This is a quite idealized situation that ignores drag caused by the sides of the box, and ignores turbulence behind the box. If the model included drag of the sides then the wind speed behind the box would not be a simple step function as shown in the figure, but rather with laminar flow there would be a ramp from the 3m/s to 5m/s region. So it seems that a simple calculation using Bernoulli's principle:
(1/2) rho v_1^2 + P_1 = (1/2 ) rho v_2^2 + P_2
where rho is the density of the air. Solve for the difference in static pressures P_1-P_2 between the front and rear of the box, and multiply by the box's frontal area to get the difference in forces exerted on the front and rear. That difference is the drag acting on the box.
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Last edited by ChipB; Apr 9th 2013 at 03:37 PM.
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