- #1

Maximusflash

I was reading about drag, and didn't quite find what I was looking for.

A general drag equation is : 0,5 x ρ x u² x Cd x A, where there are some drag coefficients that have been found during laboratory experiments.

Let's say a sphere with a radius of 1m is traveling at 2m/s under water, with buoyancy and gravity being equal.

0,5 x 1000 x 2² x 0,47 x π x 1² = 3kN

Not much, but at the same time speed is low.

But I guess that this equation has it's limitations: a free body of water with no restrictions?

What if this sphere were to travel trough a tunnel or pipe? A train going trough a tunnel at 200km/h there are a lot of factors of friction, and the size of the tunnel being a big part of it (as well as aerodynamic shape of the train).

If the underwater-tunnel is to small, even at slow speed, the sphere would be pushing the water in stead of going trough. But is it enough if the free area around the sphere is as big as the sphere, or are there other rule of thumbs here?