Mechanical Engineering - Research Publications

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    Backflow events under the effect of secondary flow of Prandtl's first kind
    Chin, RC ; Vinuesa, R ; Orlu, R ; Cardesa, J ; Noorani, A ; Chong, MS ; Schlatter, P (American Physical Society, 2020-07-30)
    A study of the backflow events in the flow through a toroidal pipe at friction Reynolds number Reτ ≈ 650 is performed and compared with the results in a straight turbulent pipe flow at Reτ ≈ 500. The statistics and topological properties of the backflow events are analysed and discussed. Conditionally averaged flow fields in the vicinity of the backflow event are obtained, and the results for the torus show a similar streamwise wall-shear stress topology which varies considerably for the azimuthal wall-shear stress when compared to the pipe flow. In the region around the backflow events, critical points are observed. The comparison between the toroidal pipe and its straight counterpart also shows fewer backflow events and critical points in the torus. This is attributed to the secondary flow of Prandtl's first kind present in the toroidal pipe, which is responsible for the convection of momentum from the inner to the outer bend through the core of the pipe, and back from outer bend to the inner bend along the azimuthal direction. These results indicate that backflow events and critical points are genuine features of wall-bounded turbulence, and are not artefacts of specific boundary or inflow conditions in simulations and/or measurement uncertainties in experiments.
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    Surface shear stress fluctuations in the atmospheric surface layer
    Monty, J. P. ; Chong, M. S. ; Hutchins, N. ; Marusic, I. ( 2006)
    A lightweight, high frequency response, floating element sensor was used to measure wall shear stress fluctuations in an atmospheric surface layer. The sensor uses a laser position measurement system to track the motion of the floating element. The measurements were taken as part of an internationally coordinated experimental program designed to make extensive spatial and temporal measurements of velocity, temperature and wall shear stress of the surface layer. Velocity measurements were made with both a 27m high vertical array and a 100m wide horizontal array of sonic anemometers; 18 anemometers in total were employed. Cross-correlations of shear stress and streamwise velocity fluctuations were analysed in an attempt to identify structure angles in the flow. The results were shown to compare favourably with experimental data from controlled, laboratory turbulent boundary layer measurements at three orders of magnitude lower Reynolds number.
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    Some predictions of the attached eddy model for a high Reynolds number boundary layer
    Nickels, T. B. ; Marusic, I. ; Hafez, S. ; Hutchins, N. ; Chong, M. S. (Royal Society Publishing, 2007-01)
    Many flows of practical interest occur at high Reynolds number, at which the flow inmost of the boundary layer is turbulent, showing apparently random fluctuations invelocity across a wide range of scales. The range of scales over which these fluctuationsoccur increases with the Reynolds number and hence high Reynolds number flows aredifficult to compute or predict. In this paper, we discuss the structure of these flows anddescribe a physical model, based on the attached eddy hypothesis, which makespredictions for the statistical properties of these flows and their variation with Reynoldsnumber. The predictions are shown to compare well with the results from recentexperiments in a new purpose-built high Reynolds number facility. The model is alsoshown to provide a clear physical explanation for the trends in the data. The limits ofapplicability of the model are also discussed.
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    Observations on high Reynolds number turbulent boundary layer measurements
    HAFEZ, SHM ; MARUSIC, I ; CHONG, MS ; JONES, MB (The University of Sydney, 2004)
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    Evidence of the -1-law in a high Reynolds number turbulent boundary layer
    Nickels, T. B. ; Hafez, S. ; Marusic, I. ; Chong, M. S. ( 2004)
    Dimensional analysis leads to a prediction of a -1-power-lawfor the streamwise velocity spectrum in a turbulent boundarylayer. This law can be derived from overlap arguments or fromphysical arguments based on the attached eddy hypothesis ofTownsend (1976). Some recent experiments have questionedthe existence of this power-law region in wall-bounded ows.In this paper experimental spectra are presented which supportthe existence of the -1-law in a high Reynolds number boundarylayer, measured in the large boundary layer facility in theWalterBasset laboratory at the University of Melbourne. The paperpresents the experimental results and discusses the theoreticaland experimental issues involved in examining the existence ofthe -1-law and reasons why it has proved so elusive.
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    Evidence of the k1-1 law in a high-Reynolds-number turbulent boundary layer
    Nickels, T. B. ; Marusic, I. ; Hafez, S. ; Chong, M. S. (The American Physical Society, 2005)
    Dimensional analysis and overlap arguments lead to a prediction of a region in the streamwise velocityspectrum of wall-bounded turbulent flows in which the dependence on the streamwise wave number, k1, isgiven by k 1-1 . Some recent experiments have questioned the existence of this region. In this Letter,experimental spectra are presented which support the existence of the k 1-1 law in a high-Reynolds-numberboundary layer. This Letter presents the experimental results and discusses the theoretical and experimentalissues involved in examining the existence of the k 1-1 law and the reasons why it has proved so elusive.
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    Turbulent channel flow: comparison of streamwise velocity data from experiments and direct numerical simulation
    Monty, JP ; Chong, MS (CAMBRIDGE UNIV PRESS, 2009-08-25)
    Recently there has been remarkable progress made in the direct numerical simulation (DNS) of wall-bounded turbulence, particularly of turbulent channel flow, with numerical data now available above Reτ ≈ 2000 (Hoyas & Jiménez, Phys. Fluids, vol. 18, 2006, p. 011702; Iwamoto et al., Proceedings of the Sixth Symposium Smart Control of Turbulence, 2005). Much knowledge has been gained from these results, particularly in the areas of flow structure and dynamics. Yet, while the value of such simulations is undoubted, only very limited comparisons with experimental data have been documented. Although the physics of the flow are captured correctly in an ideal DNS, as with any real numerical or physical experiment, there are opportunities for misrepresentation of the characteristics of turbulence. As such, this article seeks to make a comparison between a well-documented high Reynolds number (Reτ = 934), large box size (8πh × 2h × 3πh) DNS from del Álamo et al. (J. Fluid Mech., vol. 500, 2004, p. 135) and laboratory channel flow data measured by the authors. Results show that there is excellent agreement between the streamwise velocity statistics of the two data sets. The spectra are also very similar, however, throughout the logarithmic region the secondary peak in energy is clearly reduced in the DNS results. Although the source of the difference is not certain, the wavelengths concerned are close to the DNS box length, leading to the recommendation that longer box lengths should be investigated. Another large-scale spectral discrepancy near the wall results from the incorrect assumption of a constant convection velocity used to infer spatial information from the temporal. A near-wall convection velocity modification function is tentatively proposed. While the modification gives good agreement between the data sets, higher Reynolds number comparisons are required to better understand the intricate convection velocity issue.
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    A comparison of turbulent pipe, channel and boundary layer flows
    Monty, JP ; Hutchins, N ; Ng, HCH ; Marusic, I ; Chong, MS (CAMBRIDGE UNIV PRESS, 2009-08-10)
    The extent or existence of similarities between fully developed turbulent pipes and channels, and in zero-pressure-gradient turbulent boundary layers has come into question in recent years. This is in contrast to the traditionally accepted view that, upon appropriate normalization, all three flows can be regarded as the same in the near-wall region. In this paper, the authors aim to provide clarification of this issue through streamwise velocity measurements in these three flows with carefully matched Reynolds number and measurement resolution. Results show that mean statistics in the near-wall region collapse well. However, the premultiplied energy spectra of streamwise velocity fluctuations show marked structural differences that cannot be explained by scaling arguments. It is concluded that, while similarities exist at these Reynolds numbers, one should exercise caution when drawing comparisons between the three shear flows, even near the wall.