Particle-resolved simulations and dune formation in particulate Rayleigh-Bénard flow*

Fully resolved simulations of particulate Rayleigh-Bénard convection in a doubly periodic cell (aspect ratio Γ=2) with 20,000 particles at a volume fraction of 3.25% and a Rayleigh number of Ra = 10⁸ have been conducted using 2.1 billion grid points and 600 NVIDIA V100 GPUs on DOE's Summit system. The particles are 10% heavier than the fluid and tend to settle. However, near the cell bottom, the velocity of the circulating fluid acquires a horizontal component which entrains them toward the root of rising plumes. The particles gradually accumulate at these sites forming dunes which strengthen the plumes. When the dunes are sufficiently large, other particles are dragged up their slope by the flow, acquire a vertical velocity component and are entrained by the rising fluid. Thus, particle resuspension ultimately depends on drag, rather than lift, forces. The simulations demonstrate a tendency for our system to form a single large, stable dune that perpetuates the process. Most particles in the large dune remain within it, stabilizing its structure and promoting the resuspension of other particles. Other transient smaller dunes also form, but are quickly eroded by the flow.