Investigation of the rheological material properties of viscous debris flows

In various mountainous zones debris flows endanger villages and infrastructure. To define the risk areas and to investigate the efficiency of prevention-type measures numerical modeling is becoming a more and more important tool. Depending on the type of debris flow and the debris flow process a numerical tool with an appropriate physical model must be chosen. Numerical tools for the simulation of flow and deposition process of viscous debris flow often rely on a rheological model. Thus knowledge of the rheological behaviour of the whole debris flow mixture (fine to very large particles) is a prior task. However due to geometric constraints of conventional rheometric apparatus only the fine material part of the whole debris flow mixture can be analyzed so far.

In collaboration with the Institute of food science at ETH Zurich a research project at VAW focuses on a novel rheometric system that allows to analyze the rheological behaviour of mixtures containing fine and medium particles up to 5-10 mm grain size. One of the potentials of the novel system is its extension to a large scale device where an important part of the whole debris flow mixture could be analyzed and, accordingly, the parameters of the whole flow extrapolated with more accuracy.

The novel system is called “ball measuring system” and consists of a cylindrical container where the sample fluid is placed in. An eccentrically rotating sphere with a given diameter is dragged through the sample fluid. To rotate a given sphere at a specified rotational speed the torque has to be applied (fig. 1).

By its nature, the crucial task with the novel system is to derive a relation between the sphere drag flow and the original rheometric flow, which is the conversion of the measured speed and torque into the rheological parameters shear rate and shear stress. The derivation of the shear rate is found by the power characteristics of the system and applying the Metzner-Otto-theory. The relation between shear stress and torque is dependent on the sphere Reynolds number Re and the type of material.

Comparisons of rheological data of particle material with existing rheometry show that the ball measuring system is a useful new tool for the analysis of the rheological properties of large particle material (fig. 2).