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Vortex organization in a turbulent boundary layer overlying sparse roughness elements / Michele Guala in Journal of hydraulic research, Vol. 50 N° 5 (Septembre/Octobre 2012) 

Public ISBD [article]  in Journal of hydraulic research > Vol. 50 N° 5 (Septembre/Octobre 2012) . - pp. 465-481 Titre : Vortex organization in a turbulent boundary layer overlying sparse roughness elements Type de document : texte imprimé Auteurs : Michele Guala, Auteur ; Christopher D. Tomkins, Auteur ; Kenneth T. Christensen, Auteur Année de publication : 2012 Article en page(s) : pp. 465-481 Note générale : Hydraulique Langues : Anglais (eng) Mots-clés : Bed  roughness  Boundary  layer  turbulence  Particle  image  velocimetry  Wall-bounded  shear  flow  turbulence Résumé : Vortex organization in the outer layer of a turbulent boundary layer overlying sparse, hemispherical roughness elements is explored with two-component particle-image velocimetry (PIV) in multiple streamwise–wall-normal measurement planes downstream and between elements. The presence of sparse roughness elements causes a shortening of the streamwise length scale in the near-wall region. These measurements confirm that vortex packets exist in the outer layer of flow over rough walls, but that their organization is altered, and this is interpreted as the underlying cause of the length-scale reduction. In particular, the elements shed vortices which appear to align in the near-wall region, but are distinct from the packets. Further, it is observed that ejection events triggered in the element wakes are more intense compared to the ejection events in smooth wall. We speculate that this may initiate a self-sustaining mechanism leading to the formation of hairpin packets as a much more effective instability compared to those typical of smooth-wall turbulence. ISSN : 0022-1686 En ligne : http://www.tandfonline.com/doi/abs/10.1080/00221686.2012.729229 [article] Vortex organization in a turbulent boundary layer overlying sparse roughness elements [texte imprimé] / Michele Guala, Auteur ; Christopher D. Tomkins, Auteur ; Kenneth T. Christensen, Auteur . - 2012 . - pp. 465-481. Hydraulique Langues : Anglais (eng) in Journal of hydraulic research > Vol. 50 N° 5 (Septembre/Octobre 2012) . - pp. 465-481 Mots-clés : Bed  roughness  Boundary  layer  turbulence  Particle  image  velocimetry  Wall-bounded  shear  flow  turbulence Résumé : Vortex organization in the outer layer of a turbulent boundary layer overlying sparse, hemispherical roughness elements is explored with two-component particle-image velocimetry (PIV) in multiple streamwise–wall-normal measurement planes downstream and between elements. The presence of sparse roughness elements causes a shortening of the streamwise length scale in the near-wall region. These measurements confirm that vortex packets exist in the outer layer of flow over rough walls, but that their organization is altered, and this is interpreted as the underlying cause of the length-scale reduction. In particular, the elements shed vortices which appear to align in the near-wall region, but are distinct from the packets. Further, it is observed that ejection events triggered in the element wakes are more intense compared to the ejection events in smooth wall. We speculate that this may initiate a self-sustaining mechanism leading to the formation of hairpin packets as a much more effective instability compared to those typical of smooth-wall turbulence. ISSN : 0022-1686 En ligne : http://www.tandfonline.com/doi/abs/10.1080/00221686.2012.729229