ORCID Profile
0000-0001-9517-4318
Current Organisation
University of New South Wales
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Acoustics and Noise Control (excl. Architectural Acoustics) | Mechanical Engineering | Turbulent Flows | Interdisciplinary Engineering | Fluidisation and Fluid Mechanics
Management of Noise and Vibration from Transport Activities | Navy | Wind Energy | Expanding Knowledge in the Physical Sciences | Expanding Knowledge in the Biological Sciences |
Publisher: Elsevier BV
Date: 02-2022
Publisher: Cambridge University Press (CUP)
Date: 29-03-2021
DOI: 10.1017/JFM.2021.102
Publisher: Cambridge University Press (CUP)
Date: 21-05-2018
DOI: 10.1017/JFM.2018.320
Abstract: The spatial signature of spanwise velocity coherence in turbulent boundary layers has been studied using a series of unique large-field-of-view multicamera particle image velocimetry experiments, which were configured to capture streamwise/spanwise slices of the boundary layer in both the logarithmic and the wake regions. The friction Reynolds number of $Re_{\\unicode[STIX]{x1D70F}}\\approx 2600$ was chosen to nominally match the simulation of Sillero et al. ( Phys. Fluids , vol. 26 (10), 2014, 105109), who had previously reported oblique features of the spanwise coherence at the top edge of the boundary layer based on the sign of the spanwise velocity, and here we find consistent observations from experiments. In this work, we show that these oblique features in the spanwise coherence relate to the intermittent turbulent bulges at the edge of the layer, and thus the geometry of the turbulent/non-turbulent interface, with the clear appearance of two counter-oriented oblique features. Further, these features are shown to be also present in the logarithmic region once the velocity fields are deconstructed based on the sign of both the spanwise and the streamwise velocity, suggesting that the often-reported meandering of the streamwise-velocity coherence in the logarithmic region is associated with a more obvious diagonal pattern in the spanwise velocity coherence. Moreover, even though a purely visual inspection of the obliqueness in the spanwise coherence may suggest that it extends over a very large spatial extent (beyond many boundary layer thicknesses), through a conditional analysis, we show that this coherence is limited to distances nominally less than two boundary layer thicknesses. Interpretation of these findings is aided by employing synthetic velocity fields of a boundary layer constructed using the attached eddy model, where the range of eddy sizes can be prescribed. Comparisons between the model, which employs an array of self-similar packet-like eddies that are randomly distributed over the plane of the wall, and the experimental velocity fields reveal a good degree of agreement, with both exhibiting oblique features in the spanwise coherence over comparable spatial extents. These findings suggest that the oblique features in the spanwise coherence are likely to be associated with similar structures to those used in the model, providing one possible underpinning structural composition that leads to this behaviour. Further, these features appear to be limited in spatial extent to only the order of the large-scale motions in the flow.
Publisher: IOP Publishing
Date: 23-08-2023
Abstract: Over 20 years ago a report commissioned by the European Commission identified air quality in schools as a public health priority. Despite this concern, little action was taken in the following two decades. Over the last two years as classrooms were increasingly recognised as hotspots for the transmission of SARS-CoV-2, renewed interest and resources have been made available in response to this issue. Questions remain, however, over how best to achieve safer classroom air. Our analysis assessed a range of in situ interventions to remove particulate matter (PM 2.5 ) and carbon dioxide from inside a populated classroom. Our approach used saline spray and volunteers’ exhalations as our source of PM 2.5 and carbon dioxide to explore the ability of high efficiency particulate air (HEPA) filters, natural ventilation and a recirculating A/C unit to remove these air pollutants which collectively provided a novel set of data. For a total window opening of 1.86 m 2 for a 181.7 m 3 classroom with a HEPA filter with a 703m 3 /hr clean air delivery rate, our results confirmed that outdoor air was needed to purge the room to reduce carbon dioxide levels that otherwise rose to ppm in 12 min. Cross and natural ventilation reduced levels of PM 2.5 and carbon dioxide very effectively—in under 5 and 10 min respectively during low levels of outside PM 2.5 . We conclude that natural ventilation supplemented with the use of HEPA filters is the most effective way to reliably improve indoor air quality year-round, balancing the need to have easy to enact approaches to reduce the buildup of PM 2.5 , airborne viruses and carbon dioxide. These results highlight an important knowledge gap. Without having localised real-time outdoor air pollution sensing, evidence-based decisions cannot be made about how often, and for how long, windows can safely remain open to purge classrooms in times of poor quality outdoor air.
