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Homework Help: Mass flowing through fluid at constant velocity

  1. Sep 24, 2008 #1
    Hi All. This is my first post, so please forgive me if I am not following protocol somewhere.

    The question:
    A mass flows through a lets say a pipe with a rectangular cross section at a constant velocity. I am just trying to figure out all the forces acting on this mass. Of course the gravitation force is easy enough. Then, when I get to the horizontal force (force parallel to the flow) I get confused. 1/2mv^2 gives me KE, but at a constant velocity, I suppose we can assume that the force is zero (if we neglect the drag from the fluid). However, if we do account for the drag in the fluid, I think all I need to do is figure out how much frictional force there is between the fluid and the mass that flows through it. What is the best way to calculate this? I am assuming that the shape of the mass is just a cube. Do I have to do some CFD or something or is there an easy way or reference I can use to figure this one out? I just need to ballpark this figure.

    Any help would be appreciated. Thanks!
  2. jcsd
  3. Sep 24, 2008 #2
    I think the complicated answer is that it depends on the viscosity and boundary layers of the fluid. The easy answer is that if there is drag it will be proportional to the cross-sectional area and you can just put some constant out in front until you have done an experiment that could tell you what that constant is.

    Of course, you could also neglect drag depending on how massive and large the cube is.
  4. Sep 25, 2008 #3
    I think the answer is to just use the drag force equation which is

    Fd = 1/2 * p * v^2 * Cd * A

    p = density of fluid
    v = velocity
    Cd = coefficient of drag (for plate in laminar flow = 0.001)
    A = cross sectional area

    This is easily done. However, now I am thinking that if the mass is suspended in the fluid, it is not really experiencing any drag from the surrounding fluid because it is being carried by it. If the fluid and the mass were moving at different velocities, then this force drag would factor in, but if they are moving at the same velocity, then no force is exerted upon the mass in the direction of the flow. Thus, perhaps the only force really acting on the mass is gravitational? What do you all think about that? Logical?
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