For example, when we turn in a bike, we have to bend slightly inward for the centripetal force. However, considering there’s no concept of “ground” underwater, do submarines still have to bend inwards? If yes, is it for the same reason as bikes?
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You only have to bend on bikes
A) your contact area to the ground is very shallow, so even a bit of centripetal force will have your resulting g vector point outside the contact area, causing you to fall over. If you’re not **on** the ground, or have a wider contact area (like a car), the centripetal force can’t throw you of balance (as easily)
B) Bikes have a really high center of gravity, which greatly exacerbates the effects in A)
Neither of those applies to subs.
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The bike is tilted toward the center of the turn in order to generate force from the ground to push the bike into actually turning.
A sub (any boat, actually) cannot do this because there’s no solid connection between it and the ground. Instead, it uses rudders and it’s own shape to skid sideways, which generates the necessary reaction force from the water, to push the boat into the turn.
A car in a drifting or sliding turn is doing a similar thing.
On the assumption that you mean “lean in” by “bend”, then a submarine doesn’t have to. They can just turn flat, like a car, by turning the rudders at the back.
Bikes have only one way to apply continuous force, their tire contact patch. You have to tilt this to get your normal force from the ground to push you around the turn. This is roughly analagous to why airplanes roll to turn (they’re using lift, not ground normal force, but they still have to tilt it). You can turn an airplane with just rudder but it’s ugly.
Submarines, on the other hand, aren’t going particularly fast and can generate *huge* control forces because water is dense. So they can just turn with their rudder and call it good. In principle, they have all the controls needed to do a rolled turn, like an airplane, but they don’t need to.
No, submarines do not bend due to extreme structural rigidity and positive internal pressure.
If you are talking about leaning into a turn so as not to fall over due to centrifugal force then the answer is also no. Submarines have no localized points of contact to the ground which would act as pivot points of lateral forces.