## Publications

197 results found

Agostini L, Leschziner M, 2022, Auto-encoder-assisted analysis of amplitude and wavelength modulation of near-wall turbulence by outer large-scale structures in channel flow at friction Reynolds number of 5200, *Physics of Fluids*, Vol: 34, Pages: 1-18, ISSN: 1070-6631

This paper reports a novel methodology that allows the intensity of, and the underlying mechanism for, the amplitude and length-scale modulation (amplification or attenuation) of turbulent stresses in the inner layer of a channel flow at Reτ≈5200 to be clarified. A unique aspect of the present framework is the use of an auto-encoder algorithm to separate full-volume extremely large direct numerical simulation (DNS) fields into large-scale and small-scale motions. This approach is adopted in preference to the empirical mode decomposition (EMD) previously used by the present authors at the lower Reynolds number, Reτ≈1000, because resource requirements posed by the EMD quickly become untenable due to the extremely large DNS dataset and the large solution box needed to capture the wide spectrum of scales at the present Reynolds number. A second original element is a formalism that derived the modulation, conditional on large-scale fluctuations, from continuous statistical quantities represented as multivariable-joint probability-density functions, thus obviating the need for any discrete representation or binning beyond that imposed by the discrete DNS solution. A third novel aspect is the use of the length-scale-wise derivative of the second-order structure function to quantify the modulation (increase or decrease) in the length scale, again conditional on large-scale structures. Apart from illuminating the modulation itself, the study examined the validity of the quasi-steady hypothesis that proposes that the near-wall turbulence is universal when scaled by the spatially and temporally varying large-scale wall shear stress rather than its time average.

Agostini L, Leschziner M, 2021, Statistical analysis of outer large-scale/inner-layer interactions in channel flow subjected to oscillatory drag-reducing wall motion using a multiple-variable joint-probability-density function methodology, *Journal of Fluid Mechanics*, Vol: 923, Pages: 1-29, ISSN: 0022-1120

Full flow-field data derived from a direct numerical simulation for channel flow subjected to drag-reducing oscillatory spanwise motion are analysed by means of a recently developed methodology, which consolidates the entire simulation data set within multiple-variable joint-probability-density functions (PDFs). A wide variety of statistical data of interest are then extracted from the joint PDF without recourse to any of the original simulation data. The nominal friction Reynolds number of the baseline (unactuated) flow is 1025, and the actuation is effected at a wall-scaled period of 100, at which value the drag-reduction level is approximately 30 %, while any actuation-induced phase fluctuations in the streamwise direction are minimal. Interest focuses on the elucidation of the mechanisms by which the near-wall turbulence is modified by the action of footprints of large-scale structures in the outer parts of the log-law region, which tend to intensify as the Reynolds number rises. To elucidate these mechanisms, the Reynolds stresses and their production rates, conditional on the intensity of large-scale skin-friction fluctuations, are examined. The investigation includes a separation of the Reynolds stresses into large-scale and small-scale components by means the empirical mode decomposition, allowing the intensity of footprinting and of small-scale modulation of the near-wall turbulence to be quantified separately. The conditional statistical properties are presented in the form of maps in planes having the wall-normal distance and large-scale skin friction as coordinates, supplemented by wall-normal property profiles and an examination of large-scale and small-scale contributions to the skin friction. The analysis highlights the strongly asymmetric response of the production rate and the turbulence level in the buffer layer to positive vs. negative footprints, the former strongly enhancing small-scale turbulence. This is proposed to be at least a partial expla

Ricco P, Skote M, Leschziner MA, 2021, A review of turbulent skin-friction drag reduction by near-wall transverse forcing, *Progress in Aerospace Sciences*, Vol: 123, Pages: 1-58, ISSN: 0376-0421

