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Topographic perturbation of turbulent boundary layers by low-angle, early-stage aeolian dunes
journal contributionposted on 20.01.2022, 10:22 by N.R. Bristow, J.L. Best, G.F.S. Wiggs, J. Nield, Matthew BaddockMatthew Baddock, P. Delorme, K.I. Christensen
Decimeter scale early stage aeolian bedforms represent topographic features that differ notably from their mature dune counterparts, with nascent forms exhibiting more gently sloping lee sides and a reverse asymmetry in their flow parallel bed profile compared to mature dunes. Flow associated with the development of these “protodunes”, wherein the crest gradually shifts downstream towards its mature state, was investigated by studying the perturbation of the turbulent boundary layer over a succession of representative bedforms. Rigid, three dimensional models were studied in a refractive index matched experimental flume that enabled near surface quantification of mean velocities and Reynolds stresses using particle image velocimetry in wall normal and wallparallel measurement planes. Data indicate strong, topographically induced, flow perturbations over the protodunes, to a similar relative degree as that found over mature dunes, despite their low angled slopes. The shape of the crest is found to be an important factor in the development of flow perturbations, and only in the case with the flattest crest was maximal speedup of flow, and reduction in turbulent stresses, found to occur upstream of the crest. Investigation of the log-linearity of 18 the boundary layer profile over the stoss sides showed that, although the profile is strongly perturbed, a loglinear region exists, but is shifted vertically. A streamwise trend in friction velocity is thus present, showing a behavior similar to the trends in mean velocity. Analysis of the growth of the internal boundary layer on the dune stoss sides, beginning at the toe region, reveals a similar development for all dune shapes, despite clear differences in mean velocity and turbulent stress perturbations in their toe regions. The data presented herein provide the first documentation of flow over morphologies broadly characteristic of subtle, 1 low-angle, aeolian protodunes, and indicate key areas where further study is required to yield a more complete quantitative understanding of flow–form–transport couplings that govern their morphodynamics.
National Science Foundation, USA (NE/R010196NSFGEONERC, NSFGEO1829541 and NSFGEO1829513)
NSFGEO-NERC: The Origin of Aeolian Dunes (TOAD)
Natural Environment Research CouncilFind out more...
- Social Sciences and Humanities
- Geography and Environment