Suppose water is flowing in horizontal direction (positive $x$-direction) and a particle immersed in that water is also moving in the same direction.
In this case, is the drag force $F_D$ in the direction of particle motion or opposite to it?
I get from wikipedia that drag force is a frictional force and hence is opposite to particle motion, but then what is the force that is making the particle move. Because in one journal paper, I see that drag force $F_D$ is shown as force in the direction of particle motion.
This is a sketch from the paper, you can see that flow velocity and drag force are both in the same direction.