Experimental characterization of mechanical clogging of dry granular flows through sudden constrictions

A granular system is a collection of macroscopic particles that interacts through dissipative collisions and enduring contacts. It can exhibit gas, liquid or solid behaviour. These systems present phase transitions and coexistence of different phases. As for solid-liquid transitions, there is vast literature in thermal and athermal systems but no universal models of first-order or second-order phase transitions.

In particular, dry granular flows (the movement of granular material in fluids of low density and viscosity) can serve as models of debris flows. Mechanical clogging occurs when the mass of granular material is stopped in from of slits or orifices in check dams. There is currently not enough knowledge on the processes that lead to clogging.  

In this research we conducted a series of 31 laboratory experiments of dry granular flows constricted through a vertical gap, adjacent to the side wall, mimicking slit dam conditions. The granular material was composed of monosized polystyrene particles (Ø1.8 mm). The width of the slit was 2 particle diameters. The granular mass was released suddenly in a 1.5 m long chute, tilted at 20°. Instrumentation included two high-speed cameras (300 fps), located upstream, at the gate location, and downstream, at the slit location. Instantaneous velocities were obtained with PTV at the chute wall. In this work we discuss the behaviour or mean longitudinal velocities and of granular temperatures when the clogging occurs. The start of the clogging process was identified as the ts – solidification instant, this instant is defined by the moment the first particles stop moving.

It is shown that the statistical distribution of ts is probably not heavy-tailed. It has a positive asymmetry [0.410] and low flatness [-1.369]. Analysing 0.133 s before and after the solidification instant, it is shown that the mean velocity and the granular temperature of the granular system is constant up to 0.033 s before ts while the solid volume increases. It is not clear which portions of the system are in a gas phase and which are in a liquid phase.

The dissipative nature of the system becomes apparent from ts – 0.033 s. It is postulated that the rate of collisions has substantially increased with the increase of the solid fraction. It is expected that the rate of dissipation of fluctuating energy is a non-linear increasing function of the volume fraction. Hence, from ts – 0.033 onwards, a decrease in granular temperature (granular cooling) becomes evident. A reduction of the mean velocity becomes apparent at the same instant. The decrease of the fluctuating kinetic energy is continuous across the phase transition but appears stronger after ts.  

As a result of this work we will explore the hypothesis that the liquid-solid phase transition, observed in terms of mean velocities and granular temperatures is best modelled as smooth transition.

This work was funded by Portuguese Foundation for Science and Technology (FCT) through Project PTDC / ECI-EGS / 29835/2017 - POCI-01-0145-FEDER-029835, financed by FEDER funds through COMPETE2020, by National funds through FCT, IP. and partially funded by FEDER Project by the FCT Project RECI/ECM-HID/0371/2012.