Unsteady dynamics of turbulent flow in the wakes of barchan dunes modulated by overlying boundary-layer structure

Nathaniel R. Bristow, Gianluca Blois, James L. Best, Kenneth T. Christensen

Research output: Contribution to journalArticlepeer-review

Abstract

The dynamics of turbulent flow structures in the wakes of barchan dunes is investigated to understand the complex, long-range interactions that occur between these three-dimensional bedforms. High-frame-rate stereo-particle image velocimetry measurements are collected from the cross-stream plane in a refractive-index-matched flume, wherein rigid models are immersed in the logarithmic region of a turbulent boundary layer. Through application of Taylor's hypothesis, limited-domain pseudo-three-dimensional reconstructions of the flow are made, wherein coherent structures are identified in the wake of an isolated barchan that resemble hairpin-like vortices. Amplitude modulation analysis, based on wavelet decomposition of velocity fluctuations and swirling strength, suggests a correlation between the shedding of these coherent structures and the passage of large-scale motions (LSMs) aloft in the turbulent boundary layer within which the barchan dune is immersed. Impulse analysis of shear stress events shows that, while the hairpin-like structures predominantly induce ejections of low-momentum fluid, the intermittent passage of an overlying high-momentum LSM yields intense sweep events as they impact the wall. Similar flow dynamics is found in dune-dune collision configurations involving barchan dunes arranged in tandem, with the exception that significant flow asymmetries are introduced, resulting in structures resembling single-legged hairpin-like (or 'cane') vortices being identified in conditional averages. These results provide insight into both the morphodynamics of dune interactions as well as the interactions between boundary-layer structure and roughness elements protruding into the log layer.

Original languageEnglish (US)
Article numberA51
JournalJournal of Fluid Mechanics
Volume920
DOIs
StatePublished - 2021

Keywords

  • boundary layer structure
  • topographic effects
  • turbulent boundary layers

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

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