Why do tip vortices seem to 'bend' inwards at the tip of a plane wing?

Question

Why do tip vortices seem to 'bend' inwards at the tip of a plane wing?

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Here if you look closely, the tip vortices seem to bend or travel inwards very slightly after the aircraft has passed. In this picture, the effect is more exaggerated on the lower wing. What force makes the vortices bend inward?

My original thought was that the wake of the plane itself is the culprit, but I'm not sure, as the vortices seem to only bend right as they leave the wing, and then they travel straight.

It seems like if the wake was the cause then the vortices would continue to bend inward, and not just stop after the plane passed.

Answer 1

That is part of the wake rollup process.

The wake behind an airplane is caused by the downward acceleration of air while flowing around the wing. The tip vortices only show how the local streamline winds up in the wake, being made visible by local condensation. That the wake coming off the lower wing seems to "bend more inwards" is due to the downward motion of the air and the particular angle from which the photo was shot.

This answer goes into the details of wake formation. Note in particular that the distance between the cores of the trailing vortices is much more narrow than the wingspan. The air coming off the wingtips has been accelerated inwards on the upper and outwards on the lower side of the wing, and the resulting swirl causes low pressure at its core which in turn leads to the condensation that makes it visible.

What force makes the vortices bend inward?

It is important to not confuse the tip vortex with the full wake which forms behind the airplane. The downward moving air of the wake will suck in air from the sides and from above, and displace the air below it, so again a swirling motion will result. The condensed humidity of the tip vortex visualizes how this rollup evolves behind the wing. Local pressure gradients are responsible for the inward motion: This air tries to fill in the volume of lower pressure that is formed by the downward motion of the wake.

Answer 2

What you see are two visual streamlines, which contract due to converting back to undisturbed airflow.

The forward flying aeroplane occupies space and pushes the air out of the way; when it is past, the air flows back to where the plane was. As simple as that. How fast, neat and turbulence free it flows back is a function of the shape of the physical body - how streamlined it is.

Answer 3

There's at least two things going on here.

As the engine's exhaust jet entrains surrounding air, the net effect outside the entrainment region is that the jet contracts. The free stream wake contracts around it. That's what's happening well downstream of the plane.

There is also a bit of an inboard jump right behind the wing. That is due to real world engineering of sophisticated tip design and aileron settings. There are two vortex cores wrapped like a candy cane. The condensation happens to start at the tip with this aileron setting; and they roll up and combine a short distance behind the wing.

Answer 4

It might help to see video clips showing the downwards movement of wing tip vortices:

Plane flying through clouds

Owl gliding through bubble wall. The audio commentary is mis-leading, "downwards moving vortices ... helps provide lift". Clarifying: the vortices move downwards because of the downwash that coexists with lift. Although owls and some other raptor birds use their tails to generate lift more efficiently (less drag), birds like the Albatross rely on large wingspans (10 to 11 feet) for more efficient flight, and most gliders also rely on large wingspans for more efficient flight. The Nimbus 4 series rely on an 87 foot wingspan to achieve a 60:1 glide ratio at about 80 mph.