Barrel of Ilmenau
Germany
Publications

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.

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.

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.

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.

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.