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Publications (10 of 229) Show all publications
Rezaeiravesh, S., Vinuesa, R., Liefvendahl, M. & Schlatter, P. (2018). Assessment of uncertainties in hot-wire anemometry and oil-film interferometry measurements for wall-bounded turbulent flows. European journal of mechanics. B, Fluids, 72, 57-73
Open this publication in new window or tab >>Assessment of uncertainties in hot-wire anemometry and oil-film interferometry measurements for wall-bounded turbulent flows
2018 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 72, p. 57-73Article in journal (Refereed) Published
Abstract [en]

In this study, the sources of uncertainty of hot-wire anemometry (HWA) and oil-film interferometry (OFI) measurements are assessed. Both statistical and classical methods are used for the forward and inverse problems, so that the contributions to the overall uncertainty of the measured quantities can be evaluated. The correlations between the parameters are taken into account through the Bayesian inference with error-in-variable (EiV) model. In the forward problem, very small differences were found when using Monte Carlo (MC), Polynomial Chaos Expansion (PCE) and linear perturbation methods. In flow velocity measurements with HWA, the results indicate that the estimated uncertainty is lower when the correlations among parameters are considered, than when they are not taken into account. Moreover, global sensitivity analyses with Sobol indices showed that the HWA measurements are most sensitive to the wire voltage, and in the case of OFI the most sensitive factor is the calculation of fringe velocity. The relative errors in wall-shear stress, friction velocity and viscous length are 0.44%, 0.23% and0.22%, respectively. Note that these values are lower than the ones reported in other wall-bounded turbulence studies. Note that in most studies of wall-bounded turbulence the correlations among parameters are not considered, and the uncertainties from the various parameters are directly added when determining the overall uncertainty of the measured quantity. In the present analysis we account for these correlations, which may lead to a lower overall uncertainty estimate due to error cancellation Furthermore, our results also indicate that the crucial aspect when obtaining accurate inner-scaled velocity measurements is the wind-tunnel flow quality, which is more critical than the accuracy in wall-shear stress measurements.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Hot-wire anemometry, Oil-film interferometry, Uncertainty quantification, Wall-bounded turbulence
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-228692 (URN)10.1016/j.euromechflu.2018.04.012 (DOI)000447570200005 ()2-s2.0-85047057608 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20180530

Available from: 2018-05-30 Created: 2018-05-30 Last updated: 2018-11-06Bibliographically approved
Wang, Z., Örlü, R., Schlatter, P. & Chung, Y. M. (2018). Direct numerical simulation of a turbulent 90° bend pipe flow. International Journal of Heat and Fluid Flow, 73, 199-208
Open this publication in new window or tab >>Direct numerical simulation of a turbulent 90° bend pipe flow
2018 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 73, p. 199-208Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulation (DNS) has been performed for a spatially developing 90° bend pipe flow to investigate the unsteady flow motions downstream of the bend. A recycling method is implemented to generate a fully-developed turbulent inflow condition. The Reynolds number of the pipe flow is ReD=5300 and the bend curvature is γ=0.4. A long straight pipe section (40D) is attached in the downstream of the bend to allow the flow to develop. Flow oscillations downstream of the bend are measured using several methods, and the corresponding oscillation frequencies are estimated. It is found that different characteristic frequencies are obtained from various flow measurements. The stagnation point movement and single-point velocity measurements may not be good measures to determine the swirl-switching frequency. The oscillations of the lateral pressure force on the pipe wall and half-sided mass flow rate are proposed to be a more unambiguous measure of the unsteady flow motions downstream of the bend. 

Place, publisher, year, edition, pages
Elsevier B.V., 2018
Keywords
90 degrees bend, Conditional averaging, Curved pipe, DNS, Swirl switching, Turbulent pipe flow, Direct numerical simulation, Flow rate, Numerical models, Pipe flow, Reynolds number, Turbulent flow, Characteristic frequencies, Curved pipes, Oscillation frequency, Stagnation points, Turbulent inflow conditions, Oscillating flow
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-236662 (URN)10.1016/j.ijheatfluidflow.2018.08.003 (DOI)2-s2.0-85052536769 (Scopus ID)
Note

Export Date: 22 October 2018; Article; CODEN: IJHFD; Correspondence Address: Chung, Y.M.; School of Engineering and Centre for Scientific Computing, University of WarwickUnited Kingdom; email: y.m.chung@warwick.ac.uk; Funding details: University of Warwick; Funding details: EP/L000261/1, EPSRC, Engineering and Physical Sciences Research Council; Funding text: This work has been supported by the Engineering and Physical Sciences Research Council grant no EP/L000261/1 . The authors would like to thank Professor Paul Fischer for the help in using Nek5000 . Simulations were performed on ARCHER, the UK National Supercomputing Service. This work also used the HPC facilities (Tinis) at the Centre for Scientific Computing, University of Warwick. QC 20181113

