1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Aerodynamics and hydrodynamics

  1. Nov 28, 2004 #1
    Hi,
    Let's tie a sphere, drop it into a water and trawl it with a boat. How do we calculate the angle between a vertical line and rope?
    Actually I don't know much about aerodynamics and hydrodynamics so I don't know how drag force is calculated from shape, mass and speed of an object. Recently I have started using Simulink and Matlab so it would really help me to get a some formula so I could try to do some model.
    Thank you,
    Regards,
    Niko
     
  2. jcsd
  3. Nov 28, 2004 #2
    Well you need quite a bit of knowledge of these fields to properly do the calculations. For instance engineers usually model the drag force by using the empirical relation:

    [tex]F_D=C_D(Re)A\frac{1}{2}\rho v^2[/tex]

    With A the frontal surface of your sphere. rho the density of water and v your velocity. The drag coefficient ([itex]C_D[/tex]) depends on the Reynolds number so imlicitly on the velocity. If you want to find the drag force for a certain fixed velocity of the boat you could calculate the Reynolds number from its definition:

    [tex]Re=\frac{\rho v D}{\mu}[/tex]

    With D the characteristic length scale, in your case the diameter of the sphere and [itex]\mu[/tex] the dynamic viscosity of water. Next you could look up the drag coefficient in a graph or something, only for certain Reynolds numbers (<1) this can be analytically found:

    [tex]C_D= \frac{24}{Re}[/tex]

    But for Re<1 your boat would have to move very! slowly. For 1E3<Re<2E5 the drag coëfficiënt is approximately constant (~0,4). See: http://www.uh.edu/engines/spheredrag.jpg for an example of how to find the drag coefficient from the Reynolds number.

    With the drag coëfficient you can calculate the drag with the first formula. But again, I think a good calculation involves a lot of knowledge of the fields you mention and will be quite labourous, so you might still want to change your mind...
     
  4. Nov 28, 2004 #3
    Well, giving it another thought. If you discard the effect of the boat on the water, so you can evaluate the flow as homogeneous, a first approximation is not too difficult.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Aerodynamics and hydrodynamics
  1. Aerodynamics of a ball (Replies: 1)

  2. Aerodynamic heating (Replies: 13)

  3. Aerodynamic Drag (Replies: 5)

  4. Hydrodynamic limit (Replies: 2)

Loading...