ORCID Profile
0000-0002-3883-8648
Current Organisations
Monash University
,
University of Melbourne Melbourne School of Engineering
,
Princeton University
Does something not look right? The information on this page has been harvested from data sources that may not be up to date. We continue to work with information providers to improve coverage and quality. To report an issue, use the Feedback Form.
In Research Link Australia (RLA), "Research Topics" refer to ANZSRC FOR and SEO codes. These topics are either sourced from ANZSRC FOR and SEO codes listed in researchers' related grants or generated by a large language model (LLM) based on their publications.
Turbulent Flows | Interdisciplinary Engineering | Theoretical and Applied Mechanics | Geophysical Fluid Dynamics
Publisher: Cambridge University Press (CUP)
Date: 11-08-2021
DOI: 10.1017/JFM.2021.640
Publisher: The Royal Society
Date: 07-2019
Abstract: Dolphin skin has long been an inspiration for research on drag reduction mechanisms due to the presence of skin ridges that could reduce fluid resistance. We gathered in vivo three-dimensional surface data on the skin from five species of odontocetes to quantitatively examine skin texture, including the presence and size of ridges. We used these data to calculate k + values, which relate surface geometry to changes in boundary layer flow. Our results showed that while ridge size differs among species, odontocete skin was surprisingly smooth compared to the skin of other swimmers (average roughness = 5.3 µm). In addition, the presence of ridges was variable among in iduals of the same species. We predict that odontocete skin ridges do not alter boundary layer flows at cruising swimming speeds. By combining k + values and morphological data, our work provides evidence that skin ridges are unlikely to be an adaptation for drag reduction and that odontocete skin is exceptionally smooth compared to other pelagic swimmers.
Publisher: American Physical Society (APS)
Date: 08-06-2023
Publisher: Elsevier BV
Date: 06-2023
Publisher: American Physical Society (APS)
Date: 15-08-2022
Publisher: Cambridge University Press (CUP)
Date: 15-10-2020
DOI: 10.1017/JFM.2020.704
Publisher: Cambridge University Press (CUP)
Date: 28-07-2023
DOI: 10.1017/JFM.2023.498
Abstract: We present measurements of turbulent drag reduction (DR) in boundary layers at high friction Reynolds numbers in the range of $4500 \\le Re_\\tau \\le 15\\ 000$ . The efficacy of the approach, using streamwise travelling waves of spanwise wall oscillations, is studied for two actuation regimes: (i) inner-scaled actuation (ISA), as investigated in Part 1 of this study, which targets the relatively high-frequency structures of the near-wall cycle, and (ii) outer-scaled actuation (OSA), which was recently presented by Marusic et al. ( Nat. Commun. , vol. 12, 2021) for high- $Re_\\tau$ flows, targeting the lower-frequency, outer-scale motions. Multiple experimental techniques were used, including a floating-element balance to directly measure the skin-friction drag force, hot-wire anemometry to acquire long-time fluctuating velocity and wall-shear stress, and stereoscopic particle image velocimetry to measure the turbulence statistics of all three velocity components across the boundary layer. Under the ISA pathway, DR of up to 25 % was achieved, but mostly with net power saving (NPS) losses due to the high-input power cost associated with the high-frequency actuation. The low-frequency OSA pathway, however, with its lower input power requirements, was found to consistently result in positive NPS of 5–10 % for moderate DRs of 5–15 %. The results suggest that OSA is an attractive pathway for energy-efficient DR in high-Reynolds-number applications.
Publisher: Cambridge University Press (CUP)
Date: 04-07-2019
DOI: 10.1017/JFM.2019.284
Abstract: Theory and modelling remain central to improving our understanding of undulatory and oscillatory swimming. Simple models based on added mass can help to give great insight into the mechanics of undulatory swimming, as demonstrated by animals such as eels, stingrays and knifefish. To understand the swimming of oscillatory swimmers such as tuna and dolphins, models need to consider both added mass forces and circulatory forces. For all types of swimming, experiments and theory agree that the most important velocity scale is the characteristic lateral velocity of the tail motion rather than the swimming speed, which erases to a large extent the difference between results obtained in a tethered mode, compared to those obtained using a free swimming condition. There is no one-to-one connection between the integrated swimming performance and the details of the wake structure, in that similar levels of efficiency can occur with very different wake structures. Flexibility and viscous effects play crucial roles in determining the efficiency, and for isolated propulsors changing the profile shape can significantly improve both thrust and efficiency. Also, combined heave and pitch motions with an appropriate phase difference are essential to achieve high performance. Reducing the aspect ratio will always reduce thrust and efficiency, but its effects are now reasonably well understood. Planform shape can have an important mitigating influence, as do non-sinusoidal gaits and intermittent actuation.
