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B Fluid Dynamics - Calculating Coefficient of Drag

  1. Sep 20, 2016 #1
    How would one calculate the drag on a projectile (in this case a 1cm3 cube) that was launched at 0°.

    The vertical drop, initial velocity, distance, and time (taken to travel distance) where measured.

    I want to say that I could compare these experimental drop (bellow hight that projectile was shot from) to the theoretical SUVAT drop (which assumes no air resistance) and find the drag experienced by the cube that way, however I am unable to find the proper equation / an example of this.

    Also, using the drag coefficient of a cube/square wont work because the cube was erratically spinning on multiple (axis).

    Any help is appreciated. :smile:

    (this is purely an extracurricular hobby/experiment)
     
    Last edited: Sep 20, 2016
  2. jcsd
  3. Sep 20, 2016 #2

    Bystander

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    Fluid resistance is "drag." Could you restate your problem.
     
  4. Sep 20, 2016 #3
    I am aware of that.
    I guess i wasn't clear in my wording, sorry.

    What i meant is that my theoretical SUVAT values for the drop that would occur are assuming no air resistance. Compared to my experimental results which are effected by air resistance. So, by comparing the two values, the theoretical value (of drop) given no air resistance and the experimental result (of drop) with air resistance, I would think there should be some way to find the drag on the projectile.

    So i was wondering what equations or theories i should look for to do calculate the drag on the projectile from the data that i have.

    Hope this clears it up.
     
    Last edited: Sep 20, 2016
  5. Sep 20, 2016 #4

    Bystander

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    Just Googled "magnus effect;" got three hundred and some thousand hits --- first couple pages did not apply it to anything more complex than spheres and cylinders --- you might find cubes --- "+'cube shape' " trims it to
    https://www.google.com/#q="magnus+effect"+"cube+shape"
    --- and third entry refers to Magnus effect in saltation, J. Fluid Mech., 1977.
     
  6. Sep 20, 2016 #5
    Although the "Magnus Effect in Saltation" is very helpful in explaining the discrepancy between my theoretical and experimental results, it doesn't answer my main question. How would I go about finding the drag experienced by the projectile using the information I have gathered? (or is it not possible due to lack of necessary data?)

    Cheers :)

    Edit:
    upload_2016-9-21_2-49-44.png
    I was hoping I could find a way to to work out the FD. Currently using this equation I have two unknowns. Since the equation for CD is a rearrangement of the the equation above, I do not see any useful substitutions I can make to work it out.
     
    Last edited: Sep 20, 2016
  7. Sep 20, 2016 #6

    boneh3ad

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    The problem is that the drag formula you have cited is a purely empirical relationship and it is very rare that ##C_D## can be calculated analytically. Instead, it is typically measured in a number of experiments so that it can be applied later.

    Your SUVAT idea is theoretically sound, but the problem is that those equations don't deal in situations that have variable acceleration, which is the case here. The thing is, the SUVAT equations are derived from more general principles, so if you have any familiarity with calculus and, preferably, differential equations, you could go about determining this experimentally.
     
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