A Study on an Anomalous Phenomenon Occurring in the Issuing of Viscoelastic Fluids from Ducts : Flow Behavior and Pressure Fluctuation Near the Duct Exit

Abstract

When a viscoelastic fluid flows out from a horizontal duct, many cracks and protruding ridges are formed on the jet surface. The jet states are affected by the shape of the duct exit. For the knife-edge-shaped duct exit, the jet appears in three states, i. e., the stable state, the breakage state, and the multiplication state. For the semicylindrical-shaped duct exit the jet appears as another state instead of the breakage state. We call this the "stationary state". In this state, stationary ridges occur on the jet, and they become uniformly large as the shear rate increases. Anomalous pressure fluctuation is observed near the duct exit in the breakage state and the multiplication state. In the stationary state the pressure fluctuation is not observed, but the disproportional pressure distribution for the duct width direction is observed. A large vortex occurs on the end of the duct floor wall for the knife-edge-shaped duct exit. In the stable state it is steady and two-dimensional, and in the anomalous state it is unsteady and three-dimensional. But for the semicylindridal-shaped duct exit, the vortex does not appear, regardless of the jet state.