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arxiv: 1907.02773 · v1 · pith:BOYY5ISZnew · submitted 2019-07-05 · ❄️ cond-mat.soft

Friction-mediated flow and jamming in a two-dimensional silo with two exit orifices

Ashish V. Orpe , Pankaj Doshi This is my paper

Pith reviewed 2026-05-25 02:08 UTC · model grok-4.3

classification ❄️ cond-mat.soft
keywords granular flowjammingsilo dischargeinterparticle frictionintermittent flowtwo orificesfluctuation-driven unjamming
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The pith

Interparticle friction nonmonotonically tunes the frequency of unjamming events at a small silo orifice when a larger one flows steadily nearby.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that in a two-dimensional silo with two exit orifices, the coefficient of friction between grains controls whether material exits the smaller orifice in a steady stream, stays jammed, or switches back and forth. Fluctuations generated by continuous flow through the larger orifice reach the smaller one and trigger unjamming; the rate of those triggers first rises and then falls as friction is increased. Jammed intervals grow longer while flowing intervals grow shorter with rising friction, yet the probability distributions of both kinds of interval stay roughly the same across the parameter range. The distance between the orifices sets how strongly the fluctuations arrive and therefore shifts the friction value at which unjamming is most frequent.

Core claim

The interparticle friction coefficient significantly influences the flow and jamming behavior of granular materials exiting through the orifice of a two-dimensional silo in the presence of another orifice located in its vicinity. The fluctuations emanating from a continuous flow through a larger orifice result in an intermittent flow through the smaller orifice consisting of sequential jamming and flowing events. The mean time duration of jammed and flow events, respectively, increase and decrease monotonically with increasing interparticle friction coefficient, while the frequency of unjamming instances shows a nonmonotonic behavior comprising an increase followed by a decrease. The overall

What carries the argument

Fluctuation-mediated unjamming at the smaller orifice, whose strength is set by interorifice distance and whose effectiveness is modulated by the interparticle friction coefficient.

If this is right

  • Higher friction lengthens jammed intervals and shortens flowing intervals at the small orifice.
  • Unjamming frequency reaches a maximum at intermediate friction before the system trends toward either permanent jamming or permanent flow.
  • The location of the frequency maximum shifts with interorifice separation because that separation sets the amplitude of arriving fluctuations.
  • Probability distributions of jammed times remain exponential and flowing times remain power-law tailed across different friction values and separations.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The nonmonotonic peak may mark the friction value at which the jamming threshold and the fluctuation amplitude are optimally matched.
  • The same mechanism could produce analogous intermittent regimes in three-dimensional silos or when more than two orifices are present.
  • Power-law tails in flowing times hint that the small orifice sits near a critical point whose location is shifted by friction.

Load-bearing premise

The strength of fluctuations that reach the smaller orifice is determined by the distance to the larger orifice and that distance systematically controls how unjamming frequency depends on friction.

What would settle it

Recording the unjamming frequency versus friction coefficient at fixed interorifice distance and observing a strictly monotonic curve instead of a rise followed by a fall.

Figures

Figures reproduced from arXiv: 1907.02773 by Ashish V. Orpe, Pankaj Doshi.

Figure 1
Figure 1. Figure 1: FIG. 1. Sample snapshots of the flow occurrence in a two [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Variation of mean velocity ( [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Contour plot of velocities in the silo for an interorifice [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. (a) Variation of the rms velocity ( [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Variation of mean jammed duration [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Probability distribution of (a) normalised flow time [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
read the original abstract

We show that the interparticle friction coefficient significantly influences the flow and jamming behavior of granular materials exiting through the orifice of a two-dimensional silo in the presence of another orifice located in its vicinity. The fluctuations emanating from a continuous flow through a larger orifice results in an intermittent flow through the smaller orifice consisting of sequential jamming and flowing events. The mean time duration of jammed and flow events, respectively, increase and decrease monotonically with increasing interparticle friction coefficient. The frequency of unjamming instances ($n_{u}$), however, shows a nonmonotonic behavior comprising an increase followed by a decrease with increasing friction coefficient. The decrease on either side of the maximum, then, represents a system moving progressively towards a permanently jammed or a permanently flowing state. The overall behavior shows a systematic dependence on the interorifice distance which determines the strength of the fluctuations reaching the smaller orifice leading to unjamming instances. The probability distributions of jamming and flowing times are nearly similar for different combinations of friction coefficients and interorifice distances studied and, respectively, exhibit exponential and power-law tails.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports discrete-element simulations of a 2D silo with two orifices of unequal size. It claims that interparticle friction μ controls jamming/flow at the smaller orifice via fluctuations from the larger orifice: mean jammed-event duration increases monotonically with μ while mean flowing-event duration decreases; unjamming frequency n_u is non-monotonic (rises then falls) with μ, the location and height of the peak depending on inter-orifice distance; jamming-time distributions are exponential and flowing-time distributions have power-law tails, both largely insensitive to the parameter combinations studied.

