Publications 30
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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ArticleJournal of Fluid Mechanics. 2016, Vol. 786, p. 275293. DOI: 10.1017/jfm.2015.649
 Keywords turbulent boundary layers, turbulent convection, turbulent flows
 Abstract In this experimental work, the aim is to understand how turbulent thermal flows are enhanced by the destabilization of the boundary layers. Squarestud roughness elements have been added on the bottom plate of a rectangular Rayleigh–Bénard cell in air, to trigger instabilities in the boundary layers. The top plate is kept smooth. The cell proportions are identical to those of the water cell previously operated and described by Salort et al. (Phys. Fluids, vol. 26, 2014, 015112), but six times larger. The very large size of the Barrel of Ilmenau allows detailed velocity fields to be obtained using particle image velocimetry very close to the roughness elements. We found that the flow is quite different at low Rayleigh numbers, where there is no heattransfer enhancement, and at high Rayleigh numbers where there is a heattransfer enhancement due to the roughness. Below the transition, the fluid inside the notch, i.e. between the studs, is essentially at rest, though it is slowly recirculating. The velocity profiles on the top of obstacles and in grooves are fairly compatible with those obtained in the smooth case. Above the transition, on the other hand, we observe large incursions of the bulk inside the notch, and the velocity profiles on the top of obstacles are closer to the logarithmic profiles expected in the case of turbulent boundary layers.

ArticleJournal of Fluid Mechanics. 2016, Vol. 786, p. 275293. DOI: 10.1017/jfm.2015.649
 Publication URL https://www.cambridge.org/core/article/divclasstitleboundarylayerstructureinaroughrayleighbenardcellfilledwithairdiv/10160A8A9BD171A83DBE1427D7C6A2B9
 Abstract In this experimental work, the aim is to understand how turbulent thermal flows are enhanced by the destabilization of the boundary layers. Squarestud roughness elements have been added on the bottom plate of a rectangular Rayleigh–Bénard cell in air, to trigger instabilities in the boundary layers. The top plate is kept smooth. The cell proportions are identical to those of the water cell previously operated and described by Salort et al. (Phys. Fluids, vol. 26, 2014, 015112), but six times larger. The very large size of the Barrel of Ilmenau allows detailed velocity fields to be obtained using particle image velocimetry very close to the roughness elements. We found that the flow is quite different at low Rayleigh numbers, where there is no heattransfer enhancement, and at high Rayleigh numbers where there is a heattransfer enhancement due to the roughness. Below the transition, the fluid inside the notch, i.e. between the studs, is essentially at rest, though it is slowly recirculating. The velocity profiles on the top of obstacles and in grooves are fairly compatible with those obtained in the smooth case. Above the transition, on the other hand, we observe large incursions of the bulk inside the notch, and the velocity profiles on the top of obstacles are closer to the logarithmic profiles expected in the case of turbulent boundary layers.

ArticleFluid Dynamics Research. 2009, Vol. 41, Issue 2, p. 21407. DOI: 10.1088/01695983/41/2/021407
 Publication URL http://stacks.iop.org/18737005/41/i=2/a=021407
 Abstract Although the equations governing turbulent flow of fluids are well known, understanding the overwhelming richness of flow phenomena, especially in high Reynolds number turbulent flows, remains one of the grand challenges in physics and engineering. High Reynolds number turbulence is ubiquitous in aerospace engineering, ground transportation systems, flow machinery, energy production (from gas turbines to wind and water turbines), as well as in nature, e.g. various processes occurring in the planetary boundary layer. High Reynolds number turbulence is not easily obtained in the laboratory, since in order to have good spatial resolution for measurements, the size of the facility itself has to be large. In this paper, we discuss limitations of various existing facilities and propose a new facility that will allow good spatial resolution even at high Reynolds number. The work is carried out in the framework of the Center for International Cooperation in Long Pipe Experiments (CICLoPE), an international collaboration that many in the turbulence community have shown an interest to participate in.

ArticleArchives of Mechanics. 2016, Vol. 68, Issue 5, p. 395418.
 Publication URL http://am.ippt.pan.pl/am/issue/view/209
 Keywords TaylorCouette flow, torque, DNS
 Abstract In the paper the authors present the results obtained during a direct numerical simulation of the transitional TC flow in closed cavity. The spectral vanishing viscosity method is used to stabilized computations for higher Re. The attention is focused on the influence of the endwall boundary conditions on the flow structures and on statistics i.e. the radial profiles of the angular velocity, angular momentum, torque and the Reynolds stress tensor components. The obtained data are discussed in the light of the experimental results and the numerical results published in literature.

