Mechanical Engineering - Research Publications
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ItemHeat Transfer Coefficient Estimation for Turbulent Boundary Layers(The University of Queensland, 2020-12-11)Convective heat transfer in rough wall-bounded 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 non-smooth 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.
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ItemAn investigation of cold-wire spatial resolution using a DNS database(The University of Queensland, 2020-12-11)The effect of spatial resolution of cold-wire 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 smooth-wall 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 two-dimensional 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 near-wall energetic peak is observed when the filter length exceeds 100 wall units.
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ItemNo Preview AvailableThe Effects of Anisotropic Surface Roughness on Turbulent Boundary-Layer Flow(The University of Queensland, 2020-12-11)
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ItemRoughness and Reynolds Number Effects on the Flow Past a Rough-to-Smooth Step Change(Springer International Publishing, 2019)We report direct numerical simulations (DNSs) of open-channel flow with a step change from three-dimensional sinusoidal rough surface to smooth surface. We investigate the persistence of non-equilibrium behaviour beyond this step change (i.e. departures from the equilibrium smooth open-channel 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 (step-up) and above (step-down) the smooth patch. To study (2), the equivalent sand-grain roughness of the aligned case was decreased from k+s≃ks+≃ 160 to k+s≃106ks+≃106. To study (3) and (4) the step-down case at Reτ≃395Reτ≃395 was compared with a backward-facing step case at Reτ≃527Reτ≃527, and DNS of square rib rough-to-smooth 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 near-wall amplification of Reynolds shear stress, which in turn depends on ϵ/hϵ/h, i.e. higher ϵ/hϵ/h leads to higher amplification.
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ItemDirect numerical simulation of high aspect ratio spanwise-aligned bars(Cambridge University Press (CUP), 2017-01-01)We conduct minimal-channel direct numerical simulations of turbulent flow over two-dimensional 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 outer-layer length scale (pipe diameter, channel half height 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 minimal-span rough-wall 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 trough-to-crest height (k) of the roughness while keeping the width between roughness bars, W, fixed in wall units, results in non-k-type 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.
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ItemNo Preview AvailableAn experimental investigation into the breakdown of riblet drag reduction at post-optimal conditions(Australasian Fluid Mechanics Society, 2018-01-01)A long-standing question in riblet research is why drag reduction only occurs within a small, non-dimensionally scaled envelope, outside of which drag is significantly increased. For riblets with viscous-scaled spacings that are much larger than those required for drag reduction, one hypothesis is that the riblets exhibit k-type, ‘fully rough’ behaviour. However, this seems counter-intuitive 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 hot-wire traverses above a trapezoidal riblet surface, over a range of drag-increasing viscous-scaled 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 k-type roughness asymptote as hypothesised. This may be caused by the generation of time-invariant secondary flows above the riblet tips and corners of the riblet valleys, although this will require further investigation.
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ItemForm-induced stress in turbulent flow over riblets(Australasian Fluid Mechanics Society, 2018-01-01)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 high-fidelity 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 cross-sectional area ℓ+ g in both the drag-reducing and the drag-increasing 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.
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ItemKelvin–Helmholtz rollers in turbulent flow over riblets(Australasian Fluid Mechanics Society, 2018-01-01)Structures resulting from a Kelvin–Helmholtz instability have been shown to contribute to skin-friction drag in turbulent flow over blade-shaped riblets [4]. Using Direct Numerical Simulation (DNS) data, the present survey of several different riblet shapes reveals that the contribution to wall-shear stress due to the Kelvin–Helmholtz instability depends on riblet shape, in addition to a previously known dependence on riblet size. For a given drag change, sharp triangular and blade riblets promote development of the instability whilst blunt triangular and trapezoidal riblets appear to suppress it.
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ItemDEVELOPMENT AND USE OF MACHINE-LEARNT ALGEBRAIC REYNOLDS STRESS MODELS FOR ENHANCED PREDICTION OF WAKE MIXING IN LPTS(AMER SOC MECHANICAL ENGINEERS, 2018-01-01)Non-linear turbulence closures were developed that improve the prediction accuracy of wake mixing in low-pressure turbine (LPT) flows. First, Reynolds-averaged Navier-Stokes (RANS) calculations using five linear turbulence closures were performed for the T106A LPT profile at exit Mach number 0.4 and isentropic exit Reynolds numbers 60,000 and 100,000. None of these RANS models were able to accurately reproduce wake loss profiles, a crucial parameter in LPT design, from direct numerical simulation (DNS) reference data. However, the recently proposed kv2w transition model was found to produce the best agreement with DNS data in terms of blade loading and boundary layer behavior and thus was selected as baseline model for turbulence closure development. Analysis of the DNS data revealed that the linear stress-strain coupling constitutes one of the main model form errors. Hence, a geneexpression programming (GEP) based machine-learning technique was applied to the high-fidelity DNS data to train non-linear explicit algebraic Reynolds stress models (EARSM). In particular, the GEP algorithm was tasked to minimize the weighted difference between the DNS and RANS anisotropy tensors, using different training regions. The trained models were first assessed in an a priori sense (without running any CFD) and showed much improved alignment of the trained models in the region of training. Additional RANS calculations were then performed using the trained models. Importantly, to assess their robustness, the trained models were tested both on the cases they were trained for and on testing, i.e. previously not seen, cases with different flow features. The developed models improved prediction of the Reynolds stress, TKE production, wake-loss profiles and wake maturity, across all cases, in particular those trained on just the wake region.
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ItemTurbulent flow over spanwise-varying roughness in a minimal streamwise channel(IOP Publishing, 2020-06-10)All rights reserved. We report direct numerical simulations in a minimal streamwise domain of turbulent channel flow over spanwise-alternating patches of rough and smooth walls. Despite the minimal streamwise domain overpredicting streamwise-velocity fluctuations and inhibiting the meandering of long turbulent structures, it captures the rotational behaviour of mean secondary flows also observed in other studies with spanwise-varying roughness. To extend the study of spanwise-varying roughness, we prescribe a lateral velocity to the wall roughness to mimic flow over oblique patches of roughness. Far from the wall, long-lived turbulent structures are convected in the direction of the moving roughness, but their speeds are only weakly perturbed from a preferential value of around 40% of the friction velocity. The turbulence-driven secondary flows laterally convect at comparable speeds, but depend on the roughness patch width.