Mechanical Engineering - Research Publications

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    Simulation of large-eddy-break-up device (LEBU) in a moderate Reynolds number turbulent boundary layer
    Chin, C ; Monty, J ; HUTCHINS, N ; Ooi, A ; Orlu, R ; Schlatter, P (Springer, 2016-08-11)
    A well-resolved large eddy simulation (LES) of a large-eddy break-up (LEBU) device in a spatially evolving turbulent boundary layer is performed with, Reynolds number, based on free-stream velocity and momentum-loss thickness, of R e θ ≈ 4300. The implementation of the LEBU is via an immersed boundary method. The LEBU is positioned at a wall-normal distance of 0.8 δ (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, 1988). 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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    Structure Inclination Angles in the Convective Atmospheric Surface Layer
    Chauhan, K ; Hutchins, N ; Monty, J ; Marusic, I (SPRINGER, 2013-04-01)
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    The Effect of Wall Normal Actuation on a Turbulent Boundary Layer
    Schlanderer, SC ; Hutchins, N ; Sandberg, RD (SPRINGER, 2017-12-01)
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    Towards fully-resolved PIV measurements in high Reynolds number turbulent boundary layers with DSLR cameras
    de Silva, CM ; Grayson, K ; Scharnowski, S ; Kaehler, CJ ; Hutchins, N ; Marusic, I (SPRINGER, 2018-06-01)
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    Towards Reconciling the Large-Scale Structure of Turbulent Boundary Layers in the Atmosphere and Laboratory
    Hutchins, N ; Chauhan, K ; Marusic, I ; Monty, J ; Klewicki, J (SPRINGER, 2012-11-01)
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    Wall-drag measurements of smooth- and rough-wall turbulent boundary layers using a floating element
    Baars, WJ ; Squire, DT ; Talluru, KM ; Abbassi, MR ; Hutchins, N ; Marusic, I (SPRINGER, 2016)
    The mean wall shear stress, $$øverlineτ _w$$ τ ¯ w , is a fundamental variable for characterizing turbulent boundary layers. Ideally, $$øverlineτ _w$$ τ ¯ w is measured by a direct means and the use of floating elements has long been proposed. However, previous such devices have proven to be problematic due to low signal-to-noise ratios. In this paper, we present new direct measurements of $$øverlineτ _w$$ τ ¯ w where high signal-to-noise ratios are achieved using a new design of a large-scale floating element with a surface area of 3 m (streamwise) × 1 m (spanwise). These dimensions ensure a strong measurement signal, while any error associated with an integral measurement of $$øverlineτ _w$$ τ ¯ w is negligible in Melbourne’s large-scale turbulent boundary layer facility. Wall-drag induced by both smooth- and rough-wall zero-pressure-gradient flows are considered. Results for the smooth-wall friction coefficient, $$C_f \equiv øverlineτ _w/q_\infty $$ C f ≡ τ ¯ w / q ∞ , follow a Coles–Fernholz relation $$C_f = \left[ 1/κ \ln \left( Re_θ \right) + C\right] ^-2$$ C f = 1 / κ ln R e θ + C - 2 to within 3 % ( $$κ = 0.38$$ κ = 0.38 and $$C = 3.7$$ C = 3.7 ) for a momentum thickness-based Reynolds number, $$Re_θ > 15,000$$ R e θ > 15 , 000 . The agreement improves for higher Reynolds numbers to <1 % deviation for $$Re_θ > 38,000$$ R e θ > 38 , 000 . This smooth-wall benchmark verification of the experimental apparatus is critical before attempting any rough-wall studies. For a rough-wall configuration with P36 grit sandpaper, measurements were performed for $$10,500< Re_θ < 88,500$$ 10 , 500 < R e θ < 88 , 500 , for which the wall-drag indicates the anticipated trend from the transitionally to the fully rough regime.
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    Wavelet analysis of wall turbulence to study large-scale modulation of small scales
    Baars, WJ ; Talluru, KM ; Hutchins, N ; Marusic, I (SPRINGER, 2015-10-01)
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    Turbulent structures in a statistically three-dimensional boundary layer
    Kevin, ; Monty, J ; Hutchins, N (Cambridge University Press (CUP), 2019-01-25)
    We investigate the behaviour of large-scale coherent structures in a spanwise-heterogeneous turbulent boundary layer, using particle image velocimetry on multiple orthogonal planes. The statistical three-dimensionality is imposed by a herringbone riblet surface, although the key results presented here will be common to many cases of wall turbulence with embedded secondary flows in the form of mean streamwise vortices. Instantaneous velocity fields in the logarithmic layer reveal elongated low-momentum streaks located over the upwash-flow region, where their spanwise spacing is forced by the 2δ periodicity of the herringbone pattern. These streaks largely resemble the turbulence structures that occur naturally (and randomly located) in spanwise-homogeneous smooth-/rough-wall boundary layers, although here they are directly formed by the roughness pattern. In the far outer region, the large spanwise spacing permits the streaks to aggressively meander. The mean secondary flows are the time-averaged artefact of the unsteady and spanwise asymmetric large-scale roll modes that accompany these meandering streaks. Interestingly, this meandering, or instability, gives rise to a pronounced streamwise periodicity (i.e. an alternating coherent pattern) in the spatial statistics, at wavelengths of approximately 4.5 δ . Overall, the observed behaviours largely resemble the streak-instability model that has been proposed for the buffer region, only here at a much larger scale and at a forced spanwise spacing. This observation further confirms recent observations that such features may occur at an entire hierarchy of scales throughout the turbulent boundary layer.