Mechanical Engineering - Research Publications
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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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ItemManipulation of near-wall turbulence by surface slip and permeability(IOP PUBLISHING LTD, 2018-01-01)We study the effect on near-wall turbulence of tangential slip and wall-normal transpiration, typically produced by textured surfaces and other surface manipulations. For this, we conduct direct numerical simulations (DNSs) with different virtual origins for the different velocity components. The different origins result in a relative wall-normal displacement of the near-wall, quasi-streamwise vortices with respect to the mean flow, which in turn produces a change in drag. The objective of this work is to extend the existing understanding on how these virtual origins affect the flow. In the literature, the virtual origins for the tangential velocities are typically characterised by slip boundary conditions, while the wall-normal velocity is assumed to be zero at the boundary plane. Here we explore different techniques to define and implement the three virtual origins, with special emphasis on the wall-normal one. We investigate impedance conditions relating the wall-normal velocity to the pressure, and linear relations between the velocity components and their wall-normal gradients, as is typically done to impose slip conditions. These models are first tested to represent a smooth wall below the boundary plane, with all virtual origins equal, and later for different tangential and wall-normal origins. Our results confirm that the change in drag is determined by the offset between the origins perceived by mean flow and the quasi-streamwise vortices or, more generally, the near-wall turbulent cycle. The origin for the latter, however, is not set by the spanwise virtual origin alone, as previously proposed, but by a combination of the spanwise and wall-normal origins, and mainly determined by the shallowest of the two. These observations allow us to extend the existing expression to predict the change in drag, accounting for the wall-normal effect when the transpiration is not negligible.
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ItemContribution of dispersive stress to skin friction drag in turbulent flow over riblets(Darmstadt University of Technology, 2019)We carry out direct numerical simulations (DNSs) of minimal open-channel flow over riblets, which are streamwise-aligned grooves that modify the near-wall flow for drag reduction. Several riblet sizes and cross-sectional geometries are simulated, namely symmetric triangular, asymmetric triangular, blade and trapezoidal. With this unprecedented breadth and detail afforded by the DNS data, we are able to obtain more general insights into the flow physics of riblets. A generalization of the Fukagata–Iwamoto–Kasagi (FIK) identity is used to isolate the different contributions to skin friction drag changes. We show that, in the nonlinear regime of large riblet size, the dispersive contribution is comparable or larger than the turbulent one, representing an important mechanism to the breakdown of drag reduction.
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ItemCharacteristics of the entrainment velocity in a developing wake(International Symposium on Turbulence and Shear Flow Phenomena, 2015)