Hang the bucket on a long rope. Rotate the bucket to tighten the rope. Then fill the bucket with water and close the lid (shown in the blue line). At this time, the bucket and water are in a static state, and the isobaric surface in the water is plane.
Release the bucket and the torsion of the rope makes the bucket rotate.
After rotating for a period of time, water also rotates because of its viscosity. At this time, the isobaric surface in water is no longer a plane, but a parabolic surface: the pressure at the wall of the bucket is high, and the pressure at the center of the bucket is low.
Because of the rotation, the water in the center of the bucket expands, so the pressure decreases, while the water in the wall of the bucket is compressed, so the pressure increases.
The lift of the wing is the same. The airflow at the top of the wing tends to be away from the wing along the normal direction, so the airflow is expanded and the pressure is reduced. The airflow at the bottom of the wing tends to approach the wing along the normal direction, so the airflow is compressed and the pressure increases. One high, one low, there is pressure difference, so it produces lift.
I use air instead of water, then fill the air into a bucket deep enough and large enough, and then rotate the bucket, the air in the bucket will form a parabolic surface. What does this mean?