Publications 34
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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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.


ArticleEPJ Web of Conferences. 2018, Vol. 180. DOI: 10.1051/epjconf/201818002020
 Publication URL https://www.epjconferences.org/articles/epjconf/abs/2018/15/epjconf_efm2018_02020/epjconf_efm2018_02020.html
 Abstract Modelling of largescale natural (thermallygenerated) turbulent flows (such as the turbulent convection in Earth’s atmosphere, oceans, or Sun) is approached in laboratory experiments in the simplified model system called the RayleighBénard convection (RBC). We present preliminary measurements of vertical temperature profiles in the cell with the height of 4.7 m, 7.15m in diameter, obtained at the Barrel of Ilmenau (BOI), the worldwide largest experimental setup to study highly turbulent RBC, newly equipped with the Luna ODiSIB optical fibre system. In our configuration, the system permits to measure the temperature with a high spatial resolution of 5 mm along a very thin glass optical fibre with the length of 5 m and seems to be perfectly suited for measurement of time series of instantaneous vertical temperature profiles. The system was supplemented with the two Pt100 vertically movable probes specially designed by us for reference temperature profiles measurements.

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.

ArticlePhysics of Fluid. 2016, Vol. 28, p. 044108. DOI: 10.1063/1.4947261
 Abstract We report measurements of the nearwall flow field in turbulent RayleighBénard convection in air (Pr = 0.7) using particle image velocimetry. The measurements were performed in a thin, rectangular sample at fixed Rayleigh number Ra = 1.45 × 1010. In particular, we focus on the evolution of the boundary layer that a single convection roll generates along its path at the lower horizontal plate. We identify three specific flow regions along this path: (i) a region of wallnormal impingement of the down flow close to one corner of the sample, (ii) a region where a shear layer with almost constant thickness evolves, and (iii) a region in which this boundary layer grows and eventually detaches from the plate surface at the opposite corner of the sample. Our measurements with a spatial resolution better than 1/500 of the total thickness of the boundary layer show that the typical velocity field as well as its statistics qualitatively varies between the three flow regions. In particular, it could be verified that the shear layer region covering about 75 of the total area of the plate is in transition to turbulence at the Rayleigh number as low as investigated in the present work.

Conference PaperJournal of Physics C.S.. 2016.
 Publication URL http://iopscience.iop.org/article/10.1088/17426596/760/1/012035/pdf
 Keywords TaylorCouette flow, Nusselt number distribution, torque
 Abstract he TaylorCouette flow with radial temperature gradient is a canonical problem for the study of heat transfer in engineering issues. However, gaining insight into the transitional TaylorCouette flow with temperature gradient still requires detailed experimental and numerical investigations. In the present paper we have performed computations for the cavity of aspect ratio Γ= 3.76 and radii ratios η= 0.82 and 0.375 with the heated rotating bottom disk and stationary outer cylinder. We analyse the influence of the endwall boundary conditions and the thermal conditions on the flow structure, and on the distributions of the Nusselt number and torque along the inner and outer cylinders. The averaged values along the inner cylinder of the Nusselt number and torque obtained for different Re are analysed in the light of the results published in [2, 16, 17].

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

Conference PaperThe IV International AMMCS Interdisciplinary Conference. 4. 2017, Vol. 2017, Issue August 2024, p. 2.
 Keywords Sparce grids, Fractal, Structure functions
 Abstract Flat 2D fractal grids alter turbulence characteristics downstream of the grid as compared to the regular grids with the same blockage ratio and mass inflow rates [1]. This has excited interest in the turbulence community for possible enhanced mixing and related applications. Recently, a new 3D multiscale grid design has been proposed [2] such that each generation of length scale of grid elements is held in its own frame, in overall coplanar arrangement, Fig. 1. 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) [3] at the European facility in the Max Planck Institute for Dynamics and SelfOrganization located in Gottingen, Germany and 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. Fig 2. shows some early analysis of the data, the temporal structure functions up to 5th order, Sn(t)=<u(t)u(0)n>, n=2,3,4,5, from velocity time series in air at a point along the channel centreline.

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.
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