Mechanical Engineering - Research Publications
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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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ItemReynolds-number-dependent turbulent inertia and onset of log region in pipe flows(CAMBRIDGE UNIV PRESS, 2014-10)Abstract A detailed analysis of the ‘turbulent inertia’ (TI) term (the wall-normal gradient of the Reynolds shear stress,$\mathrm{d} \langle -uv\rangle /\mathrm{d} y $), in the axial mean momentum equation is presented for turbulent pipe flows at friction Reynolds numbers$\delta ^{+} \approx 500$, 1000 and 2000 using direct numerical simulation. Two different decompositions for TI are employed to further understand the mean structure of wall turbulence. In the first, the TI term is decomposed into the sum of two velocity–vorticity correlations ($\langle v \omega _z \rangle + \langle - w \omega _y \rangle $) and their co-spectra, which we interpret as an advective transport (vorticity dispersion) contribution and a change-of-scale effect (associated with the mechanism of vorticity stretching and reorientation). In the second decomposition, TI is equivalently represented as the wall-normal gradient of the Reynolds shear stress co-spectra, which serves to clarify the accelerative or decelerative effects associated with turbulent motions at different scales. The results show that the inner-normalised position,$y_m^{+}$, where the TI profile crosses zero, as well as the beginning of the logarithmic region of the wall turbulent flows (where the viscous force is leading order) move outwards in unison with increasing Reynolds number as$y_m^{+} \sim \sqrt{\delta ^{+}}$because the eddies located close to$y_m^{+}$are influenced by large-scale accelerating motions of the type$\langle - w \omega _y \rangle $related to the change-of-scale effect (due to vorticity stretching). These large-scale motions of$O(\delta ^{+})$gain a spectrum of larger length scales with increasing$\delta ^{+}$and are related to the emergence of a secondary peak in the$-uv$co-spectra. With increasing Reynolds number, the influence of the$O(\delta ^{+})$motions promotes viscosity to act over increasingly longer times, thereby increasing the$y^{+}$extent over which the mean viscous force retains leading order. Furthermore, the TI decompositions show that the$\langle v \omega _z \rangle $motions (advective transport and/or dispersion of vorticity) are the dominant mechanism in and above the log region, whereas$\langle - w \omega _y \rangle $motions (vorticity stretching and/or reorientation) are most significant below the log region. The motions associated with$\langle - w \omega _y \rangle $predominantly underlie accelerations, whereas$\langle v \omega _z \rangle $primarily contribute to decelerations. Finally, a description of the structure of wall turbulence deduced from the present analysis and our physical interpretation is presented, and is shown to be consistent with previous flow visualisation studies.