Available from: 2018-11-13 Created: 2018-11-13 Last updated: 2018-11-13Bibliographically approved
Rinaldi, E., Schlatter, P. & Bagheri, S. (2018). Edge state modulation by mean viscosity gradients. Journal of Fluid Mechanics, 838, 379-403
Open this publication in new window or tab >>Edge state modulation by mean viscosity gradients
2018 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 838, p. 379-403Article in journal (Refereed) Published
Abstract [en]

Motivated by the relevance of edge state solutions as mediators of transition, we use direct numerical simulations to study the effect of spatially non-uniform viscosity on their energy and stability in minimal channel flows. What we seek is a theoretical support rooted in a fully nonlinear framework that explains the modified threshold for transition to turbulence in flows with temperature-dependent viscosity. Consistently over a range of subcritical Reynolds numbers, we find that decreasing viscosity away from the walls weakens the streamwise streaks and the vortical structures responsible for their regeneration. The entire self-sustained cycle of the edge state is maintained on a lower kinetic energy level with a smaller driving force, compared to a flow with constant viscosity. Increasing viscosity away from the walls has the opposite effect. In both cases, the effect is proportional to the strength of the viscosity gradient. The results presented highlight a local shift in the state space of the position of the edge state relative to the laminar attractor with the consequent modulation of its basin of attraction in the proximity of the edge state and of the surrounding manifold. The implication is that the threshold for transition is reduced for perturbations evolving in the neighbourhood of the edge state in the case that viscosity decreases away from the walls, and vice versa.

Place, publisher, year, edition, pages
Cambridge University Press, 2018
Keywords
nonlinear dynamical systems, nonlinear instability, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-222027 (URN)10.1017/jfm.2017.921 (DOI)2-s2.0-85040834445 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

QC 20180131

Available from: 2018-01-31 Created: 2018-01-31 Last updated: 2018-11-14Bibliographically approved
Chin, C., Vinuesa, R., Örlü, R., Cardesa, J. I., Noorani, A., Schlatter, P. & Chong, M. S. (2018). Flow topology of rare back flow events and critical points in turbulent channels and toroidal pipes. In: Journal of Physics: Conference Series. Paper presented at 3rd Madrid Summer School on Turbulence, School of Aeronautics of the Universidad Politecnica de MadridMadrid, Spain, 29 May 2017 through 30 June 2017. Institute of Physics Publishing (IOPP), 1001(1), Article ID 012002.
Open this publication in new window or tab >>Flow topology of rare back flow events and critical points in turbulent channels and toroidal pipes
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2018 (English)In: Journal of Physics: Conference Series, Institute of Physics Publishing (IOPP), 2018, Vol. 1001, no 1, article id 012002Conference paper, Published paper (Refereed)
Abstract [en]

A study of the back flow events and critical points in the flow through a toroidal pipe at friction Reynolds number Reτ ≈ 650 is performed and compared with the results in a turbulent channel flow at Reτ ≈ 934. The statistics and topological properties of the back flow events are analysed and discussed. Conditionally-averaged flow fields in the vicinity of the back flow event are obtained, and the results for the torus show a similar streamwise wall-shear stress topology which varies considerably for the spanwise wall-shear stress when compared to the channel flow. The comparison between the toroidal pipe and channel flows also shows fewer back flow events and critical points in the torus. This cannot be solely attributed to differences in Reynolds number, but is a clear effect of the secondary flow present in the toroidal pipe. A possible mechanism is the effect of the secondary flow present in the torus, which convects momentum from the inner to the outer bend through the core of the pipe, and back from the outer to the inner bend through the pipe walls. In the region around the critical points, the skin-friction streamlines and vorticity lines exhibit similar flow characteristics with a node and saddle pair for both flows. These results indicate that back flow events and critical points are genuine features of wall-bounded turbulence, and are not artifacts of specific boundary or inflow conditions in simulations and/or measurement uncertainties in experiments.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2018
Series
Journal of Physics: Conference Series, ISSN 1742-6588 ; 1001
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-238246 (URN)10.1088/1742-6596/1001/1/012002 (DOI)2-s2.0-85046100946 (Scopus ID)
Conference
3rd Madrid Summer School on Turbulence, School of Aeronautics of the Universidad Politecnica de MadridMadrid, Spain, 29 May 2017 through 30 June 2017
Note

