 Mechanical Engineering  Research Publications
Mechanical Engineering  Research Publications
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ItemHeat Transfer Coefficient Estimation for Turbulent Boundary LayersWang, S ; Xia, Y ; Abu Rowin, W ; Marusic, I ; Sandberg, R ; Chung, D ; Hutchins, N ; Tanimoto, K ; Oda, T (The University of Queensland, 20201211)Convective heat transfer in rough wallbounded turbulent flows is prevalent in many engineering applications, such as in gas turbines and heat exchangers. At present, engineers lack the design tools to accurately predict the convective heat transfer in the presence of nonsmooth boundaries. Accordingly, a new turbulent boundary layer facility has been commissioned, where the temperature of an interchangeable test surface can be precisely controlled, and conductive heat losses are minimized. Using this facility, we can estimate the heat transfer coefficient (Stanton number, St), through measurement of the power supplied to the electrical heaters and also from measurements of the thermal and momentum boundary layers evolving over this surface. These methods have been initially investigated over a shorter smooth prototype heated surface and compared with existing St prediction models. Preliminary results suggest that we can accurately estimate St in this facility.

ItemAn investigation of coldwire spatial resolution using a DNS databaseXia, Y ; Rowin, W ; Jelly, T ; Chung, D ; Marusic, I ; Hutchins, N (The University of Queensland, 20201211)The effect of spatial resolution of coldwire anemometry on both the variance and energy spectrum of temperature fluctuations is analyzed through the use of a numerical database. Temperature fluctuation snapshots from a direct numerical simulation (DNS) of a heated smoothwall turbulent channel flow are spatially averaged in the spanwise direction to simulate the wire filtering. The results show that the wire length does not affect the mean temperature while it significantly attenuates the variance of temperature fluctuations, particularly in the vicinity of the wall. As the filter length grows, the peaks of the one and twodimensional energy spectrograms are further attenuated. Limited attenuation is seen when the filter length is smaller than 30 wall units in the vicinity of the wall, whereas a complete suppression of the nearwall energetic peak is observed when the filter length exceeds 100 wall units.

ItemDispersive stresses in turbulent flow over ribletsModesti, D ; Endrikat, S ; Hutchins, N ; Chung, D (Cambridge University Press, 20210625)We carry out direct numerical simulations of turbulent flow over riblets, streamwise aligned grooves that are designed to reduce drag by modifying the nearwall flow. Twenty riblet geometries and sizes are considered, namely symmetric triangular with tip angle, and, asymmetric triangular, blade and trapezoidal. To save on computational cost, simulations are performed using the minimalchannel flow configuration. With this unprecedented breadth of highfidelity flow data near the wall, we are able to obtain more general insights into the flow physics of riblets. As observed by GarcíaMayoral & Jiménez (J. Fluid Mech., vol. 678, 2011, pp. 317347), we confirm that the dragchange curves of all the present groove geometries better collapse when reported with the viscousscaled square root of the groove area, rather than the riblet spacing. Using a twodimensional generalization of the FukagataIwamotoKasagi identity in difference form we isolate the different dragchange contributions. We show that the drag increase associated with dispersive stresses carried by secondary flows can be as important as the one associated with the turbulent stresses and the preeminence of dispersive stresses can be estimated by the groove width at the riblet mean height.

ItemInfluence of riblet shapes on the occurrence of KelvinHelmholtz rollersEndrikat, S ; Modesti, D ; GarciaMayoral, R ; Hutchins, N ; Chung, D (CAMBRIDGE UNIV PRESS, 20210302)We investigate turbulent flow over streamwisealigned riblets (grooves) of various shapes and sizes. Small riblets with spacings of typically less than 20 viscous units are known to reduce skinfriction drag compared to a smooth wall, but larger riblets allow inertialflow mechanisms to appear and cause drag reduction to break down. One of these mechanisms is a Kelvin–Helmholtz instability that GarcíaMayoral & Jiménez (J. Fluid Mech., vol. 678, 2011, pp. 317–347) identified in turbulent flow over blade riblets. In order to evaluate its dependence on riblet shape and thus gain a broader understanding of the underlying physics, we generate an extensive data set comprising 21 cases using direct numerical simulations of fully developed minimalspan channel flow. The data set contains six riblet shapes of varying sizes between maximum drag reduction and significant drag increase. Comparing the flow fields over riblets to that over a smooth wall, we find that in this data set only large sharptriangular and blade riblets have a drag penalty associated with the Kelvin–Helmholtz instability and that the mechanism appears to be absent for blunttriangular and trapezoidal riblets of any size. We therefore investigate two indicators for the occurrence of Kelvin–Helmholtz rollers in turbulent flow over riblets. First, we confirm for all six riblet shapes that the groove crosssectional area in viscous units serves as a proxy for the wallnormal permeability that is necessary for the development of Kelvin–Helmholtz rollers. Additionally, we find that the occurrence of the instability correlates with a high momentum absorption at the riblet tips. The momentum absorption can be qualitatively predicted using Stokes flow.