Publisher: Cambridge University Press (CUP)
Date: 04-09-2023
DOI: 10.1017/JFM.2023.630
Publisher: Cambridge University Press (CUP)
Date: 10-06-2019
DOI: 10.1017/JFM.2019.351
Abstract: This paper examines the recovery of the wall-shear stress of a turbulent boundary layer that has undergone a sudden transition from a rough to a smooth surface. Early work of Antonia & Luxton ( J. Fluid Mech. , vol. 53, 1972, pp. 737–757) questioned the reliability of standard smooth-wall methods for measuring wall-shear stress in such conditions, and subsequent studies show significant disagreement depending on the approach used to determine the wall-shear stress downstream. Here we address this by utilising a collection of experimental databases at $Re_{\\unicode[STIX]{x1D70F}}\\approx 4100$ that have access to both ‘direct’ and ‘indirect’ measures of the wall-shear stress to understand the recovery to equilibrium conditions of the new surface. Our results reveal that the viscous region ( $z^{+}\\lesssim 4$ ) recovers almost immediately to an equilibrium state with the new wall conditions however, the buffer region and beyond takes several boundary layer thicknesses before recovering to equilibrium conditions, which is longer than previously thought. A unique direct numerical simulation database of a wall-bounded flow with a rough-to-smooth wall transition is employed to confirm these findings. In doing so, we present evidence that any estimate of the wall-shear stress from the mean velocity profile in the buffer region or further away from the wall tends to underestimate its magnitude in the near vicinity of the rough-to-smooth transition, and this is likely to be partly responsible for the large scatter of recovery lengths to equilibrium conditions reported in the literature. Our results also reveal that smaller energetic scales in the near-wall region recover to an equilibrium state associated with the new wall conditions within one boundary layer thickness downstream of the transition, while larger energetic scales exhibit an over-energised state for several boundary layer thicknesses downstream of the transition. Based on these observations, an alternative approach to estimating the wall-shear stress from the premultiplied energy spectrum is proposed.
Publisher: Cambridge University Press (CUP)
Date: 21-01-2020
DOI: 10.1017/JFM.2020.23
Publisher: Cambridge University Press (CUP)
Date: 23-05-2022
DOI: 10.1017/JFM.2022.409
Abstract: The inertial subrange of turbulent scales is commonly reflected by a power law signature in ensemble statistics such as the energy spectrum and structure functions – both in theory and from observations. Despite promising findings on the topic of fractal geometries in turbulence, there is no accepted image for the physical flow features corresponding to this statistical signature in the inertial subrange. The present study uses boundary layer turbulence measurements to evaluate the self-similar geometric properties of velocity isosurfaces and investigate their influence on statistics for the velocity signal. The fractal dimension of streamwise velocity isosurfaces, indicating statistical self-similarity in the size of ‘wrinkles’ along each isosurface, is shown to be constant only within the inertial subrange of scales. For the transition between the inertial subrange and production range, it is inferred that the largest wrinkles become increasingly confined by the overall size of large-scale coherent velocity regions such as uniform momentum zones. The self-similarity of isosurfaces yields power-law trends in subsequent one-dimensional statistics. For instance, the theoretical 2/3 power-law exponent for the structure function can be recovered by considering the collective behaviour of numerous isosurface level sets. The results suggest that the physical presence of inertial subrange eddies is manifested in the self-similar wrinkles of isosurfaces.
