Erodible, granular beds are fragile

Area of Science:

• Geophysics
• Granular physics
• Fluid dynamics

Background:

• Geophysical flows often involve granular transport over erodible beds made of the same particles.
• Understanding these erodible beds is crucial for setting boundary conditions in continuum models.
• Rigorous definitions and constitutive relations for erodible beds are currently lacking.

Purpose of the Study:

• To investigate the transition to an erodible bed state in granular flows.
• To define the characteristics of erodible beds using discrete-element simulations.
• To compare the mechanical properties of erodible beds to shear jamming phenomena.

Main Methods:

• Utilized discrete-element simulations (DES) to model granular flow over an erodible substrate.
• Analyzed the formation and breaking of contact chains within the granular bed.
• Investigated the role of solid volume fraction and collisional flow on bed stability.

Main Results:

• Identified a critical solid volume fraction marking the transition to an erodible bed state.
• Observed intense, intermittent formation and breaking of contact chains at this transition.
• Found that the bulk stiffness of the erodible bed is three orders of magnitude lower than in shear jamming due to insufficient contact network stability.

Conclusions:

• Discrete-element simulations provide a framework for defining erodible beds in geophysical flows.
• The transition to an erodible bed is characterized by dynamic contact networks.
• Erodible beds exhibit significantly lower bulk stiffness compared to shear-jammed states, impacting flow dynamics and continuum model boundary conditions.