The quest for reductions in fuel consumption and CO2 emissions in transport has been a powerful driving force for scientific research into methods that might underpin drag-reducing technologies for a variety of vehicular transport on roads, by rail, in the air, and on or in the water. In civil aviation, skin-friction drag accounts for around 50% of the total drag in cruise conditions, thus being a preferential target for research. With laminar conditions excluded, skin friction is intimately linked to the turbulence physics in the fluid layer closest to the skin. Hence, research into drag reduction has focused on methods to depress the turbulence activity near the surface. The most effective method of doing so is to exercise active control on the near-wall layer by subjecting the drag-producing flow in this layer to an unsteady and/or spatially varying cross-flow component, either by the action of transverse wall oscillations, by embedding rotating discs into the surface or by plasma-producing electrodes that accelerate the near-wall fluid in the transverse direction. In ideal conditions, drag-reduction margins of order of 50% can thereby be achieved. The present article provides a near-exhaustive review of research into the response of turbulent near-wall layers to the imposition of unsteady and wavy transverse motion. The review encompasses experiments, simulation, analysis and modelling, mainly in channel flows and boundary layers. It covers issues such as the drag-reduction margin in a variety of actuation scenarios and for a wide range of actuation parameters, the underlying physical phenomena that contribute to the interpretation of the origin of the drag reduction, the dependence of the drag reduction on the Reynolds number, passive control methods that are inspired by active control, and a forward look towards possible future research and practical realizations. The authors hope that this review, by far the most extensive of its kind for this subject, will b

Leschziner MA, 2020, Friction-drag reduction by transverse wall motion - a review, *Journal of Mechanics*, Vol: 36, Pages: 649-663, ISSN: 1680-0893

The quest for drag reduction is driven by environmental concerns, in general, and the need to reduce fuel consumption in transport applications, in particular. Turbulent friction is especially important in civil aviation, accounting for over 50% of the total drag in cruise. In this context, spatially and/or temporally varying in-plane wall motion, while undoubtedly difficult to implement in practice, has attracted major interest, because of the large drag-reduction margins it yields. It is also a forcing method that is of fundamental interest, as it provokes intriguing interactions between the spanwise Stokes layer induced by the wall motion and the near-wall turbulence-regeneration mechanisms. This article provides a relatively brief, ‘entry-level’, review of research in this area, principally over the past two decades. While far from being exhaustive, the review conveys a reasonably detailed picture of some major physical issues as well as of the outcome of the most important computational and experimental studies. Particular emphasis is placed on the question of how results obtained in idealised laboratory conditions and by simulation at relatively low Reynolds-number values pertain to high values typical of high-speed transport.

Nicoud F, Hickel S, Tomboulides A,
et al., 2020, Progress in Engineering Turbulence Modelling, Simulation and Measurements Preface, *FLOW TURBULENCE AND COMBUSTION*, Vol: 104, Pages: 291-292, ISSN: 1386-6184

Agostini L, Leschziner M, 2019, The connection between the spectrum of turbulent scales and the skin-friction statistics in channel flow at Re-tau approximate to 1000, *Journal of Fluid Mechanics*, Vol: 871, Pages: 22-51, ISSN: 0022-1120

Data from a direct numerical simulation for channel flow at a friction Reynolds number of 1000 are analysed to derive statistical properties that offer insight into the mechanisms by which large-scale structures in the log-law region affect the small-scale turbulence field close to the wall and the statistical skin-friction properties. The data comprise full-volume velocity fields at 150 time levels separated by 50 wall-scaled viscous time units. The scales are separated into wavelength bands by means of the ‘empirical mode decomposition’, of which the two lowest modes are considered to represent the small scales and three upper modes to represent the large scales. Joint and conditional probability density functions are then derived for various scale-specific statistics, with particular emphasis placed on the streamwise and shear stresses conditional on the large-scale fluctuations of the skin friction, generally referred to as ‘footprinting’. Statistics for the small-scale stresses, conditional on the footprints, allow the amplification and attenuation of the small-scale skin friction, generally referred to as ‘modulation’, to be quantified in dependence on the footprints. The analysis leads to the conclusion that modulation does not reflect a direct interaction between small scales and large scales, but arises from variations in shear-induced production that arise from corresponding changes in the conditional velocity profile. This causal relationship also explains the wall-normal change in sign in the correlation between large scales and small scales at a wall-scaled wall distance of approximately 100. The effects of different scales on the skin friction are investigated by means of two identities that describe the relationship between the shear-stress components and the skin friction, one identity based on integral momentum and the other on energy production/dissipation. The two identities yield significant differences in the

Agostini L, Leschziner M, 2019, On the departure of near-wall turbulence from the quasi-steady state, *Journal of Fluid Mechanics*, Vol: 871, Pages: 1-12, ISSN: 0022-1120

