Fig. 4. The soap-film tunnel is mounted in an inclined frame and driven by gravity. It consists of three sections: a divergent section (i), the constant width (60 mm) test section in which the foil flaps (ii) and a convergent section (iii). The soap reservoir (a) produces a constant head by using an overflow. The soap flows from the reservoir (a) through a tuning valve (b) and an oval nozzle made out of a plastic pipette (c). At the pipette (c) the soap film starts: it runs down, driven by gravity, between two 1 mm thick Nylon wires (d) into tunnel sections (i–iii). The Nylon wires are pulled apart with 0.2 mm Dynema fishing lines (e). Finally the soap is collected in a reservoir (f) and is drained into the main soap reservoir and pumped (P) back again to the top reservoir (a).