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I wonder how a hollow cylinder at the same cross area perform vs a solid one, i.e the hollow cylinder has larger radius? I guess they have a similar drag, is that true?

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I don't have answer right now, but there is more surface for a hollow cylinder, so maybe more drag? –  fffred Jun 3 '13 at 7:05
I didnt get the question. Are you comparing two identical cylinders wherein the dimensions are identical but one is hollow, and the other is closed? –  Saurabh Raje Jun 3 '13 at 7:54
@fffred, I would say the excess area hypothesis is true only for the initial fraction of seconds –  Saurabh Raje Jun 3 '13 at 7:56
Has this answered your question? –  Killercam Jul 11 '13 at 9:36
By hollow, do you mean that there are no circular faces. i.e. air can flow through the cylinder? –  user80551 Sep 19 '13 at 17:55

3 Answers 3

If the flow is perpendicular to the cylinders axis of symmetry as long as the two cylinders have the same radius, length and are made of the same material, then the drag on them will be the same.

If the flow is parallel to the axis of symmetry then you are right to suggest that (assuming the radius, length and material of the two cylinders are the same) the larger surface area of the hollow cylinder will cause more drag.

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I disagree with the answers for the case with flow parallel to the axis of symmetry.

The drag should be higher for the solid cylinder. Yes, the friction drag will be higher for the hollow cylinder case due to there being a higher surface area - however, there are generally two main sources of drag - friction drag (caused by viscous effects at the wall) and pressure drag (caused by pressure difference between front and rear of the object). In the case of the solid cylinder, pressure drag will dominate, and contribute a far greater drag component than the friction drag (orders of magnitude greater). A wake will form behind the solid cylinder, resulting in a low pressure region behind the object. A higher pressure stagnation region would also form upstream of the cylinder. This large pressure difference would increase drag significantly. For the hollow case, there will be virtually no pressure drag.

The only case where the drag would be greater for the hollow cylinder, would be if said cylinder was verrrrry long - maybe a hundred metres long or so. In this case, the viscous drag would accumulate as you travelled downstream, and the drag force would eventually surpass the pressure drag force a solid cylinder would experience, due to the extremely large surface area.

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I disagree with the parallel flow being equal. If the cylinder's aspect ratio is very long then, yes, the drag should be very similar because the air inside the cylinder is essentially trapped, travels with the cylinder and therefore creates a virtual base to seal the ends of the cylinder.

But for stubbier cylinders (think a hula hoop) the hollow one will have different drag. Here the air has enough space to equilibrate with the outside air.

Note: I'm assuming by hollow, you mean a cylindrical shell. That is a tube like shape. Alternatively, a cylinder with the exact exterior surfaces but empty interior should have the exact same drag as a solid one.

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