Move in a liquid container a curved object

neufneufneuf

When the object move in the container, the pressure seems to be reduce, no ? Atoms seems to have an up force (Fs) and the pressure on object is reduce from black forces, no ?

dlouth15

In the bottom diagram you have a curved surface covered with molecules and arrows representing forces. However, where are the forces on those molecules due to the other molecules in the liquid? You've only got some of the forces drawn in from what I can see.

neufneufneuf

Like that ? I drawn more atoms. Difficult to draw all forces. Even the layer of atoms touching the surface reduce pressure, this need less force for enter the volume, another layer cancel this force ?

dlouth15

The overall effect of the movement of the object would be to increase pressure but you might get the beginnings of lift effect in the upper part of the curve along the same principle that gives lift to wings as they move through the air although I would expect this effect to be very small. It would need to be modelled on a computer I think.

But on your diagram you haven't taken into account all the forces on the molecules. The object is exerting pressure on the molecules on the liquid in direct contact with the object but each of these molecules is exerting and equal and opposite reaction on the object. In addition, the rest of the molecules in the liquid are exerting general pressure on the molecules in direct contact with the object.

neufneufneuf

Sure there is a net (and big) force on object due to pressure but it seems to be reduce by the curved shape. Maybe it's only 2 or 3 % less. Is it possible to compute sum of forces ?

dlouth15

neufneufneuf wrote: »

Sure there is a net (and big) force on object due to pressure but it seems to be reduce by the curved shape. Maybe it's only 2 or 3 % less. Is it possible to compute sum of forces ?

Yes, there will be the force due to pressure and this pressure will be be increased because the object is moving. You could probably compute the net force on the object using a fluid dynamics package on a computer. I'm not familar with this area but the likes of marine engineers be able to help.

neufneufneuf

If pressure is lower due to the shape, it would say if i need energy E1 for move in curved object, I can recover more energy E2 from piston ? It's not possible, so maybe pressure is not reduced ?

dlouth15

If pressure is reduced (and I'm not sure it would be) then increased energy would be required to push the object into the container. An aircraft wing travelling through a fluid creates a lower pressure on the top surface of the wing creating but this is at the expense of increased drag - lift induced drag. Therefore more energy is required to push the wing through the fluid than would be the case without lift.

neufneufneuf

If pressure is reduced (and I'm not sure it would be) then increased energy would be required to push the object into the container.

I don't understand, if pressure is lower due to the curved shape the force for move in object is lower too, no ?

dlouth15

neufneufneuf wrote: »

I don't understand, if pressure is lower due to the curved shape the force for move in object is lower too, no ?

Energy is required to create that lower pressure. In an aircraft wing there is a component of drag called lift drag. This lift drag represents the extra energy required to create the low pressure above the wing and corresponding high pressure below the wing thus providing lift.

I'm not saying there would be much in the way of any lower pressure at the top of the curved surface in your diagram but if there were, extra energy would be required to produce it.