ItemRoughness and Reynolds Number Effects on the Flow Past a RoughtoSmooth Step ChangeRouhi, A ; Chung, D ; Hutchins, N ; Orlu, R ; Talamelli, A ; Peinke, J ; Oberlack, M (Springer International Publishing, 2019)We report direct numerical simulations (DNSs) of openchannel flow with a step change from threedimensional sinusoidal rough surface to smooth surface. We investigate the persistence of nonequilibrium behaviour beyond this step change (i.e. departures from the equilibrium smooth openchannel flow) and how this depends on (1) roughness virtual origin ϵ/hϵ/h? (scaled by the channel height h), (2) roughness size k / h?, (3) roughness shape? and (4) Reynolds number ReτReτ? To study (1), the roughness origin was placed aligned with, below (stepup) and above (stepdown) the smooth patch. To study (2), the equivalent sandgrain roughness of the aligned case was decreased from k+s≃ks+≃ 160 to k+s≃106ks+≃106. To study (3) and (4) the stepdown case at Reτ≃395Reτ≃395 was compared with a backwardfacing step case at Reτ≃527Reτ≃527, and DNS of square rib roughtosmooth case at Reτ≃1160Reτ≃1160 (Ismail et al., J. Fluid Mech., vol. 843, 2018, pp. 419–449). Results showed that ϵ/hϵ/h affects the departure from equilibrium by a large extent, while k / h, roughness shape and ReτReτ have a marginal influence. The departure from equilibrium was found to be related to the nearwall amplification of Reynolds shear stress, which in turn depends on ϵ/hϵ/h, i.e. higher ϵ/hϵ/h leads to higher amplification.

ItemSecondary motion in turbulent pipe flow with threedimensional roughnessChan, L ; MacDonald, M ; Chung, D ; Hutchins, N ; Ooi, A (Cambridge University Press (CUP), 20180831)The occurrence of secondary flows is investigated for threedimensional sinusoidal roughness where the wavelength and height of the roughness elements are systematically altered. The flow spanned from the transitionally rough regime up to the fully rough regime and the solidity of the roughness ranged from a wavy, sparse roughness to a dense roughness. Analysing the timeaveraged velocity, secondary flows are observed in all of the cases, reflected in the coherent stress profile which is dominant in the vicinity of the roughness elements. The roughness sublayer, defined as the region where the coherent stress is nonzero, scales with the roughness wavelength when the roughness is geometrically scaled (proportional increase in both roughness height and wavelength) and when the wavelength increases at fixed roughness height. Premultiplied energy spectra of the streamwise velocity turbulent fluctuations show that energy is reorganised from the largest streamwise wavelengths to the shorter streamwise wavelengths. The peaks in the premultiplied spectra at the streamwise and spanwise wavelengths are correlated with the roughness wavelength in the fully rough regime. Current simulations show that the spanwise scale of roughness determines the occurrence of largescale secondary flows.

