Experimental Measurement of Enhanced and Hindered Particle Settling in Turbulent Gas‐Particle Suspensions, and Geophysical Implications

Abstract

The dynamics of geophysical dilute turbulent gas‐particles mixtures depends to a large extent on particle concentration, which in turn depends predominantly on the particle settling velocity. We experimentally investigate air‐particle mixtures contained in a vertical pipe in which the velocity of an ascending air flux matches the settling velocity of glass particles. To obtain local particle concentrations in these mixtures, we use acoustic probing and air pressure measurements and show that these independent techniques yield similar results for a range of particle sizes and particle concentrations. Moreover, we find that in suspensions of small particles (78 μm) the settling velocity increases with the local particle concentration due to the formation of particle clusters. These clusters settle with a velocity that is four times faster than the terminal settling velocity of single particles, and they double settling speeds of the suspensions. In contrast, in suspensions of larger particles (467 μm) the settling velocity decreases with increasing particle concentration. Although particle clusters are still present in this case, the settling velocity is decreased by 30%, which is captured by a hindered settling model. These results suggest an interplay between hindered settling and cluster‐induced enhanced settling, which in our experiments occur respectively at Stokes number O(100) and O(1). We discuss implications for volcanic plumes and pyroclastic currents. Our study suggests that clustering and related enhanced or hindered particle settling velocities should be considered in models of volcanic phenomena and that drag law corrections are needed for reliable predictions and hazard assessment.

Experimental Study of the Generation of Pore Gas Pressure in Pyroclastic Density Currents Resulting From Eruptive Fountain Collapse

“…Second, as discussed by Sweeney and Valentine (2017), the remaining fraction of fine particles can be expelled with the gas from the mixture emerging from the impact zone, hence creating a dilute turbulent suspension overriding the concentrated flow and possibly moving at a higher velocity. In this regard, particle clusters with high sedimentation rates in the dilute suspension can rapidly accumulate above the concentrated basal flow to generate pore pressure and hence extend the duration of pore pressure diffusion in the mixture (Breard et al., 2018; Penlou, Roche, Manga, et al., 2023).…”

“…The data and associated descriptions [Dataset] in this paper are available at (Penlou, Roche, van den, & Wildenberg, 2023).…”

Physical properties of pyroclastic density currents: relevance, challenges and future directions

“…However, laboratory apparatus size limitations (e.g., channel width) constrain the particle length scales that can be used. In the most simplified cases, analogue materials such as glass beads have been employed due to their ease of use and resistance to abrasion (Roche et al, 2013;Smith et al, 2020;Gueugneau et al, 2022;Penlou et al, 2023). When using natural samples at benchtop scales, the grain size distribution is subsampled to include only fine particles to prevent wall effects and maintain relevant scaling [e.g., pore pressure diffusion timescale, as demonstrated by Girolami et al (2008)].…”

“…Most PDCs, due to their high temperature, will be dominated by 2-way and 4-way coupling, as they become buoyant and transform into a co-PDC plume when concentrations drop below C<~10 −4 (e.g., Lube et al, 2020). Concentration and mass loading impacts the settling velocity of particles (Breard et al, 2016;Weit et al, 2018;Penlou et al, 2023), and in turn this impacts the density stratification and partitioning of mass between the basal granular layer (i.e., the bedload) and overriding turbulent suspension (Douillet et al, 2014(Douillet et al, , 2019Brosch and Lube, 2020). The concentration of particles in PDCs is expected to be heterogeneous temporally and spatially at any given location due to particle clustering.…”

Effect of fluidization in increasing the run-out distance of granular flows generated from different aspect ratio collapsing columns

“…Fluidization is the result of the differential motion between solid particles and interstitial gas, whose interplay is able to generate pore pressure, counterbalance partially or entirely the solid particle weight, and thus reduce particle friction [17,[19][20][21][22][23][24]. In the context of PDCs, fluidization is mainly generated at the impact zone of a collapsing fountain [25][26][27][28], while the temporal evolution of fluidization is the result of the coupled effect of diffusion, advection, dilatancy, compaction and air entrainment [29][30][31], which are strongly influenced by the grain-size distribution of the solid particles [18,32] and the underlying topography [11,33].…”