Abstract
Scale effects are differences in physical behavior that manifest between a large event and a geometrically scaled laboratory model and may cause misleading predictions. This study focuses on scale effects in granular slides, important in the environment and to industry. A versatile 6 m long laboratory setup has been built following Froude similarity to investigate dry granular slides at scales varied by a factor of 4, with grain Reynolds numbers Re in the range of 10 to 10 . To provide further comparison, discrete element method simulations have also been conducted. Significant scale effects were identified; the nondimensional surface velocity increased by up to 35%, while the deposit runout distance increased by up to 26% from the smallest to the largest model. These scale effects are strongly correlated with Re, suggesting that interactions between grains and air are primarily responsible for the observed scale effects. This is supported by the discrete element method data, which did not show these scale effects in the absence of air. Furthermore, the particle drag force accounted for a significant part of the observed scale effects. Cauchy number scale effects caused by unscaled particle stiffness resulting in varying dust formation with scale are found to be of secondary importance. Comparisons of the laboratory data to that of other studies and of natural events show that data normalization with Re is an effective method of quantitatively comparing laboratory results to natural events. This upscaling technique can improve hazard assessment in nature and is potentially useful for modeling industrial flows.
Highlights
Granular slides and flows are omnipresent, occurring in natural conditions such as rockfalls, landslides, pyroclastic flows, and avalanches (Pudasaini & Hutter, 2010), and in industrial contexts including heap development (Bryant et al, 2014; Markauskas & Kacianauskas, 2011; Zhang & Vu-Quoc, 2000), chutes, hoppers, rotating drums, and blenders (Turnbull, 2011; Zhu et al, 2008)
Cauchy number scale effects caused by unscaled particle stiffness resulting in varying dust formation with scale are found to be of secondary importance
This study focuses on dry granular slides, where the interstitial fluid is air
Summary
Granular slides and flows are omnipresent, occurring in natural conditions such as rockfalls, landslides, pyroclastic flows, and avalanches (Pudasaini & Hutter, 2010), and in industrial contexts including heap development (Bryant et al, 2014; Markauskas & Kacianauskas, 2011; Zhang & Vu-Quoc, 2000), chutes, hoppers, rotating drums, and blenders (Turnbull, 2011; Zhu et al, 2008). While the DEM typically uses spherical particles for improved performance, rolling-resistance modeling can be applied to better represent the energy of systems of rough particles (Ai et al, 2011; Wensrich & Katterfeld, 2012), while multisphere clumps can be used to capture shape effects more precisely if required (Kruggel-Emden et al, 2008) This allows the DEM to better represent phenomena linked to particle characteristics including size, shape, and relative displacement of particles, such as size segregation, slide dilation and contraction, contact force transference, and jamming events. For the deposit runout distance, the normalization included the data set from Davies et al (1999) and natural events, to evaluate whether the relative importance of Re changes as the scale range increases further This results in new scaling laws allowing laboratory results, under idealized conditions, to be quantitatively compared to natural slides by excluding Re scale effects, with equation (10) facilitating direct upscaling.
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