Publisher: Cambridge University Press (CUP)
Date: 28-07-2023
DOI: 10.1017/JFM.2023.499
Abstract: Turbulent drag reduction (DR) through streamwise travelling waves of the spanwise wall oscillation is investigated over a wide range of Reynolds numbers. Here, in Part 1, wall-resolved large-eddy simulations in a channel flow are conducted to examine how the frequency and wavenumber of the travelling wave influence the DR at friction Reynolds numbers $Re_\\tau = 951$ and $4000$ . The actuation parameter space is restricted to the inner-scaled actuation (ISA) pathway, where DR is achieved through direct attenuation of the near-wall scales. The level of turbulence attenuation, hence DR, is found to change with the near-wall Stokes layer protrusion height $\\ell _{0.01}$ . A range of frequencies is identified where the Stokes layer attenuates turbulence, lifting up the cycle of turbulence generation and thickening the viscous sublayer in this range, the DR increases as $\\ell _{0.01}$ increases up to $30$ viscous units. Outside this range, the strong Stokes shear strain enhances near-wall turbulence generation leading to a drop in DR with increasing $\\ell _{0.01}$ . We further find that, within our parameter and Reynolds number space, the ISA pathway has a power cost that always exceeds any DR savings. This motivates the study of the outer-scaled actuation pathway in Part 2, where DR is achieved through actuating the outer-scaled motions.
Publisher: Elsevier BV
Date: 12-2023
Publisher: The Royal Society
Date: 30-01-2023
Abstract: Taylor–Couette flow with a low aspect ratio cylinder suffers from end effects due to the finite-span of the gap between the cylinder sides and the secondary flow in the region below the inner cylinder. We experimentally explore these end effects by varying the cylinder aspect ratio between 6.67 and 40 for a range of wall gap widths and bottom gap heights. For these geometries, end effects (i.e. non-ideal Taylor–Couette flow) can be substantial due to both features of the finite-span and the bottom secondary flow. In some cases, the finite-span effects extended between 20% and 30% of the way into the Taylor–Couette flow region, and the secondary flow at the bottom accounted for nearly half of the total measured torque. By taking these effects into consideration, our high aspect ratio results agreed well with those obtained by Taylor (Taylor 1936 Proc. R. Soc. Lond. A 157 , 546–564. (doi: 10.1098/rspa.1936.0215 )) at considerably higher aspect ratios. This article is part of the theme issue ‘Taylor–Couette and related flows on the centennial of Taylor’s seminal Philosophical Transactions paper (part 1)’.
Publisher: Elsevier BV
Date: 12-2018
Publisher: Cambridge University Press (CUP)
Date: 26-09-2023
DOI: 10.1017/JFM.2023.715
Publisher: Cambridge University Press (CUP)
Date: 16-06-2023
DOI: 10.1017/JFM.2023.417
Abstract: The wall dependence of length scales used to describe large- and small-scale structures of turbulence is examined using highly resolved experiments in zero-pressure-gradient turbulent boundary layers and pipe flows spanning the range $2000 Re_\\tau \\ 700$ . Of particular interest is the influence of external intermittency on the scaling of these length scales. It is found that when suitable scaling parameters are selected and external intermittency is accounted for, the dissipative motions follow inner scaling even into the outer-scaled regions of the flow, and that certain large-scale descriptions follow outer scaling even in the inner-scaled regions of the flow. The wall dependence is the same for both internal pipe and external boundary layer flows, and the different length scales can be related to recognizable features in the longitudinal wavenumber spectrum.
Publisher: MDPI AG
Date: 04-06-2021
Abstract: Pipe flow responds to strong perturbations in ways that are fundamentally different from the response exhibited by boundary layers undergoing a similar perturbation, primarily because of the confinement offered by the pipe wall, and the need to satisfy continuity. We review such differences by examining previous literature, with a particular focus on the response of pipe flow to three different kinds of disturbances: the abrupt change in surface condition from rough to smooth, the obstruction due to presence of a single square bar roughness elements of different sizes, and the flow downstream of a streamlined body-of-revolution placed on the centerline of the pipe. In each case, the initial response is strongly influenced by the pipe geometry, but far downstream all three flows display a common feature, which is the very slow, second-order recovery that can be explained using a model based on the Reynolds stress equations. Some future directions for research are also given.
