Plan de Acción Cuatrienal “Corpoguavio Vive su Naturaleza” 2016-

Presented in this chapter are the results for a dry material under constant rotational velocities. Results are given in a qualitative fashion, then the chapter moves on to quantitative results - measurements of the thicknesses of the layers involved in a flow- ing granular material in a rotating drum, followed by velocities and measurements of avalanching behaviour and dynamic angle of friction. Preliminary discussion of these results are given at the end of the chapter. A more general synthesis of the results as a whole is given in the Discussion chapter (this starts on page 170). The equipment used in this study is shown in Figure 3.2.1.

This study also has a number of other findings, which will be presented across the subsequent Results chapters. These include quantitative and qualitative descriptions of the internal structure of a flowing material in a drum of a higher detail than previ- ously extant in the literature, discovery of potentially previously unobserved phenom- ena, quantitative comparisons of wet/dry flows, and of static velocity/dynamic velocity flows.

This chapter focuses, as previously mentioned, on the results for the constant velocity experiments, in which the material was the sieved volcanic ash. Here the ash is dry,

i.e., the interstitial fluid is air. In later chapters, water replaces the air, the velocities are varied with time, and the solid material is switched out. The sieved ash is used at all stages, facilitating cross-experiment comparison of results.

At present, the behaviour of a granular material in a rotating drum is divided into the various accepted regimes [Henein et al., 1983a; Mellmann, 2001]. These are defined by broad ranges of Froude number, crossing several orders of magnitude, and by qualita- tive descriptions of the visual appearance of the flowing material.

Is it possible to more accurately define the transition between regimes for a material? This chapter presents results for a relatively simple granular system - a dry material

5.1. INTRODUCTION 91 (i.e, the interstitial fluid is air) with a narrow but non-monodisperse size distribution at constant rotational velocities. Bulk material properties (dynamic angle of friction, layer dimensions,etc.) are measured and plotted against rotational velocity to investi- gate the relationship between end-member flow regimes and quantitative descriptions of material behaviour.

Knowledge of layer thicknesses and transitions between different flow regimes will help us to better understand erosion mechanics in a granular system, as a growing active layer (andvice versa, a shrinking passive layer) can be considered analogous to erosion in a natural flow. This chapter represents the first step in that growing under- standing.

5.1.1 Current Knowledge of the Internal Structure of a Granular Flow

in a Rotating Drum

Previous workers in this field have generally divided a granular material in a rotating drum into two main regions; first, a passive layer (adjacent to the drum wall), and sec- ondly, an active layer flowing over the passive (Fig. 2.6.1). The two layers are separated by a line known as the yield line [Dubé et al., 2013].

This study has found that the internal structure can be more complex than this. Un- der the right circumstances, a granular flow in a rotating drum can consist of (from top to bottom): an avalanche; an active region; an upper and lower sheared layer; and a passive region. The active region is further sub-divided into the zone in which the velocity is relatively consistent across the layer, and a lower region in which the veloc- ity reduces rapidly, as identified by Chou and Hsiau [2012] and Yamane et al. [1998]. The avalanches themselves are often fronted by a "splash" at the moment when the avalanche meets the downhill side of the drum wall. A splash is defined particularly diffuse region of energetically ejected granular material, named for its visual similarity to the spray from an ocean wave.

Note that while Komatsu et al. [2001] demonstrate that the passive layer, usually con- sidered to be static relative to the active region, does flow at extremely low velocities, it is assumed for the purposes of this work that the static approximation holds [Orpe and Khakhar, 2001]. This will simplify considerations of interactions between the active region and the passively rotating layer, as well as being a closer analogue for natural flows with a static bed layer. In addition, this study assumes that the material bulk density doesn’t change considerably when flowing. This approximation is valid at the rotational velocities employed in this study [Yamane et al., 1998].

A visual summary of the observations of the ash is presented in Figure 5.1.1. The full notes are given in Appendix A.

Figure 5.1.1:A summary of the observations by velocity for the ash. Coloured horizontal lines indicate at which experimentally investigated velocities various phenomena are observed in a specific material, on which more details are given in the main text. The phenomena observed are as follows: low-density regions, or LDRs, (pink) are the zones of reduced density seen as a material in the cascading regime nears the cataracting; inflection points (purple) are sharp bends in the free surface; layers (dark blue) refers to the point at which multiple layers beyond two are

observed; splashes (light blue) are the ejection of particulates, either individually or as groups, due to an avalanche reaching the drum wall at the downhill end; collapses (green) are the events which give rise to an avalanche.

5.2 Flow Description as a Function of Drum Rotational