Unsteady Vortex Dynamics of a Viscoelastic Flow in a Circular Sudden Contraction Geometry
Streakline photography technique is performed to examine the spatial and temporal structure of the vortex / vortices observed in a viscoelastic flow through a circular 4:1 sudden contraction geometry. The test (viscoelastic) fluid used in this study is optically clear polyisobutylene in polybutene (PIB/PB) Boger fluid. Flow visualization measurements are conducted for a wide range of the flow rates (or Deborah numbers) to systematically examine the different regimes of the flow. Various exposure times and time-intervals between flow photographs are applied. In determining these time-related parameters as well as interesting flow regimes for photographing, the critical conditions for the onset of the elastic instabilities determined accurately by instantaneous pressure difference measurements near the contraction plane were used. Temporal evolution of elastic vortices and pressure fluctuations in unsteady flow region are found to be closely related. Significantly large excess pressure drop across the contraction plane due to the growth of elastic vortices is obtained. Three-dimensional snapshot images of the flow field near the contraction plane are also presented here to explain the growth mechanism of elastic vortices.