Transition to turbulence over 2D and 3D periodic large-scale roughnesses

A. M. Hamed, M. Sadowski, Z. Zhang, L. P. Chamorro

Research output: Contribution to journalArticle

Abstract

A laboratory investigation was performed to study distinctive features of the laminar-to-turbulent transition over distributed roughness characterized by two-dimensional (2D) and three-dimensional (3D) periodic, low-order topographies at roughness Reynolds number . Systematic experiments were performed using high-spatial-resolution planar particle image velocimetry (PIV) in a refractive-index-matching (RIM) channel, where the roughness covered the entire length of the test section. The results show that the flow over the 2D roughness becomes turbulent much sooner than its 3D counterpart ( versus 120 000). This is attributed to the presence of a velocity inflection point resulting from flow separation within the troughs of the 2D roughness. In the transitional region, unsteady disturbances above the two roughnesses appear upstream of near-roughness disturbances. The above-roughness disturbances are associated with the inflection point in the vertically displaced boundary layer for the 2D case, and with the mean velocity deficit resulting from the interaction of the wakes of upstream elements for the 3D case. The near-roughness fluctuations are associated with the shear layer present behind the crests of both roughnesses. The transitional region is characterized by the interaction between above- and near-roughness disturbances, which merge, leading to a rapid vertical growth of the turbulent fluctuations.

Original languageEnglish (US)
Pages (from-to)R6
JournalJournal of Fluid Mechanics
Volume804
DOIs
StatePublished - Oct 10 2016

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Turbulence
roughness
Surface roughness
turbulence
disturbances
inflection points
upstream
flow separation
Flow separation
shear layers
particle image velocimetry
troughs
wakes
Velocity measurement
Topography
boundary layers
Reynolds number
Refractive index
topography
Boundary layers

Keywords

  • instability
  • transition to turbulence

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

Cite this

Transition to turbulence over 2D and 3D periodic large-scale roughnesses. / Hamed, A. M.; Sadowski, M.; Zhang, Z.; Chamorro, L. P.

In: Journal of Fluid Mechanics, Vol. 804, 10.10.2016, p. R6.

Research output: Contribution to journalArticle

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