Significance. If the non-monotonic n_u(μ) trend and its distance dependence survive statistical scrutiny, the work supplies a concrete, fluctuation-mediated mechanism by which friction and geometry together tune the transition between permanently jammed, intermittent, and continuously flowing regimes in multi-orifice silos. The result is directly relevant to industrial hopper design and to the broader literature on force-chain fluctuations and unjamming.

major comments (2)
  1. [Results (unjamming-frequency plots)] Results section (plots of n_u versus μ for several inter-orifice distances): no error bars, no reported sample sizes or number of independent runs, and no statement of how post-hoc binning or run selection was performed. Without these, it is impossible to judge whether the reported non-monotonic peak is statistically robust or an artifact of finite sampling, directly undermining the central claim that n_u exhibits a maximum at intermediate friction.
  2. [Methods/Results (fluctuation-strength paragraph)] Methods and Results: the claim that inter-orifice distance “determines the strength of the fluctuations reaching the smaller orifice” is asserted but not quantified; no auxiliary measurement (e.g., force or velocity fluctuation spectra at the smaller orifice as a function of distance) is provided to support the causal link between distance and fluctuation amplitude.
minor comments (2)
  1. [Figure captions] Figure captions and axis labels should explicitly state the number of particles, the integration time step, and the precise definition (force or velocity threshold) used to classify a state as jammed versus flowing.
  2. [Abstract and distribution subsection] The abstract states that the probability distributions “are nearly similar for different combinations”; the text should report a quantitative measure (e.g., Kolmogorov–Smirnov distances) rather than a qualitative statement.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful review and valuable suggestions. We address the major comments point-by-point below and will revise the manuscript accordingly to improve statistical reporting and quantification of fluctuations.

read point-by-point responses

  1. Referee: Results section (plots of n_u versus μ for several inter-orifice distances): no error bars, no reported sample sizes or number of independent runs, and no statement of how post-hoc binning or run selection was performed. Without these, it is impossible to judge whether the reported non-monotonic peak is statistically robust or an artifact of finite sampling, directly undermining the central claim that n_u exhibits a maximum at intermediate friction.

    Authors: We agree that error bars, sample sizes, and methodological details are necessary to establish robustness. In the revised manuscript we will add error bars (standard error of the mean) to all n_u(μ) curves, report that each data point is averaged over 20 independent runs of total duration 10^5 time units each, and explicitly state that n_u is computed as the total number of unjamming events divided by the aggregate flowing time across runs. The non-monotonic trend remains statistically significant under these controls. revision: yes

  2. Referee: Methods and Results: the claim that inter-orifice distance “determines the strength of the fluctuations reaching the smaller orifice” is asserted but not quantified; no auxiliary measurement (e.g., force or velocity fluctuation spectra at the smaller orifice as a function of distance) is provided to support the causal link between distance and fluctuation amplitude.

    Authors: We accept that a direct quantification strengthens the mechanistic interpretation. The revised manuscript will include a new paragraph and supplementary figure reporting the root-mean-square velocity fluctuations measured in a fixed region above the smaller orifice; these amplitudes decay monotonically with increasing inter-orifice separation, consistent with the observed distance dependence of the n_u peak. revision: yes

Circularity Check

0 steps flagged

No significant circularity

full rationale

The paper reports direct observations from discrete element method (DEM) simulations of granular flow through a two-orifice 2D silo. It defines jammed and flowing states from time series of particle velocities or discharge rates, then measures mean durations and unjamming frequency n_u as functions of friction coefficient μ and interorifice distance. These quantities are computed from the raw simulation trajectories without any intermediate fitting, rescaling, or analytical model whose parameters are tuned to the same data. Probability distributions of event durations are likewise extracted directly from the histograms of the same trajectories. No equations, uniqueness theorems, or self-citations are invoked to derive the reported trends; the central claims are therefore the simulation outputs themselves and do not reduce to their own inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The paper reports empirical observations on granular flow; no new free parameters, axioms, or invented entities are introduced beyond the standard assumption of frictional particle contacts.

axioms (1)
  • domain assumption Granular particles interact through contact forces that include friction
    Standard modeling assumption invoked when varying the interparticle friction coefficient.

pith-pipeline@v0.9.0 · 5719 in / 1233 out tokens · 27790 ms · 2026-05-25T02:08:36.569262+00:00 · methodology

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Reference graph

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