Numerical Simulations of Sand Dunes

CFD-DEM simulations of sand dunes revealing grain-scale dynamics inaccessible to experiments

Laboratory experiments provide direct observations of dune dynamics, but many quantities remain inaccessible — forces acting on individual grains, instantaneous flow fields, and grain-scale interactions between colliding dunes, for example. Numerical simulations bridge this gap.

In this project, sand dunes were simulated by coupling two complementary methods: Large-Eddy Simulations (LES) for the continuous fluid phase and the Discrete Element Method (DEM) for the granular phase. Each grain is tracked individually, allowing access to grain-scale quantities that experiments cannot directly measure. Simulations involved up to 100,000 grains, capturing the full complexity of dune morphodynamics from formation to interaction.

Formation of a Subaqueous Barchan Dune

A turbulent water flow acts on an initially conical pile of grains, which progressively deforms into a barchan dune. The flow direction is from top to bottom.

CFD-DEM simulation of a turbulent water flow acting on a pile of grains deforming into a barchan dune. Flow direction: top to bottom. Video is sped up 1.1x.
  • Alvarez, C.A., & Franklin, E.M. (2020). Shape evolution of numerically obtained subaqueous barchan dunes. Physical Review E, 101, 012905. DOI

Grain-Scale Force Fields

One of the key advantages of numerical simulations is direct access to forces acting on each grain. This simulation shows the instantaneous streamwise force field — the force component aligned with the flow direction (top to bottom) — over an evolving barchan dune.

Instantaneous streamwise force fields over a barchan dune. Each grain is colored by the magnitude of the force aligned with the flow direction (top to bottom).
  • Alvarez, C.A., & Franklin, E.M. (2021). Force distribution within a barchan dune. Physics of Fluids, 33, 013313. DOI

Interacting Barchan Dunes

When two barchan dunes travel in the same direction, they interact in a complex dance of grain exchange. This simulation captures two dunes chasing each other, revealing the back-and-forth transmission of grains between the bedforms at the grain scale.

CFD-DEM simulation of two barchan dunes chasing each other. Grain transmission back and forth between the dunes is visible at the grain scale. Credit: Nicolao C. Lima.
  • Lima, N.C., Assis, W.R., Alvarez, C.A., & Franklin, E.M. (2024). Barchan-barchan dune repulsion investigated at the grain scale. Journal of Geophysical Research: Earth Surface, 129, e2024JF007741. DOI