ItemDirect numerical simulation of high aspect ratio spanwisealigned barsMacDonald, M ; Ooi, A ; GarciaMayoral, R ; Hutchins, N ; Chung, D (Cambridge University Press (CUP), 20180319)We conduct minimalchannel direct numerical simulations of turbulent flow over twodimensional rectangular bars aligned in the spanwise direction. This roughness has often been described as d type, as the roughness function ΔU+ is thought to depend only on the outerlayer length scale (pipe diameter, channel halfheight or boundary layer thickness). This is in contrast to conventional engineering rough surfaces, named k type, for which ΔU+ depends on the roughness height, k. The minimalspan roughwall channel is used to circumvent the high cost of simulating high Reynolds number flows, enabling a range of bars with varying aspect ratios to be investigated. The present results show that increasing the troughtocrest height, k, of the roughness while keeping the width between roughness bars, W, fixed in viscous units, results in non k type behaviour although this does not necessarily indicate d type behaviour. Instead, for deep surfaces with k/W≳3, the roughness function appears to depend only on W in viscous units. In these situations, the flow no longer has any information about how deep the roughness is and instead can only ‘see’ the width of the fluid gap between the bars.

ItemDirect numerical simulation of high aspect ratio spanwisealigned barsMacDonald, M ; Ooi, A ; Hutchins, N ; Chung, D (Cambridge University Press (CUP), 20170101)We conduct minimalchannel direct numerical simulations of turbulent flow over twodimensional rectangular bars aligned in the spanwise direction. This roughness has been often described as dtype, as the roughness function ΔU+ is thought to depend only on the outerlayer length scale (pipe diameter, channel half height or boundary layer thickness). This is in contrast to conventional engineering rough surfaces, named ktype, for which ΔU+ depends on the roughness height, k. The minimalspan roughwall channel is used to circumvent the high cost of simulating high Reynolds number flows, enabling a range of bars with varying aspect ratios to be investigated. The present results show that increasing the troughtocrest height (k) of the roughness while keeping the width between roughness bars, W, fixed in wall units, results in nonktype behaviour. The roughness function appears to scale with W, suggesting that this is the only relevant parameter for very deep rough surfaces with k/W≥ 3. In these situations, the flow no longer has any information about how deep the roughness is and instead can only 'see' the width of the fluid gap between the bars.

ItemNo Preview AvailableAn experimental investigation into the breakdown of riblet drag reduction at postoptimal conditionsNewton, R ; Chung, D ; Hutchins, N (Australasian Fluid Mechanics Society, 20180101)A longstanding question in riblet research is why drag reduction only occurs within a small, nondimensionally scaled envelope, outside of which drag is significantly increased. For riblets with viscousscaled spacings that are much larger than those required for drag reduction, one hypothesis is that the riblets exhibit ktype, ‘fully rough’ behaviour. However, this seems counterintuitive since fully rough behaviour is typically associated with a dominance of pressure drag over viscous drag, and yet riblets can sustain no pressure drag. This study aims to investigate this issue by conducting single normal hotwire traverses above a trapezoidal riblet surface, over a range of dragincreasing viscousscaled riblet spacings. Novelty was added by also measuring within the riblet valleys, providing a unique look at the turbulent behaviour within them. Previously proposed mechanisms for the breakdown in drag reduction have included lodgement of turbulence within the riblet valleys, and the development of a Kelvin–Helmholtz instability, but neither mechanism appears active in our results. They instead show a reduction in turbulent energy as riblet spacing increases, despite a significant increase in drag, which does seem to be approaching a ktype roughness asymptote as hypothesised. This may be caused by the generation of timeinvariant secondary flows above the riblet tips and corners of the riblet valleys, although this will require further investigation.

ItemForminduced stress in turbulent flow over ribletsModesti, D ; Endrikat, S ; GarcíaMayoral, R ; Hutchins, N ; Chung, D (Australasian Fluid Mechanics Society, 20180101)We carry out direct numerical simulation of minimal openchannel flow over riblets. Several riblet geometries are simulated, namely symmetric triangular, asymmetric triangular, blade and trapezoidal, and with this unprecedented highfidelity dataset, we are able to obtain broad insights into the flow physics of riblets. We find that the roughness sublayer thickness, above which the flow is statistically homogeneous, is proportional to the square root of the riblet groove crosssectional area ℓ+ g in both the dragreducing and the dragincreasing regime, consistent with the ability of this parameter to collapse the roughness function corresponding to different groove geometries. Large grooves are associated with mean secondary velocities and they carry additional stress that contributes up to 40% of the total shear stress at the crest, comparable to the contribution from the turbulent fluctuations.