Digital particle imaging velocimetry of viscoelastic fluids

We investigate the utility of digital particle imaging velocimetry (DPIV) for performing kinematic measurements in non‐Newtonian flows. With the advances in numerical techniques for simulation of viscoelastic flows, acquisition of spatially dense 2‐D kinematic data in steady and time‐dependent deformations can be useful in verifying predictions of the corresponding computational studies. Furthermore, kinematic measurements of the velocity field and rate of deformation in prototypical industrial processes can significantly enhance the rational design and optimization of polymer processing unit operations. Application of a high seeding density DPIV technique in viscoelastic media is discussed, and quantitative data are obtained in a number of industrially relevant flow geometries. The issues of velocity‐position assignment and the effects of a velocity gradient across DPIV correlation regions are discussed. A simple yet effective averaging technique preserves the order of accuracy and assigns the velocity vectors to their appropriate positions using an overlapping discretization scheme. The examples studied experimentally include steady flow in circular pipes, flow past obstructions, flow in a lid‐driven cavity, and time‐dependent free‐surface extensional flows in a liquid filament. With the exception of the first example, these flow geometries constitute an important collection of configurations in which quantitative experimental data for non‐Newtonian fluids are scarce or nonexistent.