Wake Induced Long Range Repulsion of Aqueous Dunes

Abstract

Desert sand dunes rarely occur in isolation, but usually form vast dune fields. The large-scale dynamics of these fields is hitherto poorly understood, not least due to the lack of long-time observations. Theoretical models usually abstract dunes in a field as self-propelled autonomous agents, exchanging mass, either remotely or as a consequence of collisions. In contrast to the spirit of these models, here we present experimental evidence that aqueous dunes interact over large distances without the necessity of exchanging mass. Interactions are mediated by turbulent structures forming in the wake of a dune, and lead to dune-dune repulsion, which can prevent collisions. We conjecture that a similar mechanism may be present in wind-driven dunes, potentially explaining the observed robust stability of dune fields in different environments.

Spatial and Temporal Development of Incipient Dunes

“…Similar to Phillips et al (2019), we also recognize the coexistence of multiple wavelengths at the upwind side of the profiles (associated with different celerities and growth rates or lengths), and these are partly the cause of the distribution widths in Figure 3. We could not, however, infer from these data signs of interactions, such as collisions, coalescence or ejection (Bacik et al, 2020; Gao et al, 2015; Hersen & Douady, 2005; Katsuki et al, 2005).…”

Unsteady dynamics of turbulent flow in the wakes of barchan dunes modulated by overlying boundary-layer structure

“…Recent studies have increasingly shown that the morphodynamics of interactions between dunes in close proximity is linked to enhanced turbulence produced in the wake of an upstream bedform impinging on a downstream bedform (Fernandez et al 2006;Wang et al 2017;Bristow et al 2018;Wang & Anderson 2018Bristow et al 2019Bristow et al , 2020Assis & Franklin 2020;Bacik et al 2020;Khosronejad et al 2020). Bacik et al (2020) showed for transverse dunes that the turbulent wake of the upstream bedform influences the migration speed of the downstream bedform by enhancing downstream sediment flux. Assis & Franklin (2020) demonstrated the role that other factors play in such interactions, including bedform alignment and sediment size and shape, while also emphasizing the importance of wake turbulence in the short-range interaction dynamics via a flow strength parameter.…”

Macroscopic Flow Disequilibrium Over Aeolian Dune Fields

“…For a single dune, this coupling is clear: flow accelerates on the stoss which erodes sand, and sand deposits on the lee where flow separates (Pye & Tsoar, 2008). For a pair of dunes, the flow wake generated by an upwind dune influences the flow over a dune downwind, creating short‐range interaction (Bacik et al, 2020; Bristow et al, 2019). For many dunes, the sum of their wakes imparts a collective drag (Stevens et al, 2015), creating a roughness sublayer in the ABL a few dune‐heights high (Ghisalberti, 2009).…”