Fluctuations of the wall shear stress vector in a large-scale natural convection cell

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 (Bruecker and Mikulich 2017, PLoS ONE 12, e0179253), 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=1.58e10 and a Prandtl number Pr=0.7. The total sampling period of 1,5 hours allowed to capture the dynamics of the magnitude and the orientation of the vector over several orders of characteristic time-scales 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 time-scale of which is coupled to the characteristic eddy turn-over 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

Sensing of minute airflow motions near walls using pappus-type nature-inspired sensors

This work describes the development and use of pappus-like structures as sensitive sensors to detect minute air-flow motions. We made such sensors from pappi taken from nature-grown seed, whose filiform hairs' length-scale is suitable for the study of large-scale turbulent convection flows. The stem with the pappus on top is fixated on an elastic membrane on the wall and tilts under wind-load proportional to the velocity magnitude in direction of the wind, similar as the biological sensory hairs found in spiders, however herein the sensory hair has multiple filiform protrusions at the tip. As the sensor response is proportional to the drag on the tip and a low mass ensures a larger bandwidth, lightweight pappus structures similar as those found in nature with documented large drag are useful to improve the response of artificial sensors. The pappus of a Dandelion represents such a structure which has evolved to maximize wind-driven dispersion, therefore it is used herein as the head of our sensor. Because of its multiple hairs arranged radially around the stem it generates uniform drag for all wind directions. While still being permeable to the flow, the hundreds of individual hairs on the tip of the sensor head maximize the drag and minimize influence of pressure gradients or shear-induced lift forces on the sensor response as they occur in non-permeable protrusions. In addition, the flow disturbance by the sensor itself is limited. The optical recording of the head-motion allows continuously remote-distance monitoring of the flow fluctuations in direction and magnitude. Application is shown for the measurement of a reference flow under isothermal conditions to detect the early occurrence of instabilities

Statistical and Temporal Characterization of Turbulent Raleigh-Bénard Convection Boundary Layers using Time-Resolved PIV Measurements

This contribution reports on near-wall flow field measurements in turbulent Rayleigh-Bénard convection (RBC) in air at a fixed Prandtl number Pr=0.7 and Rayleigh number Ra = 1.45E10. For the experiment the large scale convection (LSC) was confined to a rectangular box of 2.5x2.5x0.65 m^3 made of transparent acrylic sheets. Prior video-graphic visualizations of the bottom boundary layer flow by means of laser light sheet illumination of small particles indicated the presence of highly dynamic flow behaviour at flow conditions that classical stability analysis predict to still be in the laminar regime. While theory predicts a transition to turbulence at Reynolds numbers R_\delta \appox 420 the present investigation exhibits highly unsteady flow at a much lower Reynolds number of Re_\delta \appox 260 based on boundary layer thickness. With the help of the PIV data it can be demonstrated that the entrainment of turbulent structures from the mean wind into the boundary layer acts, alongside with the destabilization due to inner shear, as a second mechanism on its path to turbulence. Both contributions must be considered when predicting the critical bound towards the ultimate regime of thermal convection. The measurements rely on the acquisition of long, continuous sequences of particle image velocimetry (PIV) data from which both statistical and spectral information can be retrieved. Contrary to conventional implementation of the PIV technique the field of view is restricted to a narrow strip, generally extending in wall-normal direction. In this way both the acquisition frequency and the total number images of the employed high speed camera are proportionally increased. The temporally oversampled data allows the use of multi-frame PIV processing algorithms which reduces measurement uncertainties with respect to standard dual-frame analysis

Transition on local temperature fluctuations in highly turbulent convection

In 1997, a new turbulent regime has been observed in a Rayleigh-Bénard cell and has been interpreted as the “Ultimate Regime” of convection. This observation was based on global heat transfer measurements at very high Rayleigh numbers (Ra). Using a set-up similar to the one used in 1997, we examine the signature of this regime from within the flow itself. A systematic study of probe-size corrections shows that the earlier local temperature measurements within the flow were altered by an excessive size of thermometer, but not according to a theoretical model proposed in the literature. Using a probe one order of magnitude smaller than the one used previously, we find evidence that the transition to the very-high-Ra regime is indeed accompanied with a clear change in the statistics of temperature fluctuations in the flow