- Mechanical Engineering - Research Publications
Mechanical Engineering - Research Publications
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ItemIdentifying regions of importance in wall-bounded turbulence through explainable deep learning.Cremades, A ; Hoyas, S ; Deshpande, R ; Quintero, P ; Lellep, M ; Lee, WJ ; Monty, JP ; Hutchins, N ; Linkmann, M ; Marusic, I ; Vinuesa, R (Springer Science and Business Media LLC, 2024-05-13)Despite its great scientific and technological importance, wall-bounded turbulence is an unresolved problem in classical physics that requires new perspectives to be tackled. One of the key strategies has been to study interactions among the energy-containing coherent structures in the flow. Such interactions are explored in this study using an explainable deep-learning method. The instantaneous velocity field obtained from a turbulent channel flow simulation is used to predict the velocity field in time through a U-net architecture. Based on the predicted flow, we assess the importance of each structure for this prediction using the game-theoretic algorithm of SHapley Additive exPlanations (SHAP). This work provides results in agreement with previous observations in the literature and extends them by revealing that the most important structures in the flow are not necessarily the ones with the highest contribution to the Reynolds shear stress. We also apply the method to an experimental database, where we can identify structures based on their importance score. This framework has the potential to shed light on numerous fundamental phenomena of wall-bounded turbulence, including novel strategies for flow control.
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ItemNo Preview AvailableThe effect of cleaning and repainting on the ship drag penaltyUtama, IKAP ; Nugroho, B ; Yusuf, M ; Prasetyo, FA ; Hakim, ML ; Suastika, IK ; Ganapathisubramani, B ; Hutchins, N ; Monty, JP (TAYLOR & FRANCIS LTD, 2021-07-01)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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ItemNo Preview AvailableNon-k-type behaviour of roughness when in-plane wavelength approaches the boundary layer thicknessNugroho, B ; Monty, JP ; Utama, IKAP ; Ganapathisubramani, B ; Hutchins, N (CAMBRIDGE UNIV PRESS, 2021-01-22)Abstract
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ItemA direct measure of the frequency response of hot-wire anemometers: temporal resolution issues in wall-bounded turbulenceHutchins, N ; Monty, JP ; Hultmark, M ; Smits, AJ (SPRINGER, 2015-01)
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ItemSimulation of large-eddy-break-up device (LEBU) in a moderate Reynolds number turbulent boundary layerChin, C ; Monty, J ; HUTCHINS, N ; Ooi, A ; Orlu, R ; Schlatter, P (Springer, 2016-08-11)A well-resolved large eddy simulation (LES) of a large-eddy break-up (LEBU) device in a spatially evolving turbulent boundary layer is performed with, Reynolds number, based on free-stream velocity and momentum-loss thickness, of R e θ ≈ 4300. The implementation of the LEBU is via an immersed boundary method. The LEBU is positioned at a wall-normal distance of 0.8 δ (δ denoting the local boundary layer thickness at the location of the LEBU) from the wall. The LEBU acts to delay the growth of the turbulent boundary layer and produces global skin friction reduction beyond 180δ downstream of the LEBU, with a peak local skin friction reduction of approximately 12 %. However, no net drag reduction is found when accounting for the device drag of the LEBU in accordance with the towing tank experiments by Sahlin et al. (Phys. Fluids 31, 2814, 1988). Further investigation is performed on the interactions of high and low momentum bulges with the LEBU and the corresponding output is analysed, showing a ‘break-up’ of these large momentum bulges downstream of the LEBU. In addition, results from the spanwise energy spectra show consistent reduction in energy at spanwise length scales for λ+z>1000 independent of streamwise and wall-normal location when compared to the corresponding turbulent boundary layer without LEBU.
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ItemStructure Inclination Angles in the Convective Atmospheric Surface LayerChauhan, K ; Hutchins, N ; Monty, J ; Marusic, I (SPRINGER, 2013-04)
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ItemTowards Reconciling the Large-Scale Structure of Turbulent Boundary Layers in the Atmosphere and LaboratoryHutchins, N ; Chauhan, K ; Marusic, I ; Monty, J ; Klewicki, J (SPRINGER, 2012-11)
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ItemValidating under-resolved turbulence intensities for PIV experiments in canonical wall-bounded turbulenceLee, JH ; Kevin, ; Monty, JP ; Hutchins, N (SPRINGER, 2016-08)
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ItemNo Preview AvailableThe Effects of Anisotropic Surface Roughness on Turbulent Boundary-Layer FlowRamani, A ; Nugroho, B ; Busse, A ; Monty, JP ; Hutchins, N ; Jelly, TO (The University of Queensland, 2020-01-01)
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ItemTurbulent structures in a statistically three-dimensional boundary layerKevin, ; Monty, J ; Hutchins, N (Cambridge University Press (CUP), 2019-01-25)We investigate the behaviour of large-scale coherent structures in a spanwise-heterogeneous turbulent boundary layer, using particle image velocimetry on multiple orthogonal planes. The statistical three-dimensionality is imposed by a herringbone riblet surface, although the key results presented here will be common to many cases of wall turbulence with embedded secondary flows in the form of mean streamwise vortices. Instantaneous velocity fields in the logarithmic layer reveal elongated low-momentum streaks located over the upwash-flow region, where their spanwise spacing is forced by the 2δ periodicity of the herringbone pattern. These streaks largely resemble the turbulence structures that occur naturally (and randomly located) in spanwise-homogeneous smooth-/rough-wall boundary layers, although here they are directly formed by the roughness pattern. In the far outer region, the large spanwise spacing permits the streaks to aggressively meander. The mean secondary flows are the time-averaged artefact of the unsteady and spanwise asymmetric large-scale roll modes that accompany these meandering streaks. Interestingly, this meandering, or instability, gives rise to a pronounced streamwise periodicity (i.e. an alternating coherent pattern) in the spatial statistics, at wavelengths of approximately 4.5 δ . Overall, the observed behaviours largely resemble the streak-instability model that has been proposed for the buffer region, only here at a much larger scale and at a forced spanwise spacing. This observation further confirms recent observations that such features may occur at an entire hierarchy of scales throughout the turbulent boundary layer.