Simultaneous photobleaching molecular tagging velocimetry for density-stratified flows of suspensions

Abstract

To gain insight into the roll wave instability in particulate flow systems, the spatiotemporal velocities of particle and liquid phases in a clear-fluid layer are simultaneously measured by means of a photobleaching molecular tagging velocimetry (PB-MTV). A grid pattern tag is generated by intermittent irradiation of two intense laser beams, which facilitates measurement of a wall vicinity flow induced by the Boycott effect. To cope with a plurality of tagline intersections, which largely deform in shear flows, a novel spatial correlation technique is developed with a template-matching method. A priori study using artificial images of tags in a temporally oscillating flow is performed to examine the effects of the signal to noise (S/N) ratio and the degree of unsteadiness on the accuracy of the velocimetry. An experimental validation of the present PB-MTV is also conducted on the statistical values of the streamwise velocity through comparison with a laser-induced fluorescence and particle tracking velocimetry (LIF-PTV). A series of experiments are carried out using a suspension, which consists of tap water and spherical hollow light particles, varying the particle diameter and the particle volume fraction. The liquid velocity fluctuation tends to increase with decrease in the inter-particle distance compared to the thickness of the clear-fluid layer. We conclude that the roll wave instability occurs only under the fluid-like condition, where the suspension behaves as a continuum, and is affected by the sharpness of flow stratification between the clear-fluid and particle-rich phases.

Graphic abstract