Mechanical Engineering - Research Publications
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ItemStudy of the near-wall-turbulent region of the high-Reynolds-number boundary layer using an atmospheric flow(CAMBRIDGE UNIV PRESS, 2006-02-10)
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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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ItemStrategies for the visualization of multiple 2D vector fields(IEEE COMPUTER SOC, 2006)
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ItemStreamwise turbulence intensity formulation for flat-plate boundary layers(AMER INST PHYSICS, 2003-08)A similarity formulation is proposed to describe the streamwise turbulence intensity across the entire smooth-wall zero-pressure-gradient turbulent boundary layer. The formulation is an extension of the Marusic, Uddin, and Perry [Phys. Fluids 9, 3718 (1997)] formulation that was restricted to the outer region of the boundary layer, including the logarithmic region. The new formulation is found to agree very well with experimental data over a large range of Reynolds numbers varying from laboratory to atmospheric flows. The formulation is founded on physical arguments based on the attached eddy hypothesis, and suggests that the boundary layer changes significantly with Reynolds number, with an outer flow influence felt all the way down to the viscous sublayer. The formulation may also be used to explain why the empirical mixed scaling of DeGraaff and Eaton [J. Fluid Mech. 422, 319 (2000)] appears to work.
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ItemReynolds number invariance of the structure inclination angle in wall turbulence(AMER PHYSICAL SOC, 2007-09-14)Cross correlations of the fluctuating wall-shear stress and the streamwise velocity in the logarithmic region of turbulent boundary layers are reported over 3 orders of magnitude change in Reynolds number. These results are obtained using hot-film and hot-wire anemometry in a wind tunnel facility, and sonic anemometers and a purpose-built wall-shear stress sensor in the near-neutral atmospheric surface layer on the salt flats of Utah's western desert. The direct measurement of fluctuating wall-shear stress in the atmospheric surface layer has not been available before. Structure inclination angles are inferred from the cross correlation results and are found to be invariant over the large range of Reynolds number. The findings justify the prior use of low Reynolds number experiments for obtaining structure angles for near-wall models in the large-eddy simulation of atmospheric surface layer flows.
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ItemInvestigation of three dimensionality in the near field of a round jet using stereo PIV(TAYLOR & FRANCIS LTD, 2002-03-21)
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ItemLaminar and turbulent comparisons for channel flow and flow control(CAMBRIDGE UNIV PRESS, 2007-01-10)A formula is derived that shows exactly how much the discrepancy between the volume flux in laminar and in turbulent flow at the same pressure gradient increases as the pressure gradient is increased. We compare laminar and turbulent flows in channels with and without flow control. For the related problem of a fixed bulk-Reynolds-number flow, we seek the theoretical lowest bound for skin-friction drag for control schemes that use surface blowing and suction with zero-net volume-flux addition. For one such case, using a crossflow approach, we show that sustained drag below that of the laminar-Poiseuille-flow case is not possible. For more general control strategies we derive a criterion for achieving sublaminar drag and use this to consider the implications for control strategy design and the limitations at high Reynolds numbers.
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ItemTurbulence wall-shear stress sensor for the atmospheric surface layer(IOP PUBLISHING LTD, 2005-08)
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ItemA comparison of turbulent pipe, channel and boundary layer flows(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.