EGU2020-4259
https://doi.org/10.5194/egusphere-egu2020-4259
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

The Origin of Aeolian Dunes – PIV measurements of flow structure over early stage protodunes in a refractive-index-matching flume

Nathaniel Bristow1, James Best2, Kenneth Christensen1, Matthew Baddock3, Giles Wiggs4, Pauline Delorme5, and Joanna Nield
Nathaniel Bristow et al.
  • 1University of Notre Dame, Department of Aerospace and Mechanical Engineering, Notre Dame, Indiana, United States of America (nbristow@nd.edu)
  • 2University of Illinois at Urbana-Champaign, Departments of Geology, Geography and GIS, Mechanical Science and Engineering, Champaign, Illinois, United States of America
  • 3Loughborough University, Geography and Environment, Loughborough, United Kingdom
  • 4University of Oxford, School of Geography and the Environment, Oxford, United Kingdom
  • 5University of Southampton, School of Geography and Environmental Science, Southampton, United Kingdom

Understanding the initiation 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. The structure of the turbulent flow field drives the mobilization or deposition of sediment, thus controlling the initial formation of sand patches, yet is also strongly influenced itself by local conditions such as surface roughness and moisture. Furthermore, an additional feedback on the flow and transport is exerted by the sand patches themselves once they begin to form.

As protodunes begin to develop from this initial deposition, their morphologies possess unique characteristics involving a reverse asymmetry of the stoss and lee sides, wherein the crest begins upstream, close to the toe, and gradually shifts downstream toward the "regular" asymmetric profile exhibited by more mature dunes. However, these early stages of development also involve very gentle slopes and low profiles which make field measurements of the associated flow particularly challenging.

The current research effort involves a combination of field measurements, documenting the initiation and morphological development of sand patches and protodunes, in concert with detailed measurements of the flow-form interactions in a laboratory flume. The work presented herein focuses primarily on experiments conducted in a unique flow facility wherein high-resolution measurements of the turbulent flow field associated with the early stages of protodune development 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 unimpeded optical access which are critical to understanding the flow-form coupling. A series idealized, fixed-bed models are 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.

How to cite: Bristow, N., Best, J., Christensen, K., Baddock, M., Wiggs, G., Delorme, P., and Nield, J.: The Origin of Aeolian Dunes – PIV measurements of flow structure over early stage protodunes in a refractive-index-matching flume, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4259, https://doi.org/10.5194/egusphere-egu2020-4259, 2020

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Presentation version 3 – uploaded on 04 May 2020
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  • CC1: Comment on EGU2020-4259, Nathalie Vriend, 04 May 2020

    Hi Nathaniel,

    Very nice work in your presentation. I got a few questions.

    * With the work done in the laboratory of Ken, I always wonder whether you are able to capture all important physics of bedform development as you use acrylic dunes. I realize that the resolution of your flow measurements is absolutely top-notch, but you are missing the interaction and feedback between grain and flow. Can you please highlight what physical phenomena you are able to cover/measure/capture, and where you miss out as you don't do experiments with loose sediment?

    * The acrylic photodune you are using is not symmetric, although the stage 1 sand patch is. What type of flow features are you able to observe due to the asymmetry? What changes fundamentally in the flow characteristics in the proto-dune development when you switch from symmetric to asymmetric? Do you see differences in the swirling orientation, or perhaps intermittency strength?

    * What are the implications from your measured flow structures for dune initiation in the field: when do sand patches develop into protodunes and protodunes into real dunes, and when do they die down again? Is the answer just in the flow structure, or are sediment properties or other factors critical here as well?

     

    Thanks a lot, all best,

    Nathalie

    • AC1: Reply to CC1, Nathaniel Bristow, 04 May 2020

      Hi Nathalie,

      Thanks for the questions. I will do my best to answer them in brief here.

      On the first point, it is indeed important to recognize the absence of moving grains as a feedback on the flow, and so those aspects of the flow are not present here. What we are able to capture in our experiments are the large-scale features of the flow in the boundary layer, and how the "protodune" modifies them (i.e., flow-form feedback). We cannot capture the effects of a transport layer at the surface and how this might modify the turbulence, and we also are not quite fully capturing the flow-form feedback in its entirety since the "form" is fixed. The hope is that by coupling the lab measurements with the field measurements, and comparing the two, we can build some understanding of what the lab is missing and where the two differ most strongly. It is likely that in the earliest stages of protodune development, when the bedforms are so small that they are immersed within the saltation layer, that the differences between field and lab (where there is no transport layer) will be most pronounced. 

      I should have been more clear perhaps with my slide about stage 1-5. As a whole for the project, between both the lab and the field, we are most interested in the first three stages, but the lab models more so resemble stages 2-3, where there is a transition from reverse asymmetry to the more regular, mature asymmetry. I have not shown this here, as I am still working with data taken over additional bedform models which map out this transition, but having a stronger reverse asymmetry (where the crest is farther upstream) shifts the changes induced on the flow upstream. So instead of the perturbation to the flow developing mainly beyond the bedform, the flow is perturbed more strongly close to the toe, and the bedform feels those changes to the flow more strongly. That is my main thinking at this point.

      As far as implications, I think that the lab results are able to give us a sense for how strongly the flow will be perturbed by this very small bedform. This then feeds back into our understanding of how transport characteristics will be changed over and around the bedform, influencing its development. However, to answer your question, the flow alone is not the whole picture, and that is why we need the field measurements of how morphologies are changing and the transport as well. These lab measurements couple with the field work, and give us a sense that, yes, the protodune does influence the flow on its stoss side in an appreciable manner despite its gentle profile, but this will be most meaningful when we can close the loop with field data. So this work is ongoing, and these are some early results.

      Pauline Delorme, one of our collaborators from the field, has a presentation in this session also where she has been looking at describing 2D bedform initiation from the perspective of a linear stability analysis, and you might find her work interesting also! I'd encourage you to check that out also.

      Best wishes,

      Nate

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