A turbulent boundary layer over a two-dimensional rough wall |
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Authors: | Lyazid Djenidi Robert A Antonia Muriel Amielh Fabien Anselmet |
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Institution: | (1) School of Engineering, University of Newcastle, Newcastle, Australia;(2) IRPHE, Marseille, France |
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Abstract: | Particle image velocimetry (PIV) measurements and planar laser induced fluorescence (PLIF) visualizations have been made in
a turbulent boundary layer over a rough wall. The wall roughness consisted of square bars placed transversely to the flow
at a pitch to height ratio of λ/k = 11 for the PLIF experiments and λ/k = 8 and 16 for the PIV measurements. The ratio between the boundary layer thickness and the roughness height k/δ was about 20 for the PLIF and 38 for the PIV. Both the PLIF and PIV data showed that the near-wall region of the flow was
populated by unstable quasi-coherent structures which could be associated to shear layers originating at the trailing edge
of the roughness elements. The streamwise mean velocity profile presented a downward shift which varied marginally between
the two cases of λ/k, in agreement with previous measurements and DNS results. The data indicated that the Reynolds stresses normalized by the
wall units are higher for the case λ/k = 16 than those for λ/k = 8 in the outer region of the flow, suggesting that the roughness density effects could be felt well beyond the near-wall
region of the flow. As expected the roughness disturbed dramatically the sublayer which in turn altered the turbulence production
mechanism. The turbulence production is maximum at a distance of about 0.5k above the roughness elements. When normalized by the wall units, the turbulence production is found to be smaller than that
of a smooth wall. It is argued that the production of turbulence is correlated with the form drag. |
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