An examination is undertaken of the validity and limitations of the quasi-steady hypothesis of near-wall turbulence. This hypothesis is based on the supposition that the statistics of the turbulent fluctuations are universal if scaled by the local, instantaneous, wall shear when its variations are determined from footprints of large-scale, energetic, structures that reside in the outer part of the logarithmic layer. The examination is performed with the aid of direct numerical simulation data for a single Reynolds number, which are processed in a manner that brings out the variability of locally scaled statistics when conditioned on the local value of the wall friction. The key question is to what extent this variability is insignificant, thus reflecting universality. It is shown that the validity of the quasi-steady hypothesis is confined, at best, to a thin layer above the viscous sublayer. Beyond this layer, substantial variations in the conditioned shear-induced production rate of large-scale turbulence cause substantial departures from the hypothesis. Even within the wall-proximate layer, moderate departures are provoked by large-scale distortions in the conditioned strain rate that result in variations in small-scale production of turbulence down to the viscous sublayer.

Ghebali S, Chernyshenko SI, Leschziner MA, 2019, Turbulent-drag reduction by oblique wavy wall undulations, ERCOFTAC Series, Pages: 545-551

Reducing the turbulent skin-friction drag over civilian aircraft is a potentially high-reward target, as this drag component accounts for about half of the total drag in cruise conditions. Thus, even modest reductions convert into material savings, resulting in significant cuts in costs. Active-control techniques can be remarkably effective at suppressing turbulence and drag, but pose major engineering challenges in terms of actuation, efficient operation, reliability and maintainability. In contrast, passive techniques based on riblets are easier to implement, but face important durability and maintenance limitations related to the extremely small spacing of the grooves. The alternative passive-control method that is the subject of the present paper was first proposed in Chernyshenko (Drag reduction by a solid wavy wall emulating spanwise oscillations. Part 1. [physics.flu-dyn](arXiv:1304.4638 ), (2013), [1]). The key characteristic of the method is that it involves wavy surface undulations directed obliquely to the mean flow and having wave lengths two orders of magnitude larger than riblets, and would thus be much more practical to manufacture and maintain.

Agostini L, Leschziner M, 2018, The impact of footprints of large-scale outer structures on the near-wall layer in the presence of drag-reducing spanwise wall motion, *Flow, Turbulence and Combustion*, Vol: 100, Pages: 1037-1061, ISSN: 1386-6184

This study is motivated by the observation that the drag-reduction effectiveness achieved by the imposition of oscillatory spanwise wall motion declines with Reynolds number. The question thus posed is whether the decline is linked to the increasingly strong influence of large-scale outer structures in the log layer on the near-wall turbulence, in general, and the streak strength in the viscosity-affected layer, in particular – a process referred to as modulation. This question is addressed via an extensive statistical analysis of DNS data for a channel flow at a friction Reynolds number 1020, subjected to oscillatory spanwise wall motion at a nominal wall-scaled period of 100. The analysis rests on a separation of turbulent scales by means of the Empirical Mode Decomposition. This method is used to derive conditional statistics of small-scale motions and skin friction subject to prescribed intensity of large-scale motions – referred to as footprinting. It is shown that the large-scale fluctuations are responsible, directly on their own, for roughly 30% to the skin friction. Positive large-scale fluctuations are also shown to be the cause of a major amplification of small-scale streaks, relative to weak attenuation by negative fluctuations. This highly asymmetric process is likely to be indirectly influential on the drag-reduction process, although it is not possible to identify this indirect effect in quantitative terms as part of the present analysis.

Ghebali S, Chernyshenko SI, Leschziner M, 2017, Can large-scale oblique undulations on a solid wall reduce the turbulent drag?, *Physics of Fluids*, Vol: 29, Pages: 105102-1-105102-15, ISSN: 1070-6631

