Instabilities in viscoelastic flows in conduits with abrupt changes.

摘要

Kinematics and nonlinear dynamics of viscoelastic flow in various flow conduits with abrupt changes are investigated. Polyisobutylene (PIB)-based polymer solutions are used as the test fluid. Flow visualization (streakline photography) and instantaneous pressure measurements are employed to examine the spatio-temporal nature of the flow for a wide range of flow rates. Flow transitions in viscoelastic cavity and entry flows and in rectangular and circular conduits with various abrupt changes in the cross-sections are reported. The critical conditions for the onset of elastic flow transitions and secondary flows produced by hydrodynamic instabilities are determined.; Cellular-type instabilities are observed for flow in short planar cavities beyond a critical flow rate. Large cellular structures and small-scale eddies are formed in the cavity. Interaction between the large cells and the small eddies are observed, resulting in the formation of a complex three-dimensional secondary flow structure. At high flow rates, the flow becomes time-dependent, resulting in pressure fluctuations as high as 60 percent of the nominal value. In short cavities, cavity-dominated flow behavior exists, characterized by a strong interaction between large cellular structures and small-scales eddies in the cavity. In longer cavities, contraction-dominated flow is observed, which is characterized by the formation of a lip vortex near the contraction plane. Beyond a critical flow rate, pulsating-type instabilities occur for flow in longer cavities and for flow past a planar contraction. The periodic and irregular axial growth of the lip vortex results in pulsation of the flow and governs the dynamics of the flow. Cellular transitions do not occur in these geometries. Pressure measurements indicate an onset of multiple temporal instabilities in both planar cavity and contraction flows.; Similar flow transitions occur for flow in axisymmetric cavities. At flow rates beyond criticality, cellular transitions are expected to occur in the shorter cavities. For longer cavities, pulsating-type instabilities again exist, dominated by an elastic lip vortex near the contraction plane. In cavity-dominated flows (short cavities), the small-scale eddies formed near the contraction plane create a time-dependent flow. In contraction-dominated flows (long cavities), the elastic lip vortex determines the time-dependent behavior of the flow. The flow curve becomes double-valued at the onset of each temporal instability, which indicates that the flow becomes nonlinear and very complex at high flow rates.