Publisher: Springer Science and Business Media LLC
Date: 26-12-2017
Publisher: Elsevier BV
Date: 12-2021
Publisher: AIP Publishing
Date: 03-2023
DOI: 10.1063/5.0144934
Abstract: Microfluidics devices are gaining significant interest in biomedical applications. However, in a micron-scale device, reaction speed is often limited by the slow rate of diffusion of the reagents. Several active and passive micro-mixers have been fabricated to enhance mixing in microfluidic devices. Here, we demonstrate external control of mixing by rotating a rod-shaped bacterial cell. This rotation is driven by ion transit across the bacterial flagellar stator complex. We first measured the flow fields generated by rotating a single bacterial cell rotationally locked to rotate either clockwise (CW) or counterclockwise (CCW). Micro-particle image velocimetry (μPIV) and particle tracking velocimetry results showed that a bacterial cell of ∼ 2.75 μm long, rotating at 5.75 ± 0.39 Hz in a counterclockwise direction could generate distinct micro-vortices with circular flow fields with a mean velocity of 4.72 ± 1.67 μm/s and maximum velocity of 7.90 μm/s in aqueous solution. We verified our experimental data with a numerical simulation at matched flow conditions, which revealed vortices of similar dimensions and speed. We observed that the flow-field diminished with increasing z-height above the plane of the rotating cell. Lastly, we showed that we could activate and tune rotational mixing remotely using strains engineered with proteorhodopsin, where rotation could be activated by controlled external illumination using green laser light (561 nm).
Publisher: American Physical Society (APS)
Date: 07-09-2023
Publisher: Cambridge University Press (CUP)
Date: 23-09-2022
DOI: 10.1017/JFM.2022.731
Abstract: In this study, we develop an analytical model to predict the turbulent boundary layer downstream of a step-change in the surface roughness where upstream flow conditions are given. We first revisit the classical model of Elliott ( Trans. Am. Geophys. Union , vol. 39, 1958, pp. 1048–1054), who modelled the velocity distribution within and above the internal layer with a simple piecewise logarithmic profile, and evolved the velocity profile using the streamwise momentum equation. Elliott's model was originally developed for an atmospheric surface layer, and to make the model applicable to a spatially developing turbulent boundary layer with finite thickness, we propose a number of more physical refinements, including adding a wake function to the velocity profile, considering the growth of the entire boundary layer in the streamwise direction, and using a more realistic shear stress profile in the momentum equation. In particular, we implement the blending model (Li et al. , J. Fluid Mech. , vol. 923, 2021, p. A18) to account for the deviation of the mean flow within the internal layer from a canonical velocity profile based on the local wall condition. These refinements lead to improved agreement between the prediction and the measurement, especially in the vicinity of the rough-to-smooth change.
Publisher: American Physical Society (APS)
Date: 07-10-2022
Publisher: Cambridge University Press (CUP)
Date: 21-06-2017
DOI: 10.1017/JFM.2017.315
Abstract: The scaling behaviour of the longitudinal velocity structure functions $\\langle (\\unicode[STIX]{x1D6E5}_{r}u)^{2p}\\rangle ^{1 }$ (where $2p$ represents the order) is studied for various wall-bounded turbulent flows. It has been known that for very large Reynolds numbers within the logarithmic region, the structure functions can be described by $\\langle (\\unicode[STIX]{x1D6E5}_{r}u)^{2p}\\rangle ^{1 }/U_{\\unicode[STIX]{x1D70F}}^{2}\\approx D_{p}\\ln (r/z)+E_{p}$ (where $r$ is the longitudinal distance, $z$ the distance from the wall, $U_{\\unicode[STIX]{x1D70F}}$ the friction velocity and $D_{p}$ , $E_{p}$ are constants) in accordance with Townsend’s attached eddy hypothesis. Here we show that the ratios $D_{p}/D_{1}$ extracted from plots between structure functions – in the spirit of the extended self-similarity hypothesis – have further reaching universality for the energy containing range of scales. Specifically, we confirm that this description is universal across wall-bounded flows with different flow geometries, and also for both the longitudinal and transversal structure functions, where previously the scaling has been either difficult to discern or differences have been reported when examining the direct representation of $\\langle (\\unicode[STIX]{x1D6E5}_{r}u)^{2p}\\rangle ^{1 }$ . In addition, we present evidence of this universality at much lower Reynolds numbers, which opens up avenues to examine structure functions that are not readily available from high Reynolds number databases.