QC 20181101

Available from: 2018-11-01 Created: 2018-11-01 Last updated: 2018-11-01Bibliographically approved
Otero, E., Vinuesa, R., Marin, O., Laure, E. & Schlatter, P. (2018). Lossy Data Compression Effects on Wall-bounded Turbulence: Bounds on Data Reduction. Flow Turbulence and Combustion, 101(2), 365-387
Open this publication in new window or tab >>Lossy Data Compression Effects on Wall-bounded Turbulence: Bounds on Data Reduction
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2018 (English)In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 101, no 2, p. 365-387Article in journal (Refereed) Published
Abstract [en]

Postprocessing and storage of large data sets represent one of the main computational bottlenecks in computational fluid dynamics. We assume that the accuracy necessary for computation is higher than needed for postprocessing. Therefore, in the current work we assess thresholds for data reduction as required by the most common data analysis tools used in the study of fluid flow phenomena, specifically wall-bounded turbulence. These thresholds are imposed a priori by the user in L (2)-norm, and we assess a set of parameters to identify the minimum accuracy requirements. The method considered in the present work is the discrete Legendre transform (DLT), which we evaluate in the computation of turbulence statistics, spectral analysis and resilience for cases highly-sensitive to the initial conditions. Maximum acceptable compression ratios of the original data have been found to be around 97%, depending on the application purpose. The new method outperforms downsampling, as well as the previously explored data truncation method based on discrete Chebyshev transform (DCT).

Place, publisher, year, edition, pages
Springer, 2018
Keywords
Lossy data compression, Data reduction, Turbulence statistics, Orthogonal polynomials, Resilience
National Category
Bioinformatics (Computational Biology)
Identifiers
urn:nbn:se:kth:diva-237136 (URN)10.1007/s10494-018-9923-5 (DOI)000446583900006 ()2-s2.0-85047423287 (Scopus ID)
Funder
Swedish Foundation for Strategic Research Knut and Alice Wallenberg FoundationSwedish Research Council
Note

QC 20181025

Available from: 2018-10-25 Created: 2018-10-25 Last updated: 2018-10-25Bibliographically approved
Otero, E., Gong, J., Min, M., Fischer, P., Schlatter, P. & Laure, E. (2018). OpenACC accelerator for the Pn-Pn-2 algorithm in Nek5000. In: Proceedings of the 5th International Conference on Exascale Applications and Software: . Paper presented at The 5th International Conference on Exascale Applications and Software, 17th to 19th April 2018 in Edinburgh, Scotland.
Open this publication in new window or tab >>OpenACC accelerator for the Pn-Pn-2 algorithm in Nek5000
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2018 (English)In: Proceedings of the 5th International Conference on Exascale Applications and Software, 2018Conference paper, Oral presentation with published abstract (Refereed)
National Category
Computer and Information Sciences
Identifiers
urn:nbn:se:kth:diva-232362 (URN)978-0-9926615-3-3 (ISBN)
Conference
The 5th International Conference on Exascale Applications and Software, 17th to 19th April 2018 in Edinburgh, Scotland
Note

QC 20180725

Available from: 2018-07-20 Created: 2018-07-20 Last updated: 2018-07-25Bibliographically approved
Vidal, A., Nagib, H. M., Schlatter, P. & Vinuesa, R. (2018). Secondary flow in spanwise-periodic in-phase sinusoidal channels. Journal of Fluid Mechanics, 851, 288-316
Open this publication in new window or tab >>Secondary flow in spanwise-periodic in-phase sinusoidal channels
2018 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 851, p. 288-316Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulations (DNSs) are performed to analyse the secondary flow of Prandtl's second kind in fully developed spanwise-periodic channels with in-plane sinusoidal walls. The secondary flow is characterized for different combinations of wave parameters defining the wall geometry at Re-h = 2500 and 5000, where h is the half-height of the channel. The total cross-flow rate in the channel Q(yz) is defined along with a theoretical model to predict its behaviour. Interaction between the secondary flows from opposite walls is observed if lambda similar or equal to h similar or equal to A, where A and lambda are the amplitude and wavelength of the sinusoidal function defining the wall geometry. As the outer-scaled wavelength (lambda/h) is reduced, the secondary vortices become smaller and faster, increasing the total cross-flow rate per wall. However, if the inner-scaled wavelength (lambda(+)) is below 130 viscous units, the cross-flow decays for smaller wavelengths. By analysing cases in which the wavelength of the wall is much smaller than the half-height of the channel lambda << h, we show that the cross-flow distribution depends almost entirely on the separation between the scales of the instantaneous vortices, where the upper and lower bounds are determined by lambda/h and lambda(+), respectively. Therefore, the distribution of the secondary flow relative to the size of the wave at a given Re-h can be replicated at higher Re-h by decreasing lambda/h and keeping lambda(+) constant. The mechanisms that contribute to the mean cross-flow are analysed in terms of the Reynolds stresses and using quadrant analysis to evaluate the probability density function of the bursting events. These events are further classified with respect to the sign of their instantaneous spanwise velocities. Sweeping events and ejections are preferentially located in the valleys and peaks of the wall, respectively. The sweeps direct the instantaneous cross-flow from the core of the channel towards the wall, turning in the wall-tangent direction towards the peaks. The ejections drive the instantaneous cross-flow from the near-wall region towards the core. This preferential behaviour is identified as one of the main contributors to the secondary flow.

