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Homework Help: How to find velocity of air at certain point if car is moving at certain speed (Fluid Mechanics)?

  1. Nov 13, 2014 #1
    1. The problem statement, all variables and given/known data

    The shape of a car windshield is approximated in the figure below; its length is 2.0 ft and height is 1.5 ft. Obtain an equation of the windshield shape r as a function of
    r(θ), in the polar coordinate system shown in the picture.

    When the car moves at 55 mph, determine the velocity of the air at points A and B.


    2. Relevant equations

    Not really sure how to approach this...maybe Bernoulli's Equation? We've been studying the method of repeating variables, but I'm unsure how I would apply that. I really don't know where to begin with this one!

    3. The attempt at a solution

    (P/rho + v^2/2 + gz = constant)
    But I don't know the pressure difference...I'm hopelessly stuck :( If anyone could help shed some light on this, and just give me an idea where to even begin, that would be very appreciated!
  2. jcsd
  3. Nov 14, 2014 #2
    This seems to be a trick question. Have you heard of the "no slip" boundary condition?

  4. Nov 14, 2014 #3
    Ah, so v_A = 0.
  5. Nov 14, 2014 #4
    Relative to the car. Relative to the ground, it's U = 55 mph.

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