Direct numerical simulations of fully developed turbulent channel flows with wavy walls are undertaken. The wavy walls, skewed with respect to the mean flow direction, are introduced as a means of emulating a Spatial Stokes Layer (SSL) induced by in-plane wall motion. The transverse shear strain above the wavy wall is shown to be similar to that of a SSL, thereby affecting the turbulent flow and leading to a reduction in the turbulent skin-friction drag. However, some important differences with respect to the SSL case are brought to light too. In particular, the phase variations of the turbulent properties are accentuated and, unlike in the SSL case, there is a region of increased turbulence production over a portion of the wall, above the leeward side of the wave, thus giving rise to a local increase in dissipation. The pressure- and friction-drag levels are carefully quantified for various flow configurations, exhibiting a combined maximum overall-drag reduction of about 0.6%. The friction-drag reduction is shown to behave approximately quadratically for small wave slopes and then linearly for higher slopes, whilst the pressure-drag penalty increases quadratically. The transverse shear-strain layer is shown to be approximately Reynolds-number independent when the wave geometry is scaled in wall units.

Ghebali S, Chernyshenko SI, Leschziner MA, 2017, Can large-scale oblique undulations on a solid wall reduce the turbulent drag?, *Physics of Fluids*, Vol: 29, Pages: 105102-105102

Direct numerical simulations of fully-developed turbulent channel flows withwavy walls are undertaken. The wavy walls, skewed with respect to the mean flowdirection, are introduced as a means of emulating a Spatial Stokes Layer (SSL)induced by in-plane wall motion. The transverse shear strain above the wavywall is shown to be similar to that of a SSL, thereby affecting the turbulentflow, and leading to a reduction in the turbulent skin-friction drag. Thepressure- and friction-drag levels are carefully quantified for various flowconfigurations, exhibiting a combined maximum overall-drag reduction of about0.5%. The friction-drag reduction is shown to behave approximatelyquadratically for small wave slopes and then linearly for higher slopes, whilstthe pressure-drag penalty increases quadratically. Unlike in the SSL case,there is a region of increased turbulence production over a portion of thewall, above the leeward side of the wave, thus giving rise to a local increasein dissipation. The transverse shear-strain layer is shown to be approximatelyReynolds-number independent when the wave geometry is scaled in wall units.

Ghebali S, Chernyshenko S, Leschziner M, 2017, Turbulent drag reduction by wavy wall, 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017

Fully-developed turbulent flow in channels with oblique wavy walls is analysed, from a drag-reduction perspective, by means of Direct Numerical Simulations (DNS). The wavy geometry is chosen to emulate the shear strain produced by a Spatial Stokes Layer (SSL) generated by oscillatory wall motion. As the cost of performing a parametric optimisation is prohibitive, an alternate solution is presented, based on a linear model of a perturbed plane-channel flow, using a turbulent viscosity. Flow properties and levels of drag reduction or increase are reported for various configurations.

Agostini L, Leschziner M, Poggie J,
et al., 2017, Multi-scale interactions in a compressible boundary layer, *Journal of Turbulence*, Vol: 18, Pages: 760-780, ISSN: 1468-5248

The properties of spectral subranges of scales in a boundary layer at Mach=2.3 and friction Reynolds number Reτ = 570 are investigated by analysing DNS data. One major aim is to examine whether footprinting and modulation of small-scale near-wall motions by outer large structures, observed at high Reynolds numbers, also pertain to this low-Reynolds-number case, or whether the logarithmic layer simply contains a continuous hierarchy of motions without specific outer scales playing a distinctive role. To this end, the spectrum of scales is decomposed into modes by application of the “Empirical Mode Decomposition”. The properties of different scales are then investigated by means of spectra, maps of isotropy/anisotropy parameters, the premultiplied derivative of the second-order structure function, correlation coefficients and joint probability density function (PDF), the last constructed from conditionally sampled data for the small-scale motions within the large-scale footprints. A clear commonality is identified between interactions in high-Reynolds-number channel flow and the present low-Reynolds-number boundary layer.

