Publications 12
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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ArticleNew Journal of Physics. 2013, Vol. 15, Issue 1, p. 13040. DOI: 10.1088/13672630/15/1/013040
 Publication URL http://stacks.iop.org/13672630/15/i=1/a=013040
 Abstract We report highly resolved temperature measurements in turbulent Rayleigh–Bénard convection in air at a fixed Prandtl number \( Pr = 0.7 \). Extending our previous work (du Puits et al 2007 J. Fluid Mech. 572 231–54), we carried out measurements at various aspect ratios while keeping the Rayleigh number constant. We demonstrate that the temperature field inside the convective boundary layers of both horizontal plates is virtually independent on the global flow pattern accompanying the variation in the aspect ratio. Thanks to technical upgrades of the experimental facility as well as a significant improvement of the accuracy and reliability of our temperature measurement—and unlike in our previous work—we find that the measured profiles of the timeaveraged temperature field neither follow a clear powerlaw trend nor fit a linear or a logarithmic scaling over a significant fraction of the boundarylayer thickness. Analyzing the temperature data simultaneously acquired at both horizontal plates, various transitions in the crosscorrelation and the autocorrelation function of the temperature signals are observed while varying the aspect ratio \( \Gamma \). These transitions might be associated with a change in the global flow pattern from a singleroll mode at \( \Gamma = 1 \) toward a double or a multiroll mode pattern at higher aspect ratios.

ArticleReview of Scientific Instruments. 2011, Vol. 82, Issue 2. DOI: 10.1063/1.3548924

ArticlePhys. Rev. Lett.. 2014, Vol. 112, Issue 12, p. 124301. DOI: 10.1103/PhysRevLett.112.124301
 Publication URL http://link.aps.org/doi/10.1103/PhysRevLett.112.124301
 Abstract Flow visualizations and particle image velocimetry measurements in the boundary layer of a RayleighBénard experiment are presented for the Rayleigh number \( \text{Ra} = 1.4 \times 10^{10} \). Our visualizations indicate that the appearance of the flow structures is similar to ordinary (isothermal) turbulent boundary layers. Our particle image velocimetry measurements show that vorticity with both positive and negative sign is generated and that the smallest flow structures are 1 order of magnitude smaller than the boundary layer thickness. Additional local measurements using laser Doppler velocimetry yield turbulence intensities up to \( I = 0.4 \) as in turbulent atmospheric boundary layers. From our observations, we conclude that the convective boundary layer becomes turbulent locally and temporarily although its Reynolds number \( \text{Re} \approx 200 \) is considerably smaller than the value 420 underlying existing phenomenological theories. We think that, in turbulent RayleighBénard convection, the transition of the boundary layer towards turbulence depends on subtle details of the flow field and is therefore not universal.


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.

ArticleReview of Scientific Instruments. 2014, Vol. 85, Issue 7. DOI: 10.1063/1.4884717
 Publication URL http://scitation.aip.org/content/aip/journal/rsi/85/7/10.1063/1.4884717
 Abstract The CoLaPipe is a novel test facility at the Department of Aerodynamics and Fluid Mechanics, Brandenburg University of Technology CottbusSenftenberg (BTU CottbusSenftenberg), set up to investigate fully developed pipe flow at high Reynolds numbers (\( Re_{m} \leq 1.5 \times 10^{6} \)). The design of the CoLaPipe is closedreturn with two available test sections providing a lengthtodiameter ratio of \( L/D = 148 \) and \( L/D = 79 \). Within this work, we introduce the CoLaPipe and describe the various components in detail, i.e., the settling chamber, the inlet contraction, the blower, bends, and diffusers as well as the cooling system. A special feature is the numerically optimized contraction design. The applications of different measuring techniques such as hotwire anemometry and static pressure measurements to quantitatively evaluate the mean flow characteristics and turbulence statistics are discussed as well. In addition, capabilities and limitations of available and new pipe flow facilities are presented and reconsidered based on their lengthtodiameter ratio, the achieved Reynolds numbers, and the resulting spatial resolution. Here, the focus is on the facility design, the presentation of some basic characteristics, and its contribution to a reviewed list of specific questions still arising, e.g., scaling and structural behavior of turbulent pipe flow as well as the influence of the development length on turbulence investigations.

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.

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.

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.

ArticleNature. 2000, Vol. 404, Issue 6780, p. 837840. DOI: 10.1038/35009036
 Abstract Turbulent convection occurs when the Rayleigh number (Ra)—which quantifies the relative magnitude of thermal driving to dissipative forces in the fluid motion—becomes sufficiently high. Although many theoretical and experimental studies of turbulent convection exist, the basic properties of heat transport remain unclear. One important question concerns the existence of an asymptotic regime that is supposed to occur at very high Ra. Theory predicts that in such a state the Nusselt number (Nu), representing the global heat transport, should scale as \( \text{Nu} \propto \text{Ra}^{\beta }\) with \( \beta = 1/2 \). Here we investigate thermal transport over eleven orders of magnitude of the Rayleigh number (\( 10^{6} \leq \text{Ra} \leq 10^{17} \)), using cryogenic helium gas as the working fluid. Our data, over the entire range of Ra, can be described to the lowest order by a single powerlaw with scaling exponent \( \beta \) close to 0.31. In particular, we find no evidence for a transition to the \( \text{Ra}^{1/2} \) regime. We also study the variation of internal temperature fluctuations with Ra, and probe velocity statistics indirectly.