Mechanical Engineering - Research Publications
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ItemStructure Inclination Angles in the Convective Atmospheric Surface Layer(SPRINGER, 2013-04)
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ItemSpatial averaging of velocity measurements in wall-bounded turbulence: single hot-wires(IOP Publishing Ltd, 2013-11)
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ItemSpatial averaging of streamwise and spanwise velocity measurements in wall-bounded turbulence using ν- and x-probes(IOP PUBLISHING LTD, 2013-11)
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ItemThe turbulent/non-turbulent interface and entrainment in a boundary layer(CAMBRIDGE UNIV PRESS, 2014-03)Abstract The turbulent/non-turbulent interface in a zero-pressure-gradient turbulent boundary layer at high Reynolds number ($\mathit{Re}_\tau =14\, 500$) is examined using particle image velocimetry. An experimental set-up is utilized that employs multiple high-resolution cameras to capture a large field of view that extends $2\delta \times 1.1\delta $ in the streamwise/wall-normal plane with an unprecedented dynamic range. The interface is detected using a criteria of local turbulent kinetic energy and proves to be an effective method for boundary layers. The presence of a turbulent/non-turbulent superlayer is corroborated by the presence of a jump for the conditionally averaged streamwise velocity across the interface. The steep change in velocity is accompanied by a discontinuity in vorticity and a sharp rise in the Reynolds shear stress. The conditional statistics at the interface are in quantitative agreement with the superlayer equations outlined by Reynolds (J. Fluid Mech., vol. 54, 1972, pp. 481–488). Further analysis introduces the mass flux as a physically relevant parameter that provides a direct quantitative insight into the entrainment. Consistency of this approach is first established via the equality of mean entrainment calculations obtained using three different methods, namely, conditional, instantaneous and mean equations of motion. By means of ‘mass-flux spectra’ it is shown that the boundary-layer entrainment is characterized by two distinctive length scales which appear to be associated with a two-stage entrainment process and have a substantial scale separation.
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ItemSurface shear stress fluctuations in the atmospheric surface layer( 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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ItemSome predictions of the attached eddy model for a high Reynolds number boundary layer(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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ItemLarge-scale influences in near-wall turbulence(ROYAL SOC, 2007-03-15)Hot-wire data acquired in a high Reynolds number facility are used to illustrate the need for adequate scale separation when considering the coherent structure in wall-bounded turbulence. It is found that a large-scale motion in the log region becomes increasingly comparable in energy to the near-wall cycle as the Reynolds number increases. Through decomposition of fluctuating velocity signals, it is shown that this large-scale motion has a distinct modulating influence on the small-scale energy (akin to amplitude modulation). Reassessment of DNS data, in light of these results, shows similar trends, with the rate and intensity of production due to the near-wall cycle subject to a modulating influence from the largest-scale motions.
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ItemInvestigation of large-scale coherence in a turbulent boundary layer using two-point correlations(Cambridge University Press, 2005)Stereoscopic particle image velocimetry (PIV) measurements are made in streamwise–spanwise and inclined cross-stream planes (inclined at 45◦ and 135◦ to the principal flow direction) of a turbulent boundary layer at moderate Reynolds number (Reτ ∼ 1100). Two-point spatial velocity correlations computed using the PIV data reveal results that are consistent with an earlier study in which packets of hairpin vortices were identified by a feature-detection algorithm in the log region, but not in the outerwake region. Both streamwise–streamwise (Ruu) and streamwise–wall-normal (Ruw)correlations are significant for streamwise displacements of more than 1500 wallunits. Zero crossing data for the streamwise fluctuating component u reveal that streamwise strips between zero crossings of 1500 wall units or longer occur morefrequently for negative u than positive u, suggesting that long streamwise correlations in Ruu are dominated by slower streamwise structures. Additional analysis of Rwwcorrelations suggests that the long streamwise slow-moving regions contain discrete zones of strong upwash over extended streamwise distances, as might occur withinpackets of angled hairpin vortices. At a wall-normal location outside of the log region (z/δ =0.5), the correlations are shorter in the streamwise direction and broader in the spanwise direction. Correlations in the inclined cross-stream plane data revealgood agreement with the streamwise–spanwise plane. Ruu in the 45◦ plane is more elongated along the in-plane wall-normal direction than in the 135◦ plane, which isconsistent with the presence of hairpin packets with a low-speed region lifting away from the wall.
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ItemSimultaneous orthogonal-plane particle image velocimetry measurements in a turbulent boundary layer(Cambridge University Press, 2006)Stereoscopic particle image velocimetry (PIV) measurements were taken simultaneously in streamwise–spanwise and streamwise–wall-normal planes in a zero pressure-gradient turbulent boundary layer over a flat plate. Polarization techniques were employed to allow PIV to be taken in both planes simultaneously. Image preprocessing techniques were used to improve the quality of data near the line of intersection of the planes. Linear stochastic estimation was performed on these data, revealing the streamwise, spanwise, and wall-normal extent of swirl events primarily near the top of the log region of the boundary layer. Swirl events with rotation consistent with the mean vorticity are found to have a large footprint inthe lower limit of the log region whereas swirls with opposite-signed vorticity are found to have little influence lower in the boundary layer. These long-time-averaged statistics contain features that are consistent with the hairpin packet model (or its kinematic equivalent). This model also seems to provide a reasonable description of many instantaneous events involving large-scale coherence, where long regions of streamwise momentum deficit are surrounded by vortex cores.