For the velocity field given by u=Vcosθ and v=Vsinθ and w=0 , where V and θ are constant, the stream lines are:
dxu=dyv=dzw
dxVcosθ=dyVsinθ=dz0
dydx=VsinθVcosθ=tanθ
y=xtanθ+C
Hence the streamlines are straight and inclined at angle θ