I am studying fluid dynamics on my own and it is commom to see this assumption. I am asking this here because I didn't find any satisfactory answer. For example, I am studying a problem that is as follows
''Determine the form of a jet emerging from an infinitely long slit in a plane wall. Let the wall be along the $x$-axis in the $xy$-plane, and the aperture be the a segment in the $x$-axis, the fluid ocupying the half plane $y>0$. Far from the wall ($y\to\infty$) the fluid velocity is zero, and the pressure is $p_0$.''