Cross-stream stereoscopic particle image velocimetry of a modified turbulent boundary layer over directional surface pattern

Kevin Kevin, J. P. Monty, H. L. Bai, G. Pathikonda, B. Nugroho, J. M. Barros, K. T. Christensen, N. Hutchins

Research output: Contribution to journalArticlepeer-review

46 Scopus citations

Abstract

A turbulent boundary layer developed over a herringbone patterned riblet surface is investigated using stereoscopic particle image velocimetry in the cross-stream plane at Reτ ≈ 3900.. The three velocity components resulting from this experiment reveal a pronounced spanwise periodicity in all single-point velocity statistics. Consistent with previous hot-wire studies over similar-type riblets, we observe a weak time-average secondary flow in the form of δ-filling streamwise vortices. The observed differences in the surface and secondary flow characteristics, compared to other heterogeneous-roughness studies, may suggest that different mechanisms are responsible for the flow modifications in this case. Observations of instantaneous velocity fields reveal modified and rearranged turbulence structures. The instantaneous snapshots also suggest that the time-average secondary flow may be an artefact arising from superpositions of much stronger instantaneous turbulent events enhanced by the surface texture. In addition, the observed instantaneous secondary motions seem to have promoted a free-stream-engulfing behaviour in the outer layer, which would indicate an increase turbulent/non-turbulent flow mixing. It is overall demonstrated that the presence of large-scale directionality in transitional surface roughness can cause a modification throughout the entire boundary layer, even when the roughness height is 0.5 % of the layer thickness.

Original languageEnglish (US)
Pages (from-to)412-435
Number of pages24
Journaljournal of fluid mechanics
Volume813
DOIs
StatePublished - Feb 25 2017
Externally publishedYes

Keywords

  • Boundary layer structure
  • turbulence control
  • turbulent boundary layers

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

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