Internal dynamics of Newtonian and viscoplastic fluid avalanches down a sloping bed

Abstract

We experimentally investigated the spreading of fluid avalanches (i.e., fixed volumes of fluid) down an inclined flume. Emphasis was given to the velocity field within the head. Using specific imaging techniques, we were able to measure velocity profiles within the flowing fluid far from the sidewalls. We studied the behavior of Newtonian and viscoplastic fluids for various flume inclinations and initial masses. For the Newtonian fluids tested (glycerol and Triton X100), we compared the measured velocity field with that predicted by lubrication theory. Provided that the flow Reynolds number Re was sufficiently low (typically Re < 1), there was excellent agreement between theory and experiment except for the very thin region just behind the contact line. For higher Reynolds numbers (typically Re ∼ 10), the discrepancy between theory and experiment was more marked (relative errors up to 17% for the body). As viscoplastic materials, we used Carbopol ultrez 10. For the body, agreement between theoretical and measured velocity profiles was fairly satisfactory whereas it was very poor for the tip region as the curvature of the free surface became more pronounced: the velocities were not only much lower than those predicted by lubrication theory, but there was also evidence of slipping in the flow part adjacent to the contact line.

Granular suspension avalanches. II. Plastic regime

“…For the sake of comparison, the data were nondimensionalized and plotted on a log-log diagram. At short times (t < 0.1), the front position closely followed the theoretical solutionx f = (9t/4) 1/3 representing the evolution of the front position for a homogeneous Newtonian fluid 13,15 (macro-viscous regime). At later times, there was a sudden transition to another regime, which was reflected by a kink in thê x f (t) curve.…”

Granular suspension avalanches. I. Macro-viscous behavior

“…Forφ ≤ 0.57, the suspension flowed like a viscous fluid. 27 In particular, the front position varied with time as t 1/3 , consistent with the theory of thin elongating Newtonian flows down a sloping bed 28 (see also Subsection 2 of the Appendix). Forφ ≥ 0.61, the flow rapidly came to rest.…”

“…In the absence of inertia and normal-stress effects, it is possible to determine the effect of the free-surface gradient on the shear stress profile using lubrication theory. 27 As we may meet the situation in which the normal stresses play a role, we will not go deeper into the analysis.…”

Visualization of the internal flow properties and the material exchange interface in an entraining viscous Newtonian gravity current

“…The details about the original flume can be found in [3,4]. The flume comprises a transparent, roughened PMMA base which allows the use of a laser sheet, projected from below to illuminate a slice of fluid in the vertical-streamwise plane containing a fluorescent dye such as rhodamine.…”