Publisher: Elsevier BV
Date: 05-2022
Publisher: Cambridge University Press (CUP)
Date: 12-05-2017
DOI: 10.1017/JFM.2017.197
Abstract: In this paper we examine the characteristics of the interfaces that demarcate regions of relatively uniform streamwise momentum in turbulent boundary layers. The analysis utilises particle image velocimetry databases that span more than an order of magnitude of friction Reynolds number ( $Re_{\\unicode[STIX]{x1D70F}}=10^{3}$ – $10^{4}$ ), enabling us to provide a detailed description of the interfacial layers as a function of Reynolds number. As reported by Adrian et al. ( J. Fluid Mech. , vol. 422, 2000, pp. 1–54), these interfaces appear as persistent regions of strong shear with distinct patches of vorticity consistent with a packet-like structure. Here, however, we treat these interfaces as continuous lines, thus averaging the properties of the vortical patches, and find that their geometry is highly contorted and exhibits self-similarity across a wide range of scales. Specifically, the lengths of the edges of uniform momentum zones exhibit a power-law behaviour with a fractal scaling that has a constant exponent across the boundary layer, while the topmost edge or the turbulent/non-turbulent interface shows a sudden increase in the exponent. The accompanying sharp changes in velocity that occur at these edges are found to change in magnitude as a function of wall-normal height, being larger closer to the wall. Further, a Reynolds number invariance is exhibited when the magnitude of the step-like changes in velocity is scaled by the skin-friction velocity, meanwhile, the width across which it occurs is shown to be of the order of the Taylor microscale. Based on these quantitative measures, the Reynolds number scaling observed and the persistent presence of sharp changes in momentum in turbulent boundary layers, a simple model is used to reconstruct the mean velocity profile. Insight gained from the model enhances our understanding of how instantaneous phenomena (such as a zonal-like structural arrangement) manifests in the averaged flow statistics and confirms that the instantaneous momentum in a turbulent boundary layer appears to mainly consist of a step-like profile as a function of wall-normal distance.
Publisher: Cambridge University Press (CUP)
Date: 27-07-2021
DOI: 10.1017/JFM.2021.577
Publisher: Cambridge University Press (CUP)
Date: 16-06-2021
DOI: 10.1017/JFM.2021.478
Publisher: Cambridge University Press (CUP)
Date: 11-08-2023
DOI: 10.1017/JFM.2023.566
Abstract: We present experimental evidence that the superstructures in turbulent boundary layers comprise smaller, geometrically self-similar coherent motions. The evidence comes from identifying and analysing instantaneous superstructures from large-scale particle image velocimetry datasets acquired at high Reynolds numbers, capable of capturing streamwise elongated motions extending up to 12 times the boundary layer thickness. Given the challenge in identifying the constituent motions of the superstructures based on streamwise velocity signatures, a new approach is adopted that analyses the wall-normal velocity fluctuations within these very long motions, which reveals the constituent motions unambiguously. The conditional streamwise energy spectra of the Reynolds shear stress and the wall-normal fluctuations, corresponding exclusively to the superstructure region, are found to exhibit the well-known distance-from-the-wall scaling in the intermediate-scale range. It suggests that geometrically self-similar motions are the constituent motions of these very-large-scale structures. Investigation of the spatial organization of the wall-normal momentum-carrying eddies, within the superstructures, also lends empirical support to the concatenation hypothesis for the formation of these structures. The association between the superstructures and self-similar motions is reaffirmed on comparing the vertical coherence of the Reynolds-shear-stress-carrying motions, by computing conditionally averaged two-point correlations, which are found to match with the mean correlations. The mean vertical coherence of these motions, investigated for the log region across three decades of Reynolds numbers, exhibits a unique distance-from-the-wall scaling invariant with Reynolds number. The findings support modelling of these dynamically significant motions via data-driven coherent structure-based models.
