Mechanical Engineering - Research Publications
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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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ItemApplying Machine Learnt Explicit Algebraic Stress and Scalar Flux Models to a Fundamental Trailing Edge Slot(American Society of Mechanical Engineers, 2018)A form of supervised machine learning was applied to highly resolved large-eddy simulation (LES) data to develop non linear turbulence stress and heat flux closures with increased prediction accuracy for trailing-edge cooling slot cases. The LES data were generated for a thick and a thin trailing-edge slot and shown to agree well with experimental data, thus providing suitable training data for model development. A Gene Expression Programming (GEP) based algorithm was used to symbolically regress novel nonlinear Explicit Algebraic Stress Models (EASM) and heat-flux closures based on either the gradient diffusion or the generalized gradient diffusion approaches. Following a-priori assessment, the new models were used for steady RANS calculations of both thin and thick trailing-edge slot geometries, testing their performance and robustness. Overall, the best agreement with LES data was found when training the RANS model in the near wall region where high levels of anisotropy exist and using the mean squared error of the anisotropy tensor as cost function. In the case of the thin lip geometry, combining an improved EASM model with the standard eddy-diffusivity model predicted the adiabatic wall effectiveness in good agreement with the LES and experimental data. Crucially, the obtained model was also applied to different blowing ratios of the thin lip geometry and a significant improvement in the predictive accuracy of adiabatic wall effectiveness was observed for those cases not previously seen in the training process. For the thick lip case the match with reference values deteriorated due to the presence of large-scale, relative to the slot height, vortex shedding. The machine-learning algorithm was therefore also used to ‘learn’ an appropriate closure for the turbulent heat flux vector. The constructed scalar flux model, in conjunction with a trained RANS model, was found to have the capability to further improve the prediction of the adiabatic wall effectiveness.
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ItemCFD simulations of vertical surface piercing circular cylinders and comparison against experiments(Australian fluid mechanics society, 2018)When predicting the susceptibility of a submarine to above water detection, it is important to consider the impact of the wake generated by the periscope(s). Computational Fluid Dynamics (CFD) tools can be used to predict the physical size and shape of the wake, which can be combined with periscope models for input into detectability prediction models. For this application, it is important that CFD predictions of the wake are accurate not only in the mean calculations, but that the physical characteristics of the wake are captured at instantaneous snapshots in time. In a previous experimental study, Keough et al. [10] presented time resolved measurements of the wake from vertical surface piercing cylinders, utilising an automated method of extracting these measurements as a function of time from video recordings of the experiment. In the present work, CFD simulations have been performed to model this experimental data set. The open source CFD software Caelus was used, with the improved Defence Science and Technology Group version of vofSolver—the multiphase volume of fluid solver. A numerical wave gauge is implemented in order to measure the free surface elevation during the simulation and this data is compared to bow wave data obtained from animations of the CFD results, using the same automated visual tracking technique utilised for the experimental measurements. Analysis of these time-resolved measurements is performed, comparing transient statistics and spectral characteristics of the CFD predictions against the experimental data.
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ItemDirect Numerical Simulation of Confined Wall Plumes(Australian fluid mechanics society, 2018)We present results from the direct numerical simulation (DNS) of a wall attached thermal plume in a confined box. The plume originates from a local line heat source of length, L, placed at the bottom left corner of the box. The Reynolds number of the wall plume, based on box height and buoyant velocity scale, is ReH = 14530 and a parametric study is carried out for boxes of two different aspect ratios (ratio of box width to box height) for a particular value of L. In the simulation, the plume develops along the vertical side wall while remaining attached to it before spreading across the top wall to form a buoyant fluid layer and eventually moving downwards and filling the whole box. Further, the original filling box model of Baines and Turner [1] is modified to incorporate the wall shear stress and the results from the DNS are compared against it. A reasonable agreement is observed for the volume and momentum fluxes in the quiescent uniform environment and also for the time-dependent buoyancy profile calculated far away from the plume.
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ItemLarge Eddy Simulation of Flow Over Streamwise Heterogeneous Canopies: Quadrant Analysis(Australian fluid mechanics society, 2018)Large eddy simulations of flow over heterogeneous forest canopies are performed. Each simulated forest consists of equal-sized strip canopies which alternate in the streamwise direction between sparse and dense leaf area density. Quadrant analysis is then used to investigate the eddy fluxes near the top of the forest with an eye towards developing a parameterisation of particle motion through the forest. The quadrant analysis demonstrates that the sweep-ejection cycle is modified by the canopy heterogeneities. The greatest modifications occur with the largest difference in leaf area densities. Ejections appear to be enhanced over canopy heterogeneities of intermediate length. Forests with large length-scale heterogeneities and forests with short length-scale heterogeneities are qualitatively similar to uniform canopies.
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ItemTime Resolved Measurements of Wake Characteristics from Vertical Surface-Piercing Circular Cylinders(Australasian Fluid Mechanics Society, 2016)
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ItemNo Preview AvailableNumerical investigation of the behaviour of wall shear stress in three-dimensional pulsatile stenotic flows(Australasian Fluid Mechanics Society (AFMS), 2014)