Wing tip fences and winglets
I have a query regarding the difference between the utilization of winglet devices on jet aircraft. It is my understanding that winglets help prevent the flow of high pressure air from underneath the wing from jumping around the wing tip into the low pressure air on top of the wing reducing wing tip vortices. I am wondering why Airbus would use a wing tip fence which extends both above and below the wing whilst Boeing uses a winglet which only extends above the wing. Is there a more effective design?