Publications 32
Here we present a list of publications that were a result of projects funded by EuHIT or were published by members of EuHIT consortium.
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Conference ProceedingsGeophysical Research Abstracts. EGU2017659031. 2017, Vol. 19, p. 1.
 Keywords Fractal grids, Turbulence, Sparce Flow, Intermittency
 Abstract Flat 2D fractal grids are known to alter turbulence characteristics downstream of the grid as compared to the regular grids with the same blockage ratio and the same mass inflow rates. This has excited interest in the turbulence community for possible exploitation for enhanced mixing and related applications. Recently, a new 3D multiscale grid design has been proposed, such that each generation of length scale of turbulence grid elements is held in its own frame, the overall effect is a 3D coplanar arrangement of grid elements. This produces a ‘sparse’ grid system whereby each generation of grid elements produces a turbulent wake pattern that interacts with the other wake patterns downstream. A critical motivation here is that the effective blockage ratio in the 3D Sparse Grid Turbulence (3DSGT) design is significantly lower than in the flat 2D counterpart – typically the blockage ratio could be reduced from say 20 in 2D down to 4 in the 3DSGT. If this idea can be realized in practice, it could potentially greatly enhance the efficiency of turbulent mixing and transfer processes clearly having many possible applications. Work has begun on the 3DSGT experimentally using Surface Flow Image Velocimetry (SFIV) at the European facility in the Max Planck Institute for Dynamics and SelfOrganization located in Gottingen, Germany and also at the Technical University of Catalonia (UPC) in Spain, and numerically using Direct Numerical Simulation (DNS) at King Fahd University of Petroleum Minerals (KFUPM) in Saudi Arabia and in University of Warsaw in Poland. DNS is the most useful method to compare the experimental results with, and we are studying different types of codes such as Imcompact3d, and OpenFoam. Many variables will eventually be investigated for optimal mixing conditions. For example, the number of scale generations, the spacing between frames, the size ratio of grid elements, inflow conditions, etc. We will report upon the first set of findings from the 3DSGT.

ArticlePhysical Review B. 2016, Vol. 94, Issue 21, p. 214503. DOI: 10.1103/PhysRevB.94.214503
 Publication URL https://link.aps.org/doi/10.1103/PhysRevB.94.214503
 Abstract We report recent investigations into the transition to turbulence in superfluid \(^{4}\)He, realized experimentally by measuring the drag forces acting on two custommade quartz tuning forks with fundamental resonances at 6.5 kHz and 55.5 kHz, in the temperature range 10 mK to 2.17 K. In pure superfluid in the zero temperature limit, three distinct critical velocities were observed with both tuning forks. We discuss the significance of all critical velocities and associate the third critical velocity reported here with the development of large vortical structures in the flow, which thus starts to mimic turbulence in classical fluids. The interpretation of our results is directly linked to previous experimental work with oscillators such as tuning forks, grids, and vibrating wires, focusing on the behavior of purely superfluid \(^{4}\)He at very low temperatures.

Conference Paper"Computational Methods in Science and Technology". 2015, Vol. 21, p. 211.
 Publication URL http://cmst.eu/articles/numericalinvestigationsoftaylorcouetteflowusingdnssvvmethod/
 Keywords rotating fluids, Reynolds stress tensor components, DNS/SVV

Conference PaperContributions to the Foundations of Multidisciplinary Research in Mechanics, edited by J.M. Floryan. 2017, Vol. 2, p. 1015.
 Publication URL http://iutam.org/publications/ictamproceedings/ictam_2016/
 Keywords TaylorCouette flow, DNS/SVV, PIV
 Abstract The authors present the results obtained during a numerical investigations of the TylorCouette flow in configurations of aspect ratios 11.75 and 3.76, different radii ratios and endwall boundary conditions. The main purpose of the research is to investigate the influence of geometrical parameters and endwalls boundary conditions on the flow structure, the torque fluctuations and its total values. The results are discussed in the light of Brauckmann, Eckhardt [1] and Dong [2] data obtained with periodicity condition in axial direction. The results are compared also with Wendt [3] experimental results. For the cavity of aspect ratio 3.76 the authors study the laminarturbulent transition for the radii ratio from the range: 0.375  0.821 (the comparison with the EuHIT PIV results are presented). The authors have found that the transition from the threecell structure to the onecell structure occurs only for the lower ratios Computations are performed for Re up to 3100. All presented results have been obtained with the use of DNS/SVV method in which an artificial viscous operator is added to Laplace operator to stabilize the computational process for higher Reynolds numbers.

Conference ProceedingsInternational Workshop on Complex Turbulent Flows (IWCTF2017). 2017.