Place, publisher, year, edition, pages
Cambridge University Press, 2018
Keywords
turbulence simulation, turbulent flows
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-232762 (URN)10.1017/jfm.2018.498 (DOI)000439307100010 ()2-s2.0-85050633237 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg FoundationSwedish e‐Science Research Center
Note

QC 20180803

Available from: 2018-08-03 Created: 2018-08-03 Last updated: 2018-08-06Bibliographically approved
Vinuesa, R., Schlatter, P. & Nagib, H. M. (2018). Secondary flow in turbulent ducts with increasing aspect ratio. PHYSICAL REVIEW FLUIDS, 3(5), Article ID 054606.
Open this publication in new window or tab >>Secondary flow in turbulent ducts with increasing aspect ratio
2018 (English)In: PHYSICAL REVIEW FLUIDS, ISSN 2469-990X, Vol. 3, no 5, article id 054606Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulations of turbulent duct flows with aspect ratios 1, 3, 5, 7, 10, and 14.4 at a center-plane friction Reynolds number Re-tau,Re- c similar or equal to 180, and aspect ratios 1 and 3 at Re-tau,Re- c similar or equal to 360, were carried out with the spectral-element code NEK5000. The aim of these simulations is to gain insight into the kinematics and dynamics of Prandtl's secondary flow of the second kind and its impact on the flow physics of wall-bounded turbulence. The secondary flow is characterized in terms of the cross-plane component of the mean kinetic energy, and its variation in the spanwise direction of the flow. Our results show that averaging times of around 3000 convective time units (based on duct half-height h) are required to reach a converged state of the secondary flow, which extends up to a spanwise distance of around similar or equal to 5h measured from the side walls. We also show that if the duct is not wide enough to accommodate the whole extent of the secondary flow, then its structure is modified as reflected through a different spanwise distribution of energy. Another confirmation of the extent of the secondary flow is the decay rate of kinetic energy of any remnant secondary motions for z(c)/h > 5 (where z(c) is the spanwise distance from the corner) in aspect ratios 7, 10, and 14.4, which exhibits a decreasing level of energy with increasing averaging time t(a), and in its rapid rate of decay given by similar to t(a)(-1). This is the same rate of decay observed in a spanwise-periodic channel simulation, which suggests that at the core, the kinetic energy of the secondary flow integrated over the cross-sectional area, < K >(yz), behaves as a random variable with zero mean, with rate of decay consistent with central limit theorem. Long-time averages of statistics in a region of rectangular ducts extending about the width of a well-designed channel simulation (i.e., extending about similar or equal to 3h on each side of the center plane) indicate that ducts or experimental facilities with aspect ratios larger than 10 may, if properly designed, exhibit good agreement with results obtained from spanwise-periodic channel computations.

Place, publisher, year, edition, pages
American Physical Society, 2018
Keywords
Direct Numerical-Simulation, Straight Square Duct, Boundary-Layers, Rectangular Ducts, Reynolds-Numbers, Shear, Wall
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-230444 (URN)10.1103/PhysRevFluids.3.054606 (DOI)000433036100004 ()2-s2.0-85049037220 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilSwedish e‐Science Research Center
Note

QC 20180614

Available from: 2018-06-14 Created: 2018-06-14 Last updated: 2018-10-16Bibliographically approved
Saglietti, C., Schlatter, P., Wadbro, E., Berggren, M. & Henningson, D. S. (2018). Topology optimization of heat sinks in a square differentially heated cavity. International Journal of Heat and Fluid Flow, 74, 36-52
Open this publication in new window or tab >>Topology optimization of heat sinks in a square differentially heated cavity
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2018 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 74, p. 36-52Article in journal (Refereed) Published
Abstract [en]

