# Continuous to intermittent flows in growing granular heaps

## TL;DR

This study investigates the transition from smooth to intermittent flow in growing granular heaps, revealing how critical pile size depends on input flux and feeding height, supported by experiments and a phenomenological model.

## Contribution

It introduces a phenomenological model explaining the size-dependent flow transition in granular heaps based on experimental observations.

## Key findings

- Critical pile size $X_c$ scales linearly with input flux.
- $X_c$ scales with the square root of feeding height.
- The model predicts $X_c$ for varying feeding conditions.

## Abstract

If a granular material is poured from above on a horizontal surface between two parallel, vertical plates, a sand heap grows in time. For small piles, the grains flow smoothly downhill, but after a critical pile size $X_c$, the flow becomes intermittent: sudden avalanches slide downhill from the apex to the base, followed by an "uphill front" that slowly climbs up, until a new downhill avalanche interrupts the process. By means of experiments controlling the distance between the apex of the sandpile and the container feeding it from above, we show that $X_c$ grows linearly with the input flux, but scales as the square root of the feeding height. We explain these facts based on a phenomenological model based on the experimental observation that the flowing granular phase forms a "wedge" on top of the static one, differently from the case of stationary heaps. Moreover, we demonstrate that our controlled experiments allow to predict the value of $X_c$ for the common situation in which the feeding height decreases as the pile increases in size.