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Drag Coefficient

  • 1.  Drag Coefficient

    Posted 06-15-2020 11:12
    Why the area in the definition of drag coefficient is projected area?
    It comprises wall drag and form drag, why don't we consider outer surface area as wall drag would be along that, not on projected area.

    Himanshu Singh Chauhan Chemical Engineering Sophomore
    RGIPT, India

  • 2.  RE: Drag Coefficient

    Posted 06-15-2020 20:05
    Hello Himanshu, how are you? I hope you and your family are well and safe.

    So, I asked a friend of mine for some help on this matter, because this is almost a philosophical question haha.

    Basically, the reason behind it is because it's the projected area that creates the abrupt change of the current lines (which would be the recirculation).  If you notice, the bigger the drag coefficient, the bigger is the recirculation created on the current lines.

    Taking a closer look, it's the fluid that suffers this big recirculation as soon as he "hits" the area; after that it follows the surface area of the geometry. That's why we use the frontal (projected) area of the surface, since it's this area that causes the abrupt change of the current lines and a possible recirculation (mainly in turbulent flow). Note that laminar flow has a very low drag coefficient.

    I don't know if it's totally clear to you, and I completely understand if it's not. It's very had to explain this question and we tried to do our best. I also found two links that can help you understand better this subject (besides all the theory you can find in the books). You should definetely take a look and maybe they'll help you understand better:

    I hope this can help! And don't hesitate to message or e-mail me with any other doubts. I can't guarantee I'll be 100% right but I'll definetely do my best!

    I wish you the best and good luck!

    Best regards,

    Bruno Neves
    Student Chapter President at AIChE UFRJ
    Latin America Regional Liaison - ESC
    Publications Subcommittee/Student Central - ESC
    Chemical Engineering Undergraduate - UFRJ
    Rio De Janeiro - Brazil