EP014-02
The Origin of Aeolian Dunes: the development of flow structure over low-angle protodunes
Tuesday, 8 December 2020: 17:34
Virtual
Nathaniel Bristow1, Jim Best2, Kenneth T Christensen3, Matthew C Baddock4, Pauline Delorme5, Joanna M Nield5 and Giles Wiggs6, (1)University of Notre Dame, Notre Dame, IN, United States, (2)Univ. Illinois at Urbana Champaign, Geology, Geography & GIS, Mechanical Science and Engineering and Ven Te Chow Hydrosystems Laboratory, Champaign, IL, United States, (3)University of Notre Dame, Aerospace and Mechanical Engineering, Notre Dame, IN, United States, (4)Loughborough University, Geography, Loughborough, United Kingdom, (5)University of Southampton, Southampton, SO14, United Kingdom, (6)University of Oxford, School of Geography and the Environment, Oxford, United Kingdom
Abstract:
Understanding the initiation and growth of aeolian dunes poses significant challenges due to the strong couplings between turbulent fluid flow, sediment transport, and bedform morphology. While much is known concerning the dynamics of more mature bedforms, open questions remain as to how protodunes are formed, as well as the mechanisms by which they continue to evolve. As protodunes begin to develop from this initial sand patch, their morphology has been proposed to evolve from a reverse asymmetry of the stoss and lee sides, where the crest begins upstream, close to the protodune toe, and gradually shifts downstream toward the "regular" asymmetric profile exhibited by more mature dunes. These early stages of development are characterized by very gentle slopes and low-amplitude profiles that make field measurements of the associated flow particularly challenging, if not impossible.
The work presented herein focuses on experiments conducted in a unique flow facility wherein high-resolution measurements of the turbulent flow field associated with the early stages of scaled model protodunes are obtained utilizing particle-image velocimetry (PIV) in a refractive-index-matched (RIM) environment. The RIM technique facilitates flow measurements extremely close to model surfaces as well as permitting unimpeded optical access which are critical to understanding the flow-form coupling. A series idealized, fixed-bed models have been fabricated to mimic the key morphological characteristics of early protodune development observed in the field, and the flow measurements associated with them are analyzed to reveal the mechanisms controlling the bedform dynamics. Despite the gentle topographies, all models show a perturbation of the mean velocities, Reynolds stresses, and swirling motions over both the stoss and lee sides. These trends are further analyzed in terms of mean velocity profile log-linearity, local surface curvature, and intermittent flow separation (or lack thereof).