Abstract
The experimental campaign performed during 2017 in the Low Turbulence Wind Tunnel at the MaxPlanckInstitute for Dynamics and SelfOrganization is described and some of the preliminary results presented. The results compare several types of fractal and multifractal wakes in new configurations, these are based upon previous numerical and experimental turbulence data obtained by Vassilicos et al. [1,2], but we also include different types of grids (2D and 3D). Additional long velocity time series were obtained in order to also evaluate higher order structure functions in the scope of the Euhit Funded project,ˆaTurbulence Generated by Sparse 3D MultiScale Grid” in Gottinguen, Germany. The presented turbulence velocity data has been collected by means of a single hotwire anemometry, other PIV and flow visualizations will be discussed elsewhere. We compare the properties of turbulent flow generated by plane grids such as the configuration shown in Fig. 1 as well as Sierpinski type of configurations. the concept of a sparse threedimensional multiscale grid turbulence generator (3DSG) is expected to reduce the blockage of the grids, here the layers of grids of different size (Fig. 2) in a multiplane configuration is similar to the coplanar arrangement [2], but in the case of sparce grids [3, 4] producing an overall blockage ratio which is substantially less than in a comparable 2D arrangement. The experimental results will be compared with data generated from traditional fractal grids, and also with theoretical and numerical calculations where posible. In this case in order to compare the same 2D grate located in the wind tunnel, the developmment of a procedure capable of removing part of the effect of the 2D grid, so is posible to convert the structure of a grid in either fractal type grid or as simple grid. This code is written in MATALB, and may also be used for Sierpinski grids [5]. The comparison of PDF’s, Spectra and Higher order Structure Functions [6,7] is being used in the different facilities describing nonhomogeneous turbulence, because it is important to study and compare the role of nonuniform turbulence and intermittency in the different grids, Fig. 3 shows some results of the spectra and structure functions of Sierpinski grids of order 124. The 3D sparse grid turbulence generators(3DSGT) which reduces the effective blockage ratio, seem to possess a much greater parameter space [8,9], which could allow further optimisation of the turbulence, when compared to the similar structure 2DF grids [1,2]. Here, we report some results. The blockage ratio (solidity) is greatly reduced in the 3D sparse (3DS) grid system, while the mass flow rate is greatly enhanced and is sensitive to the largest scale in the grid system. (Fig. 4 ) Within the range of parameters examined, the enhancement in the mass fluxes was found to be between 21 and 38.
References
[1] S. Laizet J. C. Vassilicos. “DNS of FractalGenerated Turbulence”. In Flow Turbulence Combustion 87 (2011) 673705.
[2] QueirosConde J. C. Vassilicos. “Turbulent Wakes in 3D Fractal Grids”.In Intermittency in Turbulent Flows. (ed. J. C. Vassilicos.) Cambridge University Press 1(2001).
[3] N. A. Malik. “Sparse 3D MultiScale Grid Turbulence Generator”. In US Patent No. US 9,599,269 B2 1 (2017).
[4] N. A. Malik. “Sparse 3D MultiScale Grid Turbulence Generator”. In EPO Patent no. 2965805 1 (2017).
[5] C. E. LeytonPavez, J. M. Redondo, A. M. TarquisAlfonso, J. C. GilMarti‘In, J. D. TellezAlvarez. “Fractal analysis of growing cities and its relationship with health centre distribution”.In Proceedings of ISP RAS 29 2 (2017), 201214.
[6] Mahjoub, O. B., Babiano A. and Redondo, J. M. “Structure functions in complex flows”. In Flow, Turbulence and Combustion 59 (1998) 299313.
[7] Tarquis, A. M., Platonov, A., Matulka, A., Grau, J., Sekula, E., Diez, M., Redondo, J. M.. “ Application of multifractal analysis to the study of SAR features and oil spills on the ocean surface”. In Nonlinear Processes in Geophysics 21 (2014) 439450.
[8] J. M. Redondo, A. M. Tarquis, J. D. Tellez. “Fractal analysis of Sierpinski Flows”.In Proceedings of ISP RAS 30 1 (2018),
[9] S. S. Strijhak, J.M. Redondo, J. Tellez. “Multifractal Analysis of a Wake for a Single Wind Turbine”.In Proceedings Topical Problems of Fluid Mechanics 2017, Prague, 2017 Edited by David Simurda and Tom´as Bodn´ar 360 pp. 275284. ISBN 9788087012611, ISSN 23365781.

