on road? and why not cuboidal ones?
Cylinders are easier to make strong enough to hold them.
I guess less number of welding joints are required in elliptical or cylindrical vessels than cubical ones.
It isn't a matter of welding joints: flat surfaces bend when there is a pressure behind them while cylinders are just in tension.
As russ_watters wrote. A flat surface will flex and so cannot oppose the internal static and dynamic pressures in the tank.
A cylinder with domed or curved ends has all surfaces constrained from wobbling. That prevents large amplitude vibration fatigue of weld seams at the corners of rectangular tanks, where the repetative vibrational energy would otherwise be concentrated.
Liquid sloshing within the tank is restricted by slightly curved internal walls or baffles. They also increase the rigidity of the structure. A chassis is not always needed with a cylindrical tank.
When I was in college, I took a course called "Laplace Transforms and Boundary Value Problems". I was not to my liking, but I learned two things in it.
(1) trampolines do no make good musical instruments
(2) if you ever get on a plane with square windows, get off again immediately.
Both of these arise from the math result that sharp square corners have some very interesting properties. One of them is that they can set up set up an infinite number of vibration modes, so if you have a sharply square steel drum, you get noise, not music. The other is that there is theoretically an infinite pressure right at the corners if there is a pressure differential.
SO ... rectangular containers holding liquids are subject to structural failure at the corners, making it unsafe to use them to hold large amount of liquid. Ever see a square/rectangular water tank or a square/rectangular tanker truck?
Well, I've seen sharply square/rectangular swimming pools... Is that any different?
Yes, they are supported from the outside, which helps. Also, although the corners inside the pool may be sharp but the surrounding support outside is likely to be more rounded. Still, you bring up a good point. I've seen those as well. although I think "sharp" may be an exaggeration of the internal curvature.
Have you seen cracks in the concrete walkway that surrounds one of those swimming pools? Oftentimes you will, and most often they'll start at a corner of the pool.
A concrete swimming pool is not a thin shell, it is a concrete retaining wall that separates the external hydrostatic pressure of the soil from the hydrostatic pressure of the water in the pool. Each pressure cycle of a seasonal pool takes one year, so vibration is not a significant problem.
A “good” way to crack a concrete pool is to drain it, which unbalances the hydrostatic pressures, then let the shadow / sun to shine on the concrete to give thermal differential expansion a chance to work at the concrete. (The water in the pool normally equilibrates the temperature of the concrete). Once a crack in the concrete begins, chlorine in the water next season can promote attack of the steel reinforcing and so allow the tension structure to fail.
This thread is wandering (from liquid transport tanks to swimming pools!), but back to the tanks ...
Everything said above about the benefits of curved walls is correct. To go a step further, in the ideal situation, the walls of the vessel should be in pure tension. An air-filled balloon illustrates this; the walls of the balloon are in pure tension. If there were shear, the wall would deform and the shape would not be a sphere.
With tanks that will hold liquid at varying depths (sometimes full, but often not entirely), the pure tension objective cannot be met. Even so, the benefits of a curved wall, where most of the stress is pure normal stress with a minimum of shear is to be preferred.
What about small scale objects where the material could handle the forces at hand. A rectangular water bottles would ship easier in bulk and could be easily modified for ergonomics. Majority of bathroom sinks are elliptical. Hygienic products such as soaps, shampoos, and conditioners are distributed in elliptical cylinders; however, like the water bottle would be more efficient to ship in rectangular containers. This might go outside of physics I'm just curious
So you do have rectangular containers for those objects. Just that they aren't used for liquids.
There is very little pressure against the wall of a small container because the height is low. For that reason container shape is not as important. Full containers do not slosh while being transported.
On the other hand, big tanks that are part filled have a significant low frequency sloshing that can apply high fluid pressures to the corners of the tank and can even overturn the vehicle.
Swimming pools don't contain much, if the load gets to high, it just splashes out, not really containing it, just gravity load
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