass can leave along multiple parts of the farfield boundary), and the flow at the boundary can adjust to account for global conservation.

Re: why this type of bc are there?
 

Also note that typically these are among the easiest and most natural quantities to define at those particular boundaries from a problem definition perspective.