 Mechanical Engineering  Research Publications
Mechanical Engineering  Research Publications
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ItemA direct measure of the frequency response of hotwire anemometers: temporal resolution issues in wallbounded turbulenceHutchins, N ; Monty, JP ; Hultmark, M ; Smits, AJ (SPRINGER, 20150101)

ItemSimulation of largeeddybreakup device (LEBU) in a moderate Reynolds number turbulent boundary layerChin, C ; Monty, J ; HUTCHINS, N ; Ooi, A ; Orlu, R ; Schlatter, P (Springer, 20160811)A wellresolved large eddy simulation (LES) of a largeeddy breakup (LEBU) device in a spatially evolving turbulent boundary layer is performed with, Reynolds number, based on freestream velocity and momentumloss thickness, of R e θ ≈ 4300. The implementation of the LEBU is via an immersed boundary method. The LEBU is positioned at a wallnormal 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 ‘breakup’ 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 wallnormal location when compared to the corresponding turbulent boundary layer without LEBU.

ItemStructure Inclination Angles in the Convective Atmospheric Surface LayerChauhan, K ; Hutchins, N ; Monty, J ; Marusic, I (SPRINGER, 20130401)

ItemThe Effect of Wall Normal Actuation on a Turbulent Boundary LayerSchlanderer, SC ; Hutchins, N ; Sandberg, RD (SPRINGER, 20171201)

ItemTowards fullyresolved PIV measurements in high Reynolds number turbulent boundary layers with DSLR camerasde Silva, CM ; Grayson, K ; Scharnowski, S ; Kaehler, CJ ; Hutchins, N ; Marusic, I (SPRINGER, 20180601)

ItemTowards Reconciling the LargeScale Structure of Turbulent Boundary Layers in the Atmosphere and LaboratoryHutchins, N ; Chauhan, K ; Marusic, I ; Monty, J ; Klewicki, J (SPRINGER, 20121101)

ItemValidating underresolved turbulence intensities for PIV experiments in canonical wallbounded turbulenceLee, JH ; Kevin, ; Monty, JP ; Hutchins, N (SPRINGER, 20160801)

ItemWalldrag measurements of smooth and roughwall turbulent boundary layers using a floating elementBaars, 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 signaltonoise ratios. In this paper, we present new direct measurements of $$øverlineτ _w$$ τ ¯ w where high signaltonoise ratios are achieved using a new design of a largescale 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 largescale turbulent boundary layer facility. Walldrag induced by both smooth and roughwall zeropressuregradient flows are considered. Results for the smoothwall 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 thicknessbased 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 smoothwall benchmark verification of the experimental apparatus is critical before attempting any roughwall studies. For a roughwall 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 walldrag indicates the anticipated trend from the transitionally to the fully rough regime.

ItemWavelet analysis of wall turbulence to study largescale modulation of small scalesBaars, WJ ; Talluru, KM ; Hutchins, N ; Marusic, I (SPRINGER, 20151001)

ItemTurbulent structures in a statistically threedimensional boundary layerKevin, ; Monty, J ; Hutchins, N (Cambridge University Press (CUP), 20190125)We investigate the behaviour of largescale coherent structures in a spanwiseheterogeneous turbulent boundary layer, using particle image velocimetry on multiple orthogonal planes. The statistical threedimensionality 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 lowmomentum streaks located over the upwashflow 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 spanwisehomogeneous smooth/roughwall 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 timeaveraged artefact of the unsteady and spanwise asymmetric largescale 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 streakinstability 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.