Mechanical Engineering - Research Publications

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    The effect of cleaning and repainting on the ship drag penalty
    Utama, IKAP ; Nugroho, B ; Yusuf, M ; Prasetyo, FA ; Hakim, ML ; Suastika, IK ; Ganapathisubramani, B ; Hutchins, N ; Monty, JP (TAYLOR & FRANCIS LTD, 2021-04-12)
    Although the hull of a recently dry-docked large ship is expected to be relatively smooth, surface scanning and experimentation reveal that it can exhibit an "orange-peel" roughness pattern with an equivalent sand-grain roughness height ks = 0. 101 mm. Using the known ks value and integral boundary layer evolution, a recently cleaned and coated full-scale ship was predicted to experience a significant increase in the average coefficient of friction %ΔC¯f and total hydrodynamic resistance %ΔR¯T during operation. Here the report also discusses two recently reported empirical estimations that can estimate ks directly from measured surface topographical parameters, by-passing the need for experiments on replicated surfaces. The empirical estimations are found to have an accuracy of 4.5 - 5 percentage points in %ΔC¯f.
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    Non-k-type behaviour of roughness when in-plane wavelength approaches the boundary layer thickness
    Nugroho, B ; Monty, JP ; Utama, IKAP ; Ganapathisubramani, B ; Hutchins, N (CAMBRIDGE UNIV PRESS, 2021-01-22)
    Abstract
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    Heat Transfer Coefficient Estimation for Turbulent Boundary Layers
    Wang, S ; Xia, Y ; Abu Rowin, W ; Marusic, I ; Sandberg, R ; Chung, D ; Hutchins, N ; Tanimoto, K ; Oda, T (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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    An investigation of cold-wire spatial resolution using a DNS database
    Xia, Y ; Rowin, W ; Jelly, T ; Chung, D ; Marusic, I ; Hutchins, N (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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    The Effects of Anisotropic Surface Roughness on Turbulent Boundary-Layer Flow
    Ramani, A ; Nugroho, B ; Busse, A ; Monty, J ; Hutchins, N ; Jelly, T (The University of Queensland, 2020-12-11)
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    Dispersive stresses in turbulent flow over riblets
    Modesti, D ; Endrikat, S ; Hutchins, N ; Chung, D (Cambridge University Press, 2021-06-25)
    We carry out direct numerical simulations of turbulent flow over riblets, streamwise- aligned grooves that are designed to reduce drag by modifying the near-wall 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 minimal-channel flow configuration. With this unprecedented breadth of high-fidelity flow data near the wall, we are able to obtain more general insights into the flow physics of riblets. As observed by García-Mayoral & Jiménez (J. Fluid Mech., vol. 678, 2011, pp. 317-347), we confirm that the drag-change curves of all the present groove geometries better collapse when reported with the viscous-scaled square root of the groove area, rather than the riblet spacing. Using a two-dimensional generalization of the Fukagata-Iwamoto-Kasagi identity in difference form we isolate the different drag-change 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 pre-eminence of dispersive stresses can be estimated by the groove width at the riblet mean height.
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    Aerosolisation in endonasal endoscopic pituitary surgery
    Dhillon, RS ; Nguyen, L ; Abu Rowin, W ; Humphries, RS ; Kevin, K ; Ward, JD ; Yule, A ; Phan, TD ; Zhao, YC ; Wynne, D ; McNeill, PM ; Hutchins, N ; Scott, DA (SPRINGER, 2021-01-19)
    PURPOSE: To determine the particle size, concentration, airborne duration and spread during endoscopic endonasal pituitary surgery in actual patients in a theatre setting. METHODS: This observational study recruited a convenience sample of three patients. Procedures were performed in a positive pressure operating room. Particle image velocimetry and spectrometry with air sampling were used for aerosol detection. RESULTS: Intubation and extubation generated small particles (< 5 µm) in mean concentrations 12 times greater than background noise (p < 0.001). The mean particle concentrations during endonasal access were 4.5 times greater than background (p = 0.01). Particles were typically large (> 75 µm), remained airborne for up to 10 s and travelled up to 1.1 m. Use of a microdebrider generated mean aerosol concentrations 18 times above baseline (p = 0.005). High-speed drilling did not produce aerosols greater than baseline. Pituitary tumour resection generated mean aerosol concentrations less than background (p = 0.18). Surgical drape removal generated small and large particles in mean concentrations 6.4 times greater than background (p < 0.001). CONCLUSION: Intubation and extubation generate large amounts of small particles that remain suspended in air for long durations and disperse through theatre. Endonasal access and pituitary tumour resection generate smaller concentrations of larger particles which are airborne for shorter periods and travel shorter distances.
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    Nasal preparation with local anesthetic should be considered an aerosol-generating procedure
    Dhillon, RS ; Nguyen, LV ; Rowin, WA ; Humphries, RS ; Kevin, K ; Ward, JD ; Yule, A ; Phan, TD ; Wynne, D ; McNeill, PM ; Hutchins, N ; Scott, DA ; Zhao, YC (WILEY, 2020-12-21)
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    Influence of riblet shapes on the occurrence of Kelvin-Helmholtz rollers
    Endrikat, S ; Modesti, D ; Garcia-Mayoral, R ; Hutchins, N ; Chung, D (CAMBRIDGE UNIV PRESS, 2021-03-02)
    We investigate turbulent flow over streamwise-aligned riblets (grooves) of various shapes and sizes. Small riblets with spacings of typically less than 20 viscous units are known to reduce skin-friction drag compared to a smooth wall, but larger riblets allow inertial-flow mechanisms to appear and cause drag reduction to break down. One of these mechanisms is a Kelvin–Helmholtz instability that García-Mayoral & 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 minimal-span 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 sharp-triangular and blade riblets have a drag penalty associated with the Kelvin–Helmholtz instability and that the mechanism appears to be absent for blunt-triangular 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 cross-sectional area in viscous units serves as a proxy for the wall-normal 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.
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    Aerosolisation during tracheal intubation and extubation in an operating theatre setting
    Dhillon, RS ; Rowin, WA ; Humphries, RS ; Kevin, K ; Ward, JD ; Phan, TD ; Nguyen, LV ; Wynne, DD ; Scott, DA (WILEY, 2020-11-03)
    Aerosol-generating procedures such as tracheal intubation and extubation pose a potential risk to healthcare workers because of the possibility of airborne transmission of infection. Detailed characterisation of aerosol quantities, particle size and generating activities has been undertaken in a number of simulations but not in actual clinical practice. The aim of this study was to determine whether the processes of facemask ventilation, tracheal intubation and extubation generate aerosols in clinical practice, and to characterise any aerosols produced. In this observational study, patients scheduled to undergo elective endonasal pituitary surgery without symptoms of COVID-19 were recruited. Airway management including tracheal intubation and extubation was performed in a standard positive pressure operating room with aerosols detected using laser-based particle image velocimetry to detect larger particles, and spectrometry with continuous air sampling to detect smaller particles. A total of 482,960 data points were assessed for complete procedures in three patients. Facemask ventilation, tracheal tube insertion and cuff inflation generated small particles 30-300 times above background noise that remained suspended in airflows and spread from the patient's facial region throughout the confines of the operating theatre. Safe clinical practice of these procedures should reflect these particle profiles. This adds to data that inform decisions regarding the appropriate precautions to take in a real-world setting.