Figure 10.

(a) Experimental set-up (1×1 m) with two cylinders symmetrically placed and a magnified cylinder (ϕ=10 cm, height= 15 cm). (b) Experimental set-up (1.2×1.2 m) with two blue Perspex discs symmetrically placed and a disc (ϕ=20 cm). In the experimental tank, the water column is 6 cm. Luminosity is ensured by 4 fluorescents lamps of 80 W placed on each side of the tank, on the floor and directed towards the walls to provide indirect lighting. The whole set-up is confined in a 2×2×2.35 m experimental chamber (cage) surrounded by white sheets to isolate the experiments and to homogenize luminosity.