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How to correctly find total drag?

  1. Oct 28, 2013 #1
    Hello,

    I just want to make sure I am doing this right. As an example, if I want to add a spoiler or hood scoop to a car, after I find the correct coefficients of drag, can I assume that the total drag force acting on the car is ƩFD=drag of the car + drag of the spoiler+ drag of scoop, etc.?

    And following that logic, when I want to find the total drag force, rather than calculating out all of the individual forces, can I correctly say that FD = 0.5(ρv2A)(ƩCD) ?

    Thanks in advance,
    Rob
     
  2. jcsd
  3. Oct 28, 2013 #2
    This does not work at all, unfortunately. The drag coefficient of a complicated shape made of several parts is neither the sum of the drag coefficients of the individual parts, nor the average, nor the area-weighted average.

    It is true that the drag force on the car will be equal to the drag on the body plus the drag on the spoiler. However, this is of no use in calculating the total drag, because you cannot use the coefficient of drag for the body-without-spoiler to calculate the drag force on the body-with-spoiler, and similarly you can't use the coefficient of drag for the spoiler alone to calculate the drag force on the spoiler once it is attached to the body.

    Determining the coefficient of drag of an object as complicated as a car pretty much requires an experiment or a computer model.
     
  4. Oct 28, 2013 #3
    Thanks eigenperson, I had a feeling it was too easy lol. Ok, so much for that. Let me just ask you this then: if I want to see if a spoiler would increase the drag enough to cause a significant decrease in MPG, and I just want to know right away if it's even worth the effort (a back-of-the-envelope kind of calculation), could I just sum the forces? And if the drag is significant disregard the idea, and if my rough estimate says it's insignificant, then do an experiment...?
     
  5. Oct 29, 2013 #4

    boneh3ad

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    Which forces are you trying to sum? Generally speaking, drag forces do not obey superposition because nothing in the process is a linear phenomenon. The only way to know for sure would be to run a CFD simulation or an experiment of some kind, e.g. a wind tunnel model or just put a spoiler on your car and drive around and comparpe it to your mileage before adding the spoiler, though that would be difficult to control properly.
     
  6. Oct 29, 2013 #5

    CWatters

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    The problem is that the airflow of one part affects another. For example you could calculate the separate drag forces on

    A car at 70mph
    A roof bar at 70mph

    But you can't just add them. The airflow over the roof of a car doing 70 is likely to be faster than 70 because it speeds up as it goes up over the car.
     
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