ArticleFluid Dyn. Res.. 2016, Vol. 48, p. 061414. DOI: 10.1088/01695983/48/6/061414
 Keywords baroclinic instability, double diffusive convection, geophysical fluid dynamics, environmental flows, rotating flows
 Abstract A waterfilled differentially heated rotating annulus with initially prepared stable vertical salinity profiles is studied in the laboratory. Based on twodimensional horizontal particle image velocimetry data and infrared camera visualizations, we describe the appearance and the characteristics of the baroclinic instability in this original configuration. First, we show that when the salinity profile is linear and confined between two nonstratified layers at top and bottom, only two separate shallow fluid layers can be destabilized. These unstable layers appear nearby the top and the bottom of the tank with a stratified motionless zone between them. This laboratory arrangement is thus particularly interesting to model geophysical or astrophysical situations where stratified regions are often juxtaposed to convective ones. Then, for more general but stable initial density profiles, statistical measures are introduced to quantify the extent of the baroclinic instability at given depths and to analyze the connections between this depthdependence and the vertical salinity profiles. We find that, although the presence of stable stratification generally hinders fulldepth overturning, doublediffusive convection can lead to development of multicellular sideways convection in shallow layers and subsequently to a multilayered baroclinic instability. Therefore we conclude that by decreasing the characteristic vertical scale of the flow, stratification may even enhance the formation of cyclonic and anticyclonic eddies (and thus, mixing) in a local sense.

ArticleBoundaryLayer Meteorology. 2014, Vol. 60, Issue 3, p. 235241. DOI: 10.1007/BF00119377

Abstract
We performed an experimental study using scale models in a hydrodynamic rotating channel, concerning the interactions between fluid flows and obstacles of different shapes. The study was meant to analyze the characteristics of the wakes observed on the lee side of quasibidimensional obstacles, in a neutral atmosphere.
The obstacles were halfcylinders (with aspect ratio 0.87), placed transversally on the channel bottom and totally submerged in the fluid. We call them “quasibidimensional” since their width was a little smaller than the channel width, thus allowing the flow to partially go round their edges.
The simulations were performed in the rotating hydraulic channel of ICGCNR in Turin, and included various conditions of rotation period and flow speed. An interesting behaviour of the wakes was found on the lee side of subsynopticscale obstacles, modelled in conditions of ReynoldsRossby similitude. More precisely, if a given threshold of flow velocity is exceeded, wake size is constant and is fully determined by the height of the obstacle.

Abstract
We performed an experimental study using scale models in a hydrodynamic rotating channel, concerning the interactions between fluid flows and obstacles of different shapes. The study was meant to analyze the characteristics of the wakes observed on the lee side of quasibidimensional obstacles, in a neutral atmosphere.

ArticleInternational Journal of Heat and Mass Transfer. 2014, Vol. 73, p. 752760. DOI: 10.1016/j.ijheatmasstransfer.2014.02.033
 Publication URL http://www.sciencedirect.com/science/article/pii/S0017931014001501
 Keywords Infrared thermography, Rayleigh–Bénard convection, Twodimensional heat flux
 Abstract We report highly resolved measurements of the local wall heat flux in turbulent Rayleigh–Bénard convection using an infrared camera. The measurements have been undertaken in a Rayleigh–Bénard cell with rectangular base of 2.50 m \( \times \) 0.65 m and a height of 2.5 m which is filled with air. First of all, it could be demonstrated that in a Rayleigh–Bénard cell with rectangular crosssection the timeaveraged wall heat flux locally deviates by 30 from its mean. Furthermore, a strong correlation between the global flow structure inside the cell and the distribution of the local wall heat flux could be identified.

ArticleJournal of Low Temperature Physics. 2016, Vol. 183, Issue 3, p. 208214. DOI: 10.1007/s1090901513974
 Publication URL http://dx.doi.org/10.1007/s1090901513974
 Abstract We present proofofconcept measurements of the vortex line density generated by a quartz tuning fork resonator probed by the attenuation of second sound in superfluid \[^{4}\] 4 He at 1.6 K. The force–velocity response of a quartz tuning fork operating at a frequency of 31 kHz exhibited the onset of extra damping at a velocity of 0.5 ms \[^{1}\]  1 . Attenuation of the 5th resonant mode of second sound was observed at the same velocity, indicating the production of vortex lines. Our measurements demonstrate that an increase of the drag coefficient corresponds to the development of quantum turbulence.

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.