Publisher: Cambridge University Press (CUP)
Date: 29-09-2022
DOI: 10.1017/JFM.2022.722
Abstract: Vortical impulse theory is used to investigate the relationship between turbine thrust and the near-wake velocity and vorticity fields. Three different hypotheses regarding the near-wake structure allow the derivation of novel expressions for the thrust on a steadily rotating wind turbine, and these are tested using stereoscopic particle-image velocimetry (PIV) data acquired just behind a rotor in a water channel. When one assumes that vortex lines and streamlines are aligned in a rotor-fixed frame of reference, one obtains a PIV-based thrust estimate that fails even to capture the trend of the directly measured thrust, and this failure is attributed to an implicit assumption that most of the generated thrust does useful work. When one neglects the axial gradients of radial velocity, the PIV-based thrust estimate captures the measured thrust trend, but underpredicts its magnitude by approximately $33\\,\\%$ . The third and most promising physical proposition treats the trailing vortices as purely ‘rolling’ structures that exhibit zero-strain rate in their cores, with the corresponding thrust estimates in close agreement with direct thrust measurements. This best-performing expression appears as a correction to the classical thrust expression from momentum theory, possessing additional squared-velocity terms that can account for the high-thrust regime of turbine operation that is typically addressed empirically.
Publisher: Cambridge University Press (CUP)
Date: 19-07-2021
DOI: 10.1017/JFM.2021.515
Publisher: Elsevier BV
Date: 2022
Publisher: Cambridge University Press (CUP)
Date: 14-09-2021
DOI: 10.1017/JFM.2021.736
Abstract: A new scaling is derived that yields a Reynolds-number-independent profile for all components of the Reynolds stress in the near-wall region of wall-bounded flows, including channel, pipe and boundary layer flows. The scaling demonstrates the important role played by the wall shear stress fluctuations and how the large eddies determine the Reynolds number dependence of the near-wall turbulence behaviour.
Publisher: Cambridge University Press (CUP)
Date: 15-08-2019
DOI: 10.1017/JFM.2019.603
Abstract: The effects of roughness on the frictional drag and pressure drop in laminar channel flow are investigated numerically. The inflow is fully developed smooth wall flow, and square rib roughness, aligned normal to the bulk flow direction, is introduced as a step change. The roughness height and spacing are systematically varied, and the flow is examined as it develops over the rough wall and becomes fully developed. The length of the development region depends primarily on the roughness height, although the effects of spacing become more important as the height decreases. In the fully developed rough wall regime, the friction coefficients always increase with roughness when compared to the smooth wall case, but the increase depends crucially on the roughness height and to a lesser extent on the spacing. Using the constricted diameter in the definition of the friction factor collapses the data on the smooth wall value to within 10 % for all roughnesses studied here, with the remaining deviation increasing linearly with roughness spacing. The friction factors scale with the inverse of the Reynolds number, as seen elsewhere. The scaling of the development length and the friction coefficient can be explained by the relative contributions made by the pressure drop on each element and the skin friction acting over the surface area. These observations are examined in terms of the flow patterns in the vicinity of the roughness elements, which leads us to propose a definition for fully rough laminar flow.
Publisher: The Royal Society
Date: 13-03-2017
Abstract: The energetic motions in direct numerical simulations of turbulent pipe flow at Re τ =685 are investigated using proper orthogonal decomposition. The procedure is extended such that a pressure component is identified in addition to the three-component velocity field for each mode. The pressure component of the modes is shown to align with the streamwise velocity component associated with the large-scale motions, where positive pressure coincides with positive streamwise velocity, and vice versa. The streamwise evolution of structures is then visualized using a conditional mode, which exhibit a strong similarity to the large-scale, low-momentum motions. A low-pressure region is present in the downstream section of the structure, and a high-pressure region is present in the upstream section. This article is part of the themed issue ‘Toward the development of high-fidelity models of wall turbulence at large Reynolds number’.
Publisher: Springer Science and Business Media LLC
Date: 09-2021
Location: Australia
Start Date: 06-2016
End Date: 12-2019
Amount: $310,000.00
Funder: Australian Research Council
View Funded Activity