Digital particle image velocimetry in flows with nearly closed pathlines: The viscoelastic Taylor-Couette instability

We describe a novel technique for applying digital particle image velocimetry (DPIV) in steady or slowly varying flows with nearly closed pathlines in which the flow of interest is small relative to the primary flow and the primary flow is normal to the plane of illumination. Despite the fact that in-plane particle displacements are negligible during video-rate image capture, DPIV can be used by implementing extremely low levels of seeding and using long 'exposure' times. An 'exposure' for subsequent DPIV processing corresponds to the image formed by taking the minimum of each pixel position from each of the frames captured during the exposure time. Here, we have used a grey scale in which a pixel value of zero corresponds to white so a bright particle that appears in any of the frames captured during the exposure time will appear in the composite exposure. Preliminary results for a viscoelastic fluid in Taylor-Couette flow (flow between concentric rotating cylinders) are shown. As long as the cylinders are adequately aligned, there is no difficulty in resolving the main features of the secondary flow. Secondary flows four to six orders of magnitude weaker than the base flow can be resolved by this method in the present experiment. This technique would be equally successful in other flows such as flow between rotating parallel disks or between a cone and plate, both of which are important for rheological characterization of fluids. Through the use of co-rotation of fixtures, this technique could also be extended to higher temporal resolution and higher secondary velocities relative to the base flow. (C) 1997 Elsevier Science B.V.