Innovative designs of heat sinks are generated in the present paper through numerical optimization, by applying a material distribution topology optimization approach. The potential of the method is demonstrated in a two-dimensional differentially heated cavity, in which the heat transfer is increased by means of introducing a solid structure that acts as a heat sink. We simulate the heat transfer in the whole system by performing direct numerical simulations of the conjugated problem, i.e. temperature diffusion and convection in the entire domain and momentum conservation in the fluid surrounding the solid. The flow is driven by the buoyancy force, under the Boussinesq approximation, and we describe the presence of solid material as the action of a Brinkman friction force in the Navier–Stokes equations. To obtain a design with a given length scale, we apply regularization techniques by filtering the material distribution. Two different types of filters are applied and compared for obtaining the most realistic solution. Given the large scale of the problem, the optimization is solved with a gradient based method that relies on adjoint sensitivity analysis. The results show the applicability of the method by presenting innovative geometries that are increasing the heat flux. Moreover, the effect of various factors is studied: We investigate the impact of boundary conditions, initial designs, and Rayleigh number. Complex tree-like structures are favored when a horizontal temperature gradient is imposed on the boundary and when we limit the amount of solid volume in the cavity. The choice of the initial design affects the final topology of the generated solid structures, but not their performance for the studied cases. Additionally, when the Rayleigh number increases, the topology of the heat exchanger is able to substantially enhance the convection contribution to the heat transfer. 

Place, publisher, year, edition, pages
Elsevier B.V., 2018
Keywords
Conjugate heat transfer, Differentially heated cavity, Direct numerical simulations, Heat sink, Natural convection, Topology optimization, Direct numerical simulation, Friction, Heat flux, Heat sinks, Navier Stokes equations, Numerical models, Optimization, Sensitivity analysis, Shape optimization, Topology, Adjoint sensitivity analysis, Boussinesq approximations, Horizontal temperature gradient, Momentum conservations, Numerical optimizations, Regularization technique
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-236567 (URN)10.1016/j.ijheatfluidflow.2018.08.004 (DOI)2-s2.0-85053787782 (Scopus ID)
Note

 Funding details: Umeå Universitet; Funding text: Huawei Sweden is acknowledged for financially supporting the main part of this research. Additional funding was provided by the Swedish e-Science Research Center (SeRC). The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at the High Performance Computer Center North (HPC2N) at the Umeå University (UMU). QC 20181127

Available from: 2018-11-27 Created: 2018-11-27 Last updated: 2018-11-27Bibliographically approved
Appelquist, E., Schlatter, P., Alfredsson, P. H. & Lingwood, R. (2018). Turbulence in the rotating-disk boundary layer investigated through direct numerical simulations. European journal of mechanics. B, Fluids, 70, 6-18
Open this publication in new window or tab >>Turbulence in the rotating-disk boundary layer investigated through direct numerical simulations
2018 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 70, p. 6-18Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulations (DNS) are reported for the turbulent rotating-disk boundary layer for the first time. Two turbulent simulations are presented with overlapping small and large Reynolds numbers, where the largest corresponds to a momentum-loss Reynolds number of almost 2000. Simulation data are compared with experimental data from the same flow case reported by Imayama et al. (2014), and also a comparison is made with a numerical simulation of a two-dimensional turbulent boundary layer (2DTBL) over a flat plate reported by Schlatter and Örlü (2010). The agreement of the turbulent statistics between experiments and simulations is in general very good, as well as the findings of a missing wake region and a lower shape factor compared to the 2DTBL. The simulations also show rms-levels in the inner region similar to the 2DTBL. The simulations validate Imayama et al.’s results showing that the rotating-disk turbulent boundary layer in the near-wall region contains shorter streamwise (azimuthal) wavelengths than the 2DTBL, probably due to the outward inclination of the low-speed streaks. Moreover, all velocity components are available from the simulations, and hence the local flow angle, Reynolds stresses and all terms in the turbulent kinetic energy equation are also discussed. However there are in general no large differences compared to the 2DTBL, hence the three-dimensional effects seem to have only a small influence on the turbulence.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Near-wall turbulence, Rotation, Turbulence statistics
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-227538 (URN)10.1016/j.euromechflu.2018.01.008 (DOI)000432105000002 ()2-s2.0-85042080446 (Scopus ID)
Funder
Swedish e‐Science Research CenterSwedish Research Council
Note

QC 20180517

Available from: 2018-05-17 Created: 2018-05-17 Last updated: 2018-05-31Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-9627-5903

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