Open Channel Siphon with Viscoelastic Fluids

Viscoelastic fluids show many remarkable phenomena, the best known of which is probably the Weissenberg effect. Because such fluids produce normal stresses in a plane perpendicular to that in which shear takes place, they will climb up a rotating shaft (for example, a stirring rod) immersed in the liquid. A more direct illustration of the elastic properties of the fluid is provided when a rotating flow comes to rest; tracer particles within the flow indicate that the fluid first stops and then flows in the opposite direction for a short duration. An apparently unreported but equally spectacular effect with this type of fluid is the open channel siphon. The sequence of pictures in Fig. 1 illustrates the phenomenon; the upper 4 l. beaker was originally filled with a viscoelastic fluid, and it was then tipped slightly to start the liquid flowing over the lip of the beaker. Once established, the stream continued. The flow rate initially increased, then slackened off and finally stopped when the distance up the beaker wall became too great