Buoyancy forces

Consider first the buoyancy forces. When a body is immersed in a fluid, it experiences local forces which oppose the pressure gradient in the fluid. For example if a table tennis ball is immersed in a swimming pool, it experiences an upward force corresponding with the pressure gradient in the water, which points vertically downwards.

By establishing an axial pressure gradient in the air in the duct, it is possible to generate corresponding axial body forces on a cable inside that duct. The force on the cable then acts in the opposite direction to the pressure gradient.