Bm. Baumert et al., DIGITAL PARTICLE IMAGE VELOCIMETRY IN FLOWS WITH NEARLY CLOSED PATHLINES - THE VISCOELASTIC TAYLOR-COUETTE INSTABILITY, Journal of non-Newtonian fluid mechanics, 69(2-3), 1997, pp. 221-237
We describe a novel technique for applying digital particle image velo
cimetry (DPIV) in steady or slowly varying flows with nearly closed pa
thlines in which the flow of interest is small relative to the primary
flow and the primary flow is normal to the plane of illumination. Des
pite the fact that in-plane particle displacements are negligible duri
ng video-rate image capture, DPIV can be used by implementing extremel
y low levels of seeding and using long 'exposure' times. An 'exposure'
for subsequent DPIV processing corresponds to the image formed by tak
ing the minimum of each pixel position from each of the frames capture
d 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 ap
pears in any of the frames captured during the exposure time will appe
ar in the composite exposure. Preliminary results for a viscoelastic f
luid in Taylor-Couette flow (flow between concentric rotating cylinder
s) are shown. As long as the cylinders are adequately aligned, there i
s no difficulty in resolving the main features of the secondary flow.
Secondary flows four to six orders of magnitude weaker than the base f
low can be resolved by this method in the present experiment. This tec
hnique would be equally successful in other flows such as flow between
rotating parallel disks or between a cone and plate, both of which ar
e important for rheological characterization of fluids. Through the us
e of co-rotation of fixtures, this technique could also be extended to
higher temporal resolution and higher secondary velocities relative t
o the base flow. (C) 1997 Elsevier Science B.V.