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Old Apr 8th 2013, 11:44 PM   #1
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Exclamation Calculate the Drag Force

Given a velocity distribution before the object, calculate the force on the object.

Last edited by simonj; Apr 10th 2013 at 08:13 AM.
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Old Apr 9th 2013, 08:05 AM   #2
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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.

Last edited by ChipB; Apr 9th 2013 at 03:37 PM.
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Old Apr 9th 2013, 12:40 PM   #3
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I see. Can't you also create a control volume around the object and use the linear momentum equation to figure out the drag force?
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