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    An experimental investigation into the breakdown of riblet drag reduction at post-optimal conditions

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    Author
    Newton, R; Chung, D; Hutchins, N
    Date
    2018-01-01
    Source Title
    Proceedings of the 21st Australasian Fluid Mechanics Conference, AFMC 2018
    Publisher
    Australasian Fluid Mechanics Society
    University of Melbourne Author/s
    Hutchins, Nicholas; Newton, Ryan; Chung, Daniel
    Affiliation
    Mechanical Engineering
    Metadata
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    Document Type
    Conference Paper
    Citations
    Newton, R., Chung, D. & Hutchins, N. (2018). An experimental investigation into the breakdown of riblet drag reduction at post-optimal conditions. Proceedings of the 21st Australasian Fluid Mechanics Conference, AFMC 2018, Australasian Fluid Mechanics Society.
    Access Status
    Access this item via the Open Access location
    URI
    http://hdl.handle.net/11343/252859
    Open Access URL
    http://people.eng.unimelb.edu.au/imarusic/proceedings/21/Contribution_738_final.pdf
    Abstract
    A long-standing question in riblet research is why drag reduction only occurs within a small, non-dimensionally scaled envelope, outside of which drag is significantly increased. For riblets with viscous-scaled spacings that are much larger than those required for drag reduction, one hypothesis is that the riblets exhibit k-type, ‘fully rough’ behaviour. However, this seems counter-intuitive since fully rough behaviour is typically associated with a dominance of pressure drag over viscous drag, and yet riblets can sustain no pressure drag. This study aims to investigate this issue by conducting single normal hot-wire traverses above a trapezoidal riblet surface, over a range of drag-increasing viscous-scaled riblet spacings. Novelty was added by also measuring within the riblet valleys, providing a unique look at the turbulent behaviour within them. Previously proposed mechanisms for the breakdown in drag reduction have included lodgement of turbulence within the riblet valleys, and the development of a Kelvin–Helmholtz instability, but neither mechanism appears active in our results. They instead show a reduction in turbulent energy as riblet spacing increases, despite a significant increase in drag, which does seem to be approaching a k-type roughness asymptote as hypothesised. This may be caused by the generation of time-invariant secondary flows above the riblet tips and corners of the riblet valleys, although this will require further investigation.

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