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Speed and distance are two main determinants in this question. A slow moving vehicle or train entering a tunnel is unlikely to be affected by the presence of solid stationery object covering them. But say the object is moving fast enough, will the presence of a tunnel requires the object to use more energy to go through it. Also the distance between the object and the wall of the tunnel, AND the length of the tunnel itself will most likely affect the energy required to go through the entire tunnel. Will it require more energy the longer the tunnel?

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  • $\begingroup$ Sure. You're correct on all these things. The main question is whether it will be significant. $\endgroup$ Commented May 20, 2016 at 11:12

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This is hard to answer quantitively.

A train in open air pushes air out of the way, and sucks air in behind it. Some air is dragged along with the train. Mostly air moves near the train.

In a New York subway, you can feel a strong breeze before the train arrives. The tunnel diameter isn't that much larger than the train, so the train acts something like a piston. It pushes air ahead of it and suck in air behind. The density of air is about 1 kg/m^3. It takes energy to accelerate that mass.

Air is not very viscous, but it does have viscosity. A thin layer of air against the train moves at the speed of the train. Likewise, a thin layer against the tunnel stays still. For a slow train, the flow might be laminar. Adjacent layers move at slightly different speeds, and air velocity varies smoothly between the train and tunnel. In that case, some kinetic energy is lost as heat.

Most flow is turbulent. This is disorganized and has much larger velocity differences across short distances. More energy is lost in turbulent flow.

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