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What is the velocity of a falling object with air resistance?

  1. Jan 15, 2017 #1
    1. The problem statement, all variables and given/known data
    I am trying to develop simulation for a falling object subject to air resistance. Object is similar to Samara seed. object is considered to be under steady vertical descend.
    know variable :
    surface area of object (A)
    weight of object (W)
    cd: drag coefficient
    object rotates while falling.
    length from center of rotation to tip of the object (L)
    inclines at an angle beta (β)
    Swept disk radius (r) = L*cos(β)
    2. Relevant equations
    terminal velocity(Vs)= √[(2*W)/(cd*ρ*A)
    Disk loading= W/A=ρ(Vs2-Vf2)/2




    3. The attempt at a solution
    I am stuck in this position and don't know how to approach the problem from here. want to know how to calculate the velocity of object and angular velocity of the tip of the object.
     
  2. jcsd
  3. Jan 16, 2017 #2

    CWatters

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    I can't help with the aerodynamics but...

    If it's descending at constant velocity then at that velocity the vertical force upwards (caused by a combination of lift and drag) is equal to the force due to gravity downwards.
     
  4. Jan 16, 2017 #3

    CWatters

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  5. Jan 18, 2017 #4
    thanks for the replies.
    I had some more doubt. Like how to draw the helix created by the object if it start rotating 1 meter above the ground.
    given that we know the velocity of fall, angular velocity of rotation and radius of helix.
     
  6. Jan 19, 2017 #5

    CWatters

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    If we assume that the seed very quickly reaches terminal velocity then that's solvable...

    The vertical velocity (v) and the height (h) give you the time it takes to fall (t)....

    t = h/v

    Then the time (t) and angular velocity (ω) gives you the number of revolutions (n) it makes before hitting the ground..

    n = ωt/2π (Note I had to edit this line)

    Then the height (h) and the number of revolutions (n) gives you the pitch of the helix (p) in meters (p = the distance it falls per revolution)..

    p = h/n

    I'm not sure how you define the radius of the helix? I think seeds with a single wing rotate about a point on the wing near the seed head. So the seed and wing tip move in a helix (different radii) but the centre of rotation descends in a straight line (ignoring wind).
     
  7. Jan 22, 2017 #6
    thank you for the help.
    center of rotation can be found by find center of mass (C.M). Most of the seed have this as center of rotation.
     
  8. Jan 27, 2017 #7
    Can you help me with finding the angular velocity of the object.
    I took angular velocity as terminal velocty (vtrmnl)*sin(Θ)/radius, since no other force is acting on the object. [Θ is the angle of attack]
    But I think there is some thing wron.
     
  9. Jan 27, 2017 #8

    CWatters

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    I make it

    Angular velocity (Rads/s) = Vtrmnl / (Radius * Tan(Θ))

    Seed fall.png

    In one revolution it falls a distance h so

    Tan(θ) = h/circumference
    so
    h = Tan(θ) * 2πR

    The time for one revolution is t..

    t = h/Vtermnl
    t = (Tan(θ) * 2πR)/Vtermnl

    Angular Velocity = angle/time
    = 2π/t
    = 2π/((Tan(θ) * 2πR)/Vtermnl)
    = Vtermnl/(Radius * Tan(Θ))
     
  10. Jan 27, 2017 #9

    CWatters

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    I should add that in my post above the angle θ is the pitch angle of the helix (not the aerodynamic angle of attack). I don't think you can easily calculate the angular velocity from the aerodynamic pitch.
     
  11. Jan 29, 2017 #10
    How to find pitch angle?
    I think we can find it using h and circumference. Using Pythagoras theorm.
     
    Last edited: Jan 29, 2017
  12. Jan 30, 2017 #11

    CWatters

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    Correct although you don't need Pythagoras...

    The pitch angle of the helix (distance descended per revolution) = Tan-1(h/circumference)

    But note that this is not the same as the aerodynamic pitch.
     
  13. Feb 4, 2017 #12
    problem in this is that i couldn't find 'h'
    .
     
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