A unique class of directional surfaces arranged in a converging-diverging (herringbone) pattern are studied experimentally in a zero pressure gradient turbulent boundary layers. Hot-wire measurements using both single and cross-wire show that these small surfaces are able to generate large-scale counter rotating roll-modes/vortices within the turbulent boundary layer, resulting in dramatic spanwise variation in the boundary layer thickness δ (50% variation for the strongest case). The results reveal that above the converging region, the local mean velocity decreases while the turbulence intensity increases, resulting in locally thicker boundary layer. Over the diverging region, the opposite situation occurs, where the mean velocity increases and the turbulence intensity decreases, resulting in a locally thinner boundary layer. The strong perturbation effect from these surfaces to the overall flow dynamics seems unusual, considering that their peak-to-trough height is approximately only 1% of the boundary layer thickness. This study, also investigates the behavior of the large-scale counter-rotating roll-modes when the surface reverts from the herringbone pattern back to the smooth wall, to see how far they persist over the smooth wall. Our preliminary results show that the roll-modes above the smooth wall still persist even at 40δ downstream. The results of this study show that the herringbone surface roughness pattern can act as a novel method of generating counter rotating roll-modes (vortices) for flow control purposes in various engineering applications. Nomenclaturex Streamwise direction/distance, m y Spanwise direction/distance, m z Wall-normal direction/distance, m U Mean streamwise velocity, m/s V Mean spanwise velocity, m/s W Mean wall-normal velocity, m/s U ∞ Freestream velocity, m/s U τ Skin friction velocity, m/s U sa Spanwise averaged mean velocity, m/s δ Boundary layer thickness, m δ s Boundary layer thickness over smooth wall, m δ r Boundary layer thickness over rough surface, m δ sa Boundary layer thickness averaged over one spanwise wavelength, m δ rs Boundary layer thickness averaged over one spanwise wavelength of rough to smooth case, m Downloaded by KUNGLIGA TEKNISKA HOGSKOLEN KTH on July 30, 2015 | http://arc.aiaa.org |

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A well-resolved large eddy simulation (LES) of a large-eddy breakup (LEBU) device in a spatially evolving turbulent boundary layer is performed with, Reynolds number, based on free-stream velocity and momentumloss thickness, of Re θ ≈ 4300. The implementation of the LEBU is via an immersed boundary method. The LEBU is positioned at a wall-normal distance of 0.8δ (δ denoting the local boundary layer thickness at the location of the LEBU) from the wall. The LEBU acts to delay the growth of the turbulent boundary layer and produces global skin friction reduction beyond 180δ downstream of the LEBU, with a peak local skin friction reduction of approximately 12%. However, no net drag reduction is found when accounting for the device drag of the LEBU in accordance with the towing tank experiments by Sahlin et al. (Phys. Fluids 31, 2814. Further investigation is performed on the interactions of high and low momentum bulges with the LEBU and the corresponding output is analysed, showing a 'break-up' of these large momentum bulges downstream of the LEBU. In addition, results from the spanwise energy spectra show consistent reduction in energy at spanwise length scales for λ + z > 1000 independent of streamwise and wall-normal location when compared to the corresponding turbulent boundary layer without LEBU.

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