Publisher: IOP Publishing
Date: 06-09-2021
Publisher: Mary Ann Liebert Inc
Date: 02-2023
DOI: 10.1089/HS.2022.0100
Publisher: American Physical Society (APS)
Date: 06-07-2021
Publisher: Oxford University Press (OUP)
Date: 27-08-2020
DOI: 10.1093/CID/CIAA1241
Abstract: Choral singing has become a major risk during the coronavirus disease 2019 (COVID-19) pandemic due to high infection rates. Our visualization and velocimetry results reveal that the majority of droplets expelled during singing follow the ambient airflow pattern. These results point toward the possibility of COVID-19 spread by small airborne droplets during singing.
Publisher: American Physical Society (APS)
Date: 10-03-2020
Publisher: AIP Publishing
Date: 11-2021
DOI: 10.1063/5.0067609
Abstract: Respiratory infections transmit through droplets and aerosols generated by the infected in idual during respiratory emissions. It is essential to study the flow dynamics of these emissions to develop strategies for mitigating the risk of infection. In particular, the dynamics of droplets expelled during violent exhalations such as sneezing is crucial, but has received little attention to date. Here, for the first time, we present the results of droplet dynamics of 35 sneezes, obtained from four volunteers, using particle tracking velocimetry experiments. Our results reveal a mean droplet velocity of 2–5.4 m/s across the different subjects. These values are significantly lower than what is usually assumed in the studies simulating or replicating sneezes. Furthermore, the large variation in droplet speeds, flow direction, spread angle, and head movement is also quantified. These findings will enable the refinement of models and simulations of sneezes toward improving infection control guidelines.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 09-2023
Publisher: Cambridge University Press (CUP)
Date: 06-10-2017
DOI: 10.1017/JFM.2017.625
Abstract: Considering structure functions of the streamwise velocity component in a framework akin to the extended self-similarity hypothesis (ESS), de Silva et al. ( J. Fluid Mech. , vol. 823, 2017, pp. 498–510) observed that remarkably the large-scale (energy-containing range) statistics in canonical wall-bounded flows exhibit universal behaviour. In the present study, we extend this universality, which was seen to encompass also flows at moderate Reynolds number, to Taylor–Couette flow. In doing so, we find that also the transversal structure function of the spanwise velocity component exhibits the same universal behaviour across all flow types considered. We further demonstrate that these observations are consistent with predictions developed based on an attached-eddy hypothesis. These considerations also yield a possible explanation for the efficacy of the ESS framework by showing that it relaxes the self-similarity assumption for the attached-eddy contributions. By taking the effect of streamwise alignment into account, the attached-eddy model predicts different behaviour for structure functions in the streamwise and in the spanwise directions and that this effect cancels in the ESS framework – both consistent with the data. Moreover, it is demonstrated here that also the additive constants, which were previously believed to be flow dependent, are indeed universal at least in turbulent boundary layers and pipe flow where high Reynolds number data are currently available.
Publisher: Springer Science and Business Media LLC
Date: 18-07-2020
Location: Australia
Start Date: 03-2020
End Date: 03-2021
Amount: $340,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2022
End Date: 12-2024
Amount: $480,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2020
End Date: 02-2024
Amount: $490,000.00
Funder: Australian Research Council
View Funded Activity