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    Turbulent structures in a statistically three-dimensional boundary layer

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    17
    1
    Author
    Kevin; Monty, J; Hutchins, N
    Date
    2019-01-25
    Source Title
    Journal of Fluid Mechanics
    Publisher
    Cambridge University Press (CUP)
    University of Melbourne Author/s
    Hutchins, Nicholas; Monty, Jason; Kevin, Kevin
    Affiliation
    Mechanical Engineering
    Metadata
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    Document Type
    Journal Article
    Citations
    Kevin, Monty, J. & Hutchins, N. (2019). Turbulent structures in a statistically three-dimensional boundary layer. Journal of Fluid Mechanics, 859, pp.543-565. https://doi.org/10.1017/jfm.2018.814.
    Access Status
    Open Access
    URI
    http://hdl.handle.net/11343/254304
    DOI
    10.1017/jfm.2018.814
    ARC Grant code
    ARC/DP160102279
    Abstract
    We investigate the behaviour of large-scale coherent structures in a spanwise-heterogeneous turbulent boundary layer, using particle image velocimetry on multiple orthogonal planes. The statistical three-dimensionality is imposed by a herringbone riblet surface, although the key results presented here will be common to many cases of wall turbulence with embedded secondary flows in the form of mean streamwise vortices. Instantaneous velocity fields in the logarithmic layer reveal elongated low-momentum streaks located over the upwash-flow region, where their spanwise spacing is forced by the 2δ periodicity of the herringbone pattern. These streaks largely resemble the turbulence structures that occur naturally (and randomly located) in spanwise-homogeneous smooth-/rough-wall boundary layers, although here they are directly formed by the roughness pattern. In the far outer region, the large spanwise spacing permits the streaks to aggressively meander. The mean secondary flows are the time-averaged artefact of the unsteady and spanwise asymmetric large-scale roll modes that accompany these meandering streaks. Interestingly, this meandering, or instability, gives rise to a pronounced streamwise periodicity (i.e. an alternating coherent pattern) in the spatial statistics, at wavelengths of approximately 4.5 δ . Overall, the observed behaviours largely resemble the streak-instability model that has been proposed for the buffer region, only here at a much larger scale and at a forced spanwise spacing. This observation further confirms recent observations that such features may occur at an entire hierarchy of scales throughout the turbulent boundary layer.

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