Sand transverse dune aerodynamics: 3D Coherent Flow Structures from a computational study

The engineering interest about dune fields is dictated by the their interaction with a number of human infrastructures in arid environments. Sand dunes dynamics is dictated by wind and its ability to induce sand erosion, transport and deposition. A deep understanding of dune aerodynamics serves then to ground effective strategies for the protection of human infrastructures from sand, the so-called sand mitigation. Because of their simple geometry and their frequent occurrence in desert area, transverse sand dunes are usually adopted in literature as a benchmark to investigate dune aerodynamics by means of both computational or experimental approaches, usually in nominally 2D setups. The present study aims at evaluating 3D flow features in the wake of a idealised transverse dune, if any, under different nominally 2D setup conditions by means of computational simulations and to compare the obtained results with experimental measurements available in literature

Numerical modeling of the wind flow over a transverse dune

Transverse dunes, which form under unidirectional winds and have fixed profile in the direction perpendicular to the wind, occur on all celestial objects of our solar system where dunes have been detected. Here we perform a numerical study of the average turbulent wind flow over a transverse dune by means of computational fluid dynamics simulations. We find that the length of the zone of recirculating flow at the dune lee --- the {\em{separation bubble}} --- displays a surprisingly strong dependence on the wind shear velocity, $u_{\ast}$: it is nearly independent of $u_{\ast}$ for shear velocities within the range between $0.2\,$ms and $0.8\,$ms but increases linearly with $u_{\ast}$ for larger shear velocities. Our calculations show that transport in the direction opposite to dune migration within the separation bubble can be sustained if $u_{\ast}$ is larger than approximately $0.39\,$ms, whereas a larger value of $u_{\ast}$ (about $0.49\,$ms) is required to initiate this reverse transport.Comment: 11 pages, 8 figure

Large eddy simulation of interacting barchan dunes in a steady, unidirectional flow

We have performed large-eddy simulations of turbulent flow 4 over barchan dunes in a channel with different interdune spacings in the downstream direction at Reynolds number, Re∞ ≃ 26000 (based on the free 6 stream velocity and channel height). Simulations are validated against ex-perimental data (at Re∞ = 55460); the largest interdune spacing (2.38λ, where λ is the length of the barchan model) presents similar characteristics to the isolated dune in the experiment, indicating that at this distance the sheltering effect of the upstream dune is rather weak. We examine 3D realizations of the mean and instantaneous flow to explain features of the flow field relevant to sediment transport. Barchan dunes induce two counter-rotating streamwise vortices, along each of the horns, which direct high-momentum fluid toward the symmetry plane and low-momentum fluid near the bed away from the centerline. The flow near the bed, upstream of the dune, diverges from the centerline plane, decelerates and then rises on the stoss side of the dune while accelerating; the flow close to the centerline plane separates at the crest and reattaches on the bed. Away from the centerline plane and along the horns, flow separation occurs intermittently. The flow in the separation bubble is routed towards the horns and leaves the dune at their tips. The separated flow at the crest reattaches on the bed, except on the centerline symmetry plane of the dune, where a weak saddle point of separation ap- pears at the bed. The distribution of the bed shear-stress, characteristics of the separation and reattachment regions, and instantaneous wall turbulence are discussed. Characteristics of the internal boundary layer developing on the bed after the reattachment region are studied. The interdune spacing isfound to affect significantly the turbulent flow over the stoss side of the downstream dunes; at smaller interdune-spacings, coherent high- and low- speed streaks are shorter but stronger, and the spanwise normal Reynolds stress is larger. The turbulent kinetic energy budgets show the importance of the pressure transport and mean-flow advection in transporting energy from the overlying wake layer to the internal boundary layer over the stoss side of the closely-spaced dunes. The characteristics of the separated-shear layer are altered slightly at smaller interdune spacing; the separation bubble is smaller, the separated-shear layer is stronger, and the bed shear-stress is larger. Away from the dunes, typical wall-turbulence structures are observed, but coher- ent eddies generated in the separated-shear layer due to the Kelvin-Helmholtz instability are dominant near the dune. Coherent structures are generated more frequently at smaller interdune spacing; they move farther away from the bed, towards the free surface, and remain in between the horns. At larger interdune spacings, these coherent structures are advected in the spanwise direction with the mean streamwise vortices and can be observed outside of the dunes

A fully Eulerian multiphase model of windblown sand coupled with morphodynamic evolution: Erosion, transport, deposition, and avalanching

Abstract Modeling unsteady windblown sand dynamics requires not only treatment of the sand present in the air as a suspended constituent of a mixture but also consideration of erosion and sedimentation phenomena and consequently of the morphodynamic evolution of the sand-bed surface, including avalanching, especially in the presence of natural or human-built obstacles, artifacts, and infrastructures. With this aim in mind, we present a comprehensive multiphase model capable of accurately simulating all the physical phenomena mentioned above, producing satisfactory results, with reasonable computational effort. As test cases, two- and three-dimensional simulations of dune evolution are reported, as is windblown sand transport over a straight vertical wall. Examples of sand transport around other obstacles are given to show the flexibility of the model and its usefulness for such engineering applications

Scale-dependent perspectives on the geomorphology and evolution of beachdune systems

Despite widespread recognition that landforms are complex Earth systems with process-response linkages that span temporal scales from seconds to millennia and spatial scales from sand grains to landscapes, research that integrates knowledge across these scales is fairly uncommon. As a result, understanding of geomorphic systems is often scale-constrained due to a host of methodological, logistical, and theoretical factors that limit the scope of how Earth scientists study landforms and broader landscapes. This paper reviews recent advances in understanding of the geomorphology of beach-dune systems derived from over a decade of collaborative research from Prince Edward Island (PEI), Canada. A comprehensive summary of key findings is provided from short-term experiments embedded within a decade-long monitoring program and a multi-decadal reconstruction of coastal landscape change. Specific attention is paid to the challenges of scale integration and the contextual limitations research at specific spatial and/or temporal scales imposes. A conceptual framework is presented that integrates across key scales of investigation in geomorphology and is grounded in classic ideas in Earth surface sciences on the effectiveness of formative events at different scales. The paper uses this framework to organize the review of this body of research in a 'scale aware' way and, thereby, identifies many new advances in knowledge on the form and function of subaerial beach-dune systems. Finally, the paper offers a synopsis of how greater understanding of the complexities at different scales can be used to inform the development of predictive models, especially those at a temporal scale of decades to centuries, which are most relevant to coastal management issues. Models at this (landform) scale require an understanding of controls that exist at both ‘landscape’ and ‘plot’ scales. Landscape scale controls such as sea level change, regional climate, and the underlying geologic framework essentially provide bounding conditions for independent variables such as winds, waves, water levels, and littoral sediment supply. Similarly, an holistic understanding of the range of processes, feedbacks, and linkages at the finer plot scale is required to inform and verify the assumptions that underly the physical modelling of beach-dune interaction at the landform scale