Maxime Farin

Fundamental changes of granular flow dynamics, deposition, and erosion processes at high slope angles: Insights from laboratory experiments

Entrainment of underlying debris by geophysical flows can significantly increase the flow deposit extent. To study this phenomenon, analog laboratory experiments have been conducted on granular column collapse over an inclined channel with and without an erodible bed made of similar granular material. Results show that for slope angles below a critical value θc, between 10° and 16°, the run out distance rf depends only on the initial column height h0 and is unaffected by the presence of an erodible bed. On steeper slopes, the flow dynamics change fundamentally, with a slow propagation phase developing after flow front deceleration, significantly extending the flow duration. This phase has characteristics similar to those of steady uniform flows. Its duration increases with increasing slope angle, column volume, column inclination with respect to the slope and channel width, decreasing column aspect ratio (height over length), and in the presence of an erodible bed. It is independent, however, of the maximum front velocity. The increase in the duration of the slow propagation phase has a crucial effect on flow dynamics and deposition. Over a rigid bed, the development of this phase leads to run out distances rf that depend on both the initial column height h0 and length r0. Over an erodible bed, as the duration of the slow propagation phase increases, the duration of bed excavation increases, leading to a greater increase in the run out distance compared with that over a rigid bed (up to 50%). This effect is even more pronounced as bed compaction decreases.

Characterization of rockfalls from seismic signal: insights from laboratory experiments

The seismic signals generated by rockfalls can provide information on their dynamics and location. However, the lack of field observations makes it difficult to establish clear relationships between the characteristics of the signal and the source. In this study, scaling laws are derived from analytical impact models to relate the mass and the speed of an individual impactor to the radiated elastic energy and the frequency content of the emitted seismic signal. It appears that the radiated elastic energy and frequencies decrease when the impact is viscoelastic or elasto-plastic compared to the case of an elastic impact. The scaling laws are validated with laboratory experiments of impacts of beads and gravels on smooth thin plates and rough thick blocks. Regardless of the involved materials, the masses and speeds of the impactors are retrieved from seismic measurements within a factor of 3. A quantitative energy budget of the impacts is established. On smooth thin plates, the lost energy is either radiated in elastic waves or dissipated in viscoelasticity when the impactor is large or small with respect to the plate thickness, respectively. In contrast, on rough thick blocks, the elastic energy radiation represents less than 5% of the lost energy. Most of the energy is lost in plastic deformation or rotation modes of the bead owing to surface roughness. Finally, we estimate the elastic energy radiated during field scale rockfalls experiments. This energy is shown to be proportional to the boulder mass, in agreement with the theoretical scaling laws.

Continuum viscoplastic simulation of a granular column collapse on large slopes: μ(I) rheology and lateral wall effects

We simulate here dry granular flows resulting from the collapse of granular columns on an inclined channel (up to 22°) and compare precisely the results with laboratory experiments. Incompressibility is assumed despite the dilatancy observed in the experiments (up to 10%). The 2-D model is based on the so-called μ(I) rheology that induces a Drucker-Prager yield stress and a variable viscosity. A nonlinear Coulomb friction term, representing the friction on the lateral walls of the channel, is added to the model. We demonstrate that this term is crucial to accurately reproduce granular collapses on slopes ≳10°, whereas it remains of little effect on the horizontal slope. Quantitative comparison between the experimental and numerical changes with time of the thickness profiles and front velocity makes it possible to strongly constrain the rheology. In particular, we show that the use of a variable or a constant viscosity does not change significantly the results provided that these viscosities are of the same order. However, only a fine tuning of the constant viscosity (η=1 Pa s) makes it possible to predict the slow propagation phase observed experimentally at large slopes. Finally, we observed that small-scale instabilities develop when refining the mesh (also called ill-posed behavior, characterized in the work of Barker et al. [“Well-posed and ill-posed behaviour of the μ(I)-rheology for granular flow,” J. Fluid Mech. 779, 794–818 (2015)] and in the present work) associated with the mechanical model. The velocity field becomes stratified and the bands of high velocity gradient appear. These model instabilities are not avoided by using variable viscosity models such as the μ(I) rheology. However we show that the velocity range, the static-flowing transition, and the thickness profiles are almost not affected by them.

Remote focusing of elastic waves in complex structures using time reversal of acoustic waves

Remotely assessing the state of damage of thin plates or beams within a complex structure is a burning issue in many industrial problems. A time-reversal technique is used to focus a short and localized elastic impulsion on a thin plate using a network of loudspeakers. The generated plate vibration is measured using a laser vibrometer. We built an experimental setup to mimic a complex structure, with several thin plates attached close to each others inside a cylinder. We show that the acoustic method allows us to put a targeted plate in vibration inside the cylinder, without exciting the other plates in the same structure. The time-reversal technique takes advantage of the strong wave reverberation caused by the presence of the complex structure around the plate, which creates virtual sound sources, and improves the focusing on the plate compared to that obtained when the plate has nothing around it. The ratio of plate vibration energy over emitted acoustic energy is about 1%. Finally, we create a small defect on a plate and assess whether the defaults can be detected from changes in the plate vibration modes. Our experimental results are compared with finite elements simulations. Remotely assessing the state of damage of thin plates or beams within a complex structure is a burning issue in many industrial problems. A time-reversal technique is used to focus a short and localized elastic impulsion on a thin plate using a network of loudspeakers. The generated plate vibration is measured using a laser vibrometer. We built an experimental setup to mimic a complex structure, with several thin plates attached close to each others inside a cylinder. We show that the acoustic method allows us to put a targeted plate in vibration inside the cylinder, without exciting the other plates in the same structure. The time-reversal technique takes advantage of the strong wave reverberation caused by the presence of the complex structure around the plate, which creates virtual sound sources, and improves the focusing on the plate compared to that obtained when the plate has nothing around it. The ratio of plate vibration energy over emitted acoustic energy is about 1%. Finally, we create a small d...