Johan Bosman, JS Chief Aerodynamicist, explains:
Whenever there is a difference in load on the wing,
a vortex will form with excess drag as a result.
Flap fences reduce the vortex size, because the
pressure difference is “capped off”. The flap fences
on the JS1 Revelation are fixed to the flaperons,
not the wing root, as the load and pressure
difference is higher on the flaperon.
There is however a trade-off for fitting flap
fences, and in this case it is the added wetted
surface area of the fences and interference drag.
Recent oil flow experiments on the root with flap
fences showed good flow lines, with no separation or
vortices forming on the surface near the fences,
which indicate no added detrimental effects.
it is truly beneficial to add flap fences is still a
good question, but a current CFD study might shed
light upon the problem in the near future.