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.
Best Answer
Motion is a very diffuse concept :) you have to add a frame of reference to make it meaningfull.
In the frame of reference of the surrounding water the force definitely tries to stop the particle.
So if you have a stone rolled along the ground by a swift stream, the force goes in the direction of motion (in the usual, external, frame of reference), since the stone is still too slow for the water; whereas for a stone falling into a deep pond, the friction will be opposite ist motion.