Collaborative Research: Coordinated Experiments and Simulations of Near-Surface Turbulent Flow over Barchan Dunes: Informing Models of Dune Migration and Interaction

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PI: Christensen, Kenneth / Best, James / Anderson, WilliamProposal Number: 1603211 / 1604155 / 1603254The focus of the proposed research is to explore the migration of sand dunes in large scale, desert environments or ocean floor environments. The proposed collaborative approach involves experiments, theory and simulations to understand the process of dune migration under turbulent flow. The findings of this work can be important for Aeolian dune migration, a problem that can be critical to predicting and mitigating desertification as it occurs in the Western US and in other places in the world.Understanding and predicting the morphodynamics of subaqueous barchan dunes is critical for management of waterways and quantifying transport of nutrients and pollutants. Similarly, understanding these processes for aeolian barchan dunes is important for characterizing desertification processes and for informing numerical weather prediction. At large dune spacings, barchan kinematics can be accurately predicted using sediment transport models predicated on mean bed shear stress. However, barchans typically occur in fields. Since dune migration rate is size dependent, a heterogeneous dune field will result in variable bedform spacing, with smaller (faster) dunes approaching larger (slower) dunes. Thus, when two dunes are in close proximity, the upstream one will produce an unsteady, turbulent wake that will sculpt the morphology of the downstream one, introducing significant morphological complexity owing to spatially-heterogeneous turbulence. Such effects are not captured by current state-of-the-art models. (These approaches simulate the bedform evolution but do not resolve the overlying flow; they instead use a spatially homogeneous representation of turbulence as the flow input). Informed by preliminary research, and compelled by deficiencies in existing dune modeling approaches, it is hypothesized that the "missing link" to elucidating the morphodynamics of interacting barchan dunes is the unsteady nature of the turbulence generated within the inter-dune space. It is proposed to pursue a collaborative research effort that leverages an innovative measurement protocol and scientific computing, complemented by observations in nature. The results will transform the understanding of dune-dune flow field interactions for fixed-bed dunes as a function of proximity and volumetric ratio and will form the basis for advancing morphodynamic models. Planar and volumetric PIV measurements will be conducted in a refractive-index-matched environment (allowing unprecedented optical access to the flow) for targeted dune configurations. These experiments will validate large-eddy simulations that will enable larger spatial volumes and a much broader suite of dune field parameters to be explored, with the latter informed by field observations. The culminating task will involve incorporating the turbulence heterogeneity identified into existing morphodynamic models to test the hypothesis. The proposed work will have an impact not only in fluid dynamics through its impact on engineering turbulent flows, but also to other disciplines including boundary-layer meteorology, geomorphology and sedimentology.