Abstract
The experimental campaign performed during 2017 in the Low Turbulence Wind Tunnel at the MaxPlanckInstitute for Dynamics and SelfOrganization is described and some of the preliminary results presented. The results compare several types of fractal and multifractal wakes in new configurations, these are based upon previous numerical and experimental turbulence data obtained by Vassilicos et al. [1,2], but we also include different types of grids (2D and 3D). Additional long velocity time series were obtained in order to also evaluate higher order structure functions in the scope of the Euhit Funded project,ˆaTurbulence Generated by Sparse 3D MultiScale Grid” in Gottinguen, Germany. The presented turbulence velocity data has been collected by means of a single hotwire anemometry, other PIV and flow visualizations will be discussed elsewhere. We compare the properties of turbulent flow generated by plane grids such as the configuration shown in Fig. 1 as well as Sierpinski type of configurations. the concept of a sparse threedimensional multiscale grid turbulence generator (3DSG) is expected to reduce the blockage of the grids, here the layers of grids of different size (Fig. 2) in a multiplane configuration is similar to the coplanar arrangement [2], but in the case of sparce grids [3, 4] producing an overall blockage ratio which is substantially less than in a comparable 2D arrangement. The experimental results will be compared with data generated from traditional fractal grids, and also with theoretical and numerical calculations where posible. In this case in order to compare the same 2D grate located in the wind tunnel, the developmment of a procedure capable of removing part of the effect of the 2D grid, so is posible to convert the structure of a grid in either fractal type grid or as simple grid. This code is written in MATALB, and may also be used for Sierpinski grids [5]. The comparison of PDF’s, Spectra and Higher order Structure Functions [6,7] is being used in the different facilities describing nonhomogeneous turbulence, because it is important to study and compare the role of nonuniform turbulence and intermittency in the different grids, Fig. 3 shows some results of the spectra and structure functions of Sierpinski grids of order 124. The 3D sparse grid turbulence generators(3DSGT) which reduces the effective blockage ratio, seem to possess a much greater parameter space [8,9], which could allow further optimisation of the turbulence, when compared to the similar structure 2DF grids [1,2]. Here, we report some results. The blockage ratio (solidity) is greatly reduced in the 3D sparse (3DS) grid system, while the mass flow rate is greatly enhanced and is sensitive to the largest scale in the grid system. (Fig. 4 ) Within the range of parameters examined, the enhancement in the mass fluxes was found to be between 21 and 38.

ArticleJournal of Geophysical Research: Atmospheres. 2014, Vol. 119, Issue 11, p. 62926305. DOI: 10.1002/2013JD021243
 Keywords physical simulation, rotating tank, microburst
 Abstract A laboratory simulation of atmospheric microbursts is presented. The physical model of the atmospheric phenomenon is reproduced at a reduced scale in a rotating tank (TURLab, Italy), the similitude is based on the Froude number. Different experiments were carried out varying the Rossby number, and the analysis of four significant cases is presented. The velocity, vorticity, and turbulent kinetic energy fields are evaluated together with the swirling strength analysis. The comparison with the natural prototype is eventually shown and discussed.

Conference PaperProc. 36th International Conference on Ocean, Offshore and Arctic Engineering (OMAE). 2017, p. OMAE201761156.
 Abstract The probability of occurrence of rogue waves in wind generated fields is investigated experimentally in an annular windwave flume. Unlike many experiments on rogue waves, where waves are mechanically generated, here the wave field is forced naturally by wind as it is in the ocean. The peculiar geometry of the flume makes waves propagating circularly in an unlimitedfetch condition. Water surface elevation was measured at specific crosssections under the effect of different wind speeds to monitor the temporal evolution of the wave field. Results show that the kurtosis of the surface elevation, the fourth order mo ment of the probability density function and a measure of the percentage of extremes in a wave record, gradually increases in time with the evolution of the wave field. Deviations from Nor mal statistics are observed to be a function of wind speed. The maximum departure from Normality resembled the one induced by quasiresonant wavewave interactions and it is observed at the final stage of wave growth and immediately before reaching the fully development regime.

Conference ProceedingsGeophysical Research Abstracts. 2017, Vol. 1.
 Keywords Diffusion, Turbulence, 3DSGT
 Abstract Presentationin the Short course, Experimental and Field Turbulence in Geophysical and Environmental Flows. SC2/AS2.8/NP9.2

ArticleFlow, Turbulence and Combustion. 2004, Vol. 72, Issue 2, p. 245271. DOI: 10.1023/B:APPL.0000044414.48888.25
 Abstract In this paper we report an experimental investigation of various statistical properties of the spatial Fourier modes of the vorticity field in turbulent jets for a large range of Reynolds numbers (530 ≤R\(\lambda \)≤ 6100). The continuous time evolution of a spatial Fourier mode of the vorticity distribution, characterized by a welldefined wavevector, is obtained from acoustic scattering measurements. The spatial enstrophy spectrum, as a function of the spatial wavevector, is determined by scanning the incoming sound frequencies. Timefrequency analysis of the turbulent vorticity fluctuations is also performed for different length scales of the flows. Vorticity timecorrelations show that the characteristic time of a Fourier mode behaves as the sweeping time. Finally, we report preliminary Lagrangian velocity measurements obtained using acoustic scattering by soap bubbles inflated with helium. Gathering a large number of passages of isolated bubbles in the scattering volume, one is able to compute the Lagrangian velocity PDF and velocity spectrum. Despite the spatial filtering due to the finite size of the bubble, the latter exhibits a power law, with the 2 exponent predicted by the Kolmogorov theory, over one decade of frequencies.

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