Agostini L, Leschziner M, 2017, Spectral analysis of near-wall turbulence in channel flow at Re-tau=4200 with emphasis on the attached-eddy hypothesis, *Physical Review Fluids*, Vol: 2, ISSN: 2469-990X

Direct numerical simulation data for channel flow at a friction Reynolds number of 4200, generated by Lozano-Durán and Jiménez [J. Fluid Mech. 759, 432 (2014)], are used to examine the properties of near-wall turbulence within subranges of eddy-length scale. Attention is primarily focused on the intermediate layer (mesolayer) covering the logarithmic velocity region within the range of wall-scaled wall-normal distance of 80–1500. The examination is based on a number of statistical properties, including premultiplied and compensated spectra, the premultiplied derivative of the second-order structure function, and three scalar parameters that characterize the anisotropic or isotropic state of the various length-scale subranges. This analysis leads to the delineation of three regions within the map of wall-normal-wise premultiplied spectra, each characterized by distinct turbulence properties. A question of particular interest is whether the Townsend-Perry attached-eddy hypothesis (AEH) can be shown to be valid across the entire mesolayer, in contrast to the usual focus on the outer portion of the logarithmic-velocity layer at high Reynolds numbers, which is populated with very-large-scale motions. This question is addressed by reference to properties in the premultiplied scalewise derivative of the second-order structure function (PMDS2) and joint probability density functions of streamwise-velocity fluctuations and their streamwise and spanwise derivatives. This examination provides evidence, based primarily on the existence of a plateau region in the PMDS2, for the qualified validity of the AEH right down the lower limit of the logarithmic velocity range.

Agostini L, Leschziner M, 2016, On the validity of the quasi-steady-turbulence hypothesis in representing the effects of large scales on small scales in boundary layers, *Physics of Fluids*, Vol: 28, ISSN: 1089-7666

The “quasi-steady hypothesis,” as understood in the context of large-scale/small-scale interactions in near-wall turbulence, rests on the assumption that the small scales near the wall react within very short time scales to changes imposed on them by energetic large scales whose length scales differ by at least one order of magnitude and whose energy reaches a maximum in the middle to the outer portion of the log-law layer. A key statistical manifestation of this assumption is that scaling the small-scale motions with the large-scale wall-friction-velocity footprints renders the small-scale statistics universal. This hypothesis is examined here by reference to direct numerical simulation (DNS) data for channel flow at Reτ ≈ 4200, subjected to a large-scale/small-scale separation by the empirical mode decomposition method. Flowproperties examined include the mean velocity, second moments, joint probability density functions, and skewness. It is shown that the validity of the hypothesis depends on the particular property being considered and on the range of length scales of structures included within the large-scale spectrum. The quasi-steady hypothesis is found to be well justified for the mean velocity and streamwise energy of the small scales up to y+∼O(80)y+∼O(80), but only up to y+∼O(30)y+∼O(30) for other properties.

Agostini L, Leschziner M, Gaitonde D, 2016, Skewness-induced asymmetric modulation of small-scale turbulence by large-scale structures, *Physics of Fluids*, Vol: 28, ISSN: 1089-7666

Agostini L, Leschziner M, 2016, Predicting the response of small-scale near-wall turbulence to large-scale outer motions, *Physics of Fluids*, Vol: 28, ISSN: 1089-7666

Leschziner M, Reeks M, Rodi W,
et al., 2015, Engineering Turbulence Modelling, Simulation and Measurements Preface, *FLOW TURBULENCE AND COMBUSTION*, Vol: 95, Pages: 191-192, ISSN: 1386-6184

Leschziner M, 2015, Statistical Turbulence Modelling for Fluid Dynamics - Demystified: An Introductory Text for Graduate Engineering Students, ISBN: 9781783266609

This book is intended for self-study or as a companion of lectures delivered to post-graduate students on the subject of the computational prediction of complex turbulent flows.

Agostini L, Touber E, Leschziner MA, 2015, The turbulence vorticity as a window to the physics of friction-drag reduction by oscillatory wall motion, *INTERNATIONAL JOURNAL OF HEAT AND FLUID FLOW*, Vol: 51, Pages: 3-15, ISSN: 0142-727X

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- Citations: 21

Agostini L, Leschziner MA, 2014, On the influence of outer large-scale structures on near-wall turbulence in channel flow (vol 26, 075107, 2014), *PHYSICS OF FLUIDS*, Vol: 26, ISSN: 1070-6631

Agostini L, Leschziner MA, 2014, On the influence of outer large-scale structures on near-wall turbulence in channel flow, *PHYSICS OF FLUIDS*, Vol: 26, ISSN: 1070-6631

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- Citations: 72

Agostini L, Touber E, Leschziner MA, 2014, Spanwise oscillatory wall motion in channel flow: drag-reduction mechanisms inferred from DNS-predicted phase-wise property variations at Re-tau=1000, *Journal of Fluid Mechanics*, Vol: 743, Pages: 606-635, ISSN: 0022-1120

