Sarno, Luca ; Papa, Maria Nicolina ; Tai, Yih-Chin ; Wang, Yongqi ; Villani, Paolo (2024)
Optical measurements of solid volume fraction and granular temperatures in granular flows over erodible bed.
In: Journal of Physics: Conference Series, 2802 (1)
doi: 10.1088/1742-6596/2802/1/012005
Artikel, Bibliographie

Kurzbeschreibung (Abstract)

Granular media are ubiquitous in geophysics and industrial applications and, hence, have attracted increasing interest in the scientific community. Yet, their dynamics is far from being completely understood and exhibits a rich variety of flow regimes, ranging from solid-like to gas-like behaviours. In the process of understanding these complex systems, experimental investigations at the laboratory scale and non-invasive measuring techniques play a crucial role. In this work, we report the velocity and solid volume fraction measurements of steady dry granular flows, carried out in a narrow flume and over an erodible bed. The sidewall profiles of flow velocity and granular temperature (i.e., a measure of the fluctuation kinetic energy of the particles) are obtained by a multi-pass window deformation particle image velocimetry (PIV) approach. Conversely, the near-wall solid volume fraction profiles are achieved by employing the stochastic optical method (SOM), proposed in [Sarno et al. Granular Matter (2016) 18: 80]. This non-invasive technique is designed to indirectly estimate the near-wall volume fraction by regarding it as a stochastic process depending on a well-determined physical quantity, called the two-dimensional volume fraction. Such a quantity is made optically measurable thanks to controlled illumination conditions, where the incidence angle of a flicker-free light source is carefully adjusted. The measurements show that granular flows over an erodible bed are generally characterized by a stratified structure with weak mass exchange from one layer to the other, which is typical of dense and quasi-static regimes. Conversely, a more collisional upper region, characterized by higher velocity fluctuations and lower volume fraction values, is systematically found in an upper region with thickness of few grain diameters below the free surface, which confirms the crucial role played by the confining pressures. Moreover, the experimental findings corroborate the existence of long-range momentum exchange mechanisms along the flow depth.

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