Ronald du Puits

We present measurements of the near-wall velocity field in turbulent Rayleigh-Bénard convection with a partially rough horizontal surface. These measurements cover Rayleigh numbers ranging from R a = 5.8 × 10 10 to R a = 8.0 × 10 11 , while the Prandtl number was fixed at P r = 0.7 . The measurements have been undertaken in the large-scale convection experiment “Barrel of Ilmenau,” which provides a very high resolution in space and time. The measurements confirm the prominent role of the ratio between the thickness of the boundary layer δ t h and the height of the roughness elements h , although transition effects only appear below δ t h / h ≈ 0.6 in our experiments. In addition, we calculated the ratio between the turbulent kinetic energy and the average kinetic energy in the boundary layer. This ratio remains virtually constant up to R a = 4.2 × 10 11 and increases beyond this value. This is another indication of a qualitative transition of the boundary layer flow field. We observed this transition above both the TOP and above the VALLEY regions of the rough surface, concluding that both regions may contribute similarly to an increase in heat flux. The transition in the flow field is purely induced by viscous effects. The Richardson number, based on the thickness of the boundary layer and the velocity of the mean wind, is much smaller than one for all Rayleigh numbers investigated. This indicates that viscous effects dominate the near-wall flow field, and buoyancy does not play any role in the variation of the flow field here. We have also analysed time series of the wall-normal velocity component w in the plane where the temperature fluctuates at its maximum. The distribution of the fluctuations of w deviates from a normal distribution for all investigated Rayleigh numbers. In particular, the distributions exhibit broader tails on both sides. However, the distributions show just a little asymmetry, which we would expect as a signature of thermal plumes.

We report first experimental data of the wall shear stress in turbulent air flow in a large-scale Rayleigh–Bénard experiment. Using a novel, nature-inspired measurement concept [C. H. Bruecker and V. Mikulich, PLoS One 12, e0179253 (2017)], we measured the mean and fluctuating part of the two components of the wall shear stress vector at the heated bottom plate at a Rayleigh number Ra = 1.58 × 10¹⁰ and a Prandtl number Pr = 0.7. The total sampling period of 1.5 h allowed us to capture the dynamics of the magnitude and the orientation of the vector over several orders of characteristic timescales of the large-scale circulation. We found the amplitude of short-term (turbulent) fluctuations to be following a highly skewed Weibull distribution, while the long-term fluctuations are dominated by the modulation effect of a quasi-regular angular precession of the outer flow around a constant mean, the timescale of which is coupled to the characteristic eddy turnover time of the global recirculation roll. Events of instantaneous negative streamwise wall shear occur when rapid twisting of the local flow happens. A mechanical model is used to explain the precession by tilting the spin moment of the large circulation roll and conservation of angular momentum. A slow angular drift of the mean orientation is observed in a phase of considerable weakening of mean wind magnitude.

We present the principle for a micro-sensor aimed at measuring local correlations of turbulent velocity and temperature. The operating principle is versatile and can be adapted for various types of flow. It is based on a micro-machined cantilever, on the tip of which a platinum resistor is patterned. The deflection of the cantilever yields an estimate for the local velocity, and the impedance of the platinum yields an estimate for the local temperature. The velocity measurement is tested in two turbulent jets: one with air at room temperature which allows us to compare with well-known calibrated reference anemometers, and another one in the GReC jet at CERN with cryogenic gaseous helium which allows a much larger range of resolved turbulent scales. The recording of temperature fluctuations is tested in the Barrel of Ilmenau which provides a controlled turbulent thermal flow in air. Measurements in the wake of a heated or cooled cylinder demonstrate the capability of the sensor to display the cross correlation between temperature and velocity correctly.

We report measurements of the near-wall flow field in turbulent Rayleigh-Bénard 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 wall-normal 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.