A direct-numerical-simulation-based study is presented, which focuses on the response of near-wall turbulence and skin friction to the imposition of an oscillatory spanwise wall motion in channel flow. One point of contrast to earlier studies is the relatively high Reynolds number of the flow, namely Reτ=1000 in the unforced baseline flow. Another is the focus on transients in the drag that are in the form of moderate oscillatory variations in the skin friction and near-wall turbulence around the low-drag state at a sub-optimal actuation period. These conditions allow phase-averaged statistics to be extracted, during the periodic drag decrease and rise, that shed light on the interaction between turbulence and the unsteady Stokes strain. Results are presented for, among others, the phase-averaged second moments of stochastic fluctuations and their budgets, enstrophy components and joint probability density functions. The study identifies velocity skewness – the wall-normal derivative of the angle of the velocity vector – as playing a significant role in the streak-damping process during the drag-reduction phase. Furthermore, the phase-wise asymmetry in the skewness is identified as the source of a distinctive hysteresis in all properties, wherein the drag decrease progresses over a longer proportion of the actuation cycle than the drag increase. This feature, coupled with the fact that the streak-generation time scale limits the ability of the streaks to re-establish themselves during the low-skewness phase when the actuation period is sufficiently short, is proposed to drive the drag-reduction process. The observations in the study thus augment a previously identified mechanism proposed by two of the present authors, in which the drag-reduction process was linked to the rate of change in the Stokes strain in the upper region of the viscous sublayer where the streaks are strongest. Furthermore, an examination of the stochastic-stress budgets and the

Bentaleb Y, Leschziner MA, 2013, The structure of a three-dimensional boundary layer subjected to streamwise-varying spanwise-homogeneous pressure gradient, 7th International Symposium on Turbulence Heat and Mass Transfer (THMT), Publisher: ELSEVIER SCIENCE INC, Pages: 109-119, ISSN: 0142-727X

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- Citations: 3

Lardeau S, Leschziner MA, 2013, The streamwise drag-reduction response of a boundary layer subjected to a sudden imposition of transverse oscillatory wall motion, *PHYSICS OF FLUIDS*, Vol: 25, ISSN: 1070-6631

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- Citations: 32

Blesbois O, Chernyshenko SI, Touber E,
et al., 2013, Pattern prediction by linear analysis of turbulent flow with drag reduction by wall oscillation, *JOURNAL OF FLUID MECHANICS*, Vol: 724, Pages: 607-641, ISSN: 0022-1120

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- Citations: 28

Agostini L, Touber E, Leschziner MA, 2013, Spanwise oscillatory wall motion in channel flow: Drag-reduction mechanisms inferred from DNS-predicted phase-wise property variations at Re<inf>τ</inf> = 1000

A DNS-based study is presented, which focuses on the response of near-wall turbulence and skin friction to the imposition of an oscillatory spanwise wall motion in channel flow. The main focus is on transients in the drag, at Reτ=1000, that are in the form of moderate oscillatory variations in the phase-averaged skin friction and near-wall turbulence around the low-drag state at non-optimal actuation conditions at which the drag reduction margin does not reach the highest possible level. The study reveals a distinctive hysteresis in the periodic fall and rise of the drag, and the results allow the interaction between drag and the turbulence response to the unsteady Stokes strain to be illuminated.

Lardeau S, Leschziner M, Zaki T, 2012, Large Eddy Simulation of Transitional Separated Flow over a Flat Plate and a Compressor Blade, *FLOW TURBULENCE AND COMBUSTION*, Vol: 88, Pages: 19-44, ISSN: 1386-6184

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- Citations: 72

Leschziner MA, Hanjalic K, Rodi W, 2012, Special Issue: Engineering Turbulence Modelling, Simulation and Measurements Preface, *FLOW TURBULENCE AND COMBUSTION*, Vol: 88, Pages: 1-2, ISSN: 1386-6184

Touber E, Leschziner MA, 2012, Near-wall streak modification by spanwise oscillatory wall motion and drag-reduction mechanisms, *Journal of Fluid Mechanics*, Vol: 693, Pages: 150-200

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