We present a new technique to study preferential concentration of inertial particles in (active) grid generated turbulence. This method, based on the use of a Taylor hypothesis combined to high-speed imaging, allows t...
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We present a new technique to study preferential concentration of inertial particles in (active) grid generated turbulence. This method, based on the use of a Taylor hypothesis combined to high-speed imaging, allows to reconstruct unprecedentedly long fields of particles and therefore to analyse new large-scale features, not easily resolvable with standard high-speed imaging methods. We first show that the new approach robustly reproduces results on particles clustering previously reported from standard methods. We then extend the analysis to show the first evidence of superclustering (existence of clusters of clusters) of inertial particles in turbulence and present the first characterization of superclusters. Copyright (C) EPLA, 2015
Two 3D-3C velocimetry techniques for micro-scale measurements are compared: tomographic particle image velocimetry (Tomo-PIV) and 3D particle-tracking velocimetry (3D-PTV). Both methods are applied to experimental dat...
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Two 3D-3C velocimetry techniques for micro-scale measurements are compared: tomographic particle image velocimetry (Tomo-PIV) and 3D particle-tracking velocimetry (3D-PTV). Both methods are applied to experimental data from a confined shear-driven liquid droplet over a moving surface. The droplet has 200 mu m height and 2 mm diameter. Micro 3D-PTV and Tomo-PIV are used to obtain the tracer particle distribution and the flow velocity field for the same set of images. It is shown that the reconstructed particle distributions are distinctly different, where Tomo-PIV returns a nearly uniform distribution over the height of the volume, as expected, and PTV reveals a clear peak in the particle distribution near the plane of focus. In Tomo-PIV, however, the reconstructed particle peak intensity decreases in proportion to the distance from the plane of focus. Due to the differences in particle distributions, the measured flow velocities are also different. In particular, we observe Tomo-PIV to be in closer agreement with mass conservation. Furthermore, the random noise level is found to increase with distance to the plane of focus at a higher rate for 3D-PTV as compared to Tomo-PIV. Thus, for a given noise threshold value, the latter method can measure reliably over a thicker volume.
In 1997, a new turbulent regime has been observed in a Rayleigh-Benard cell and has been interpreted as the "Ultimate Regime" of convection. This observation was based on global heat transfer measurements at...
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In 1997, a new turbulent regime has been observed in a Rayleigh-Benard 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. Copyright (C) EPLA, 2009
We use an extended laser Doppler technique to track optically the velocity of individual particles in a high Reynolds number turbulent flow. The particle sizes are of the order of the Kolmogorov scale and the time res...
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We use an extended laser Doppler technique to track optically the velocity of individual particles in a high Reynolds number turbulent flow. The particle sizes are of the order of the Kolmogorov scale and the time resolution, 30 microseconds, resolves the fastest scales of the fluid motion. Particles are tracked for mean durations of the order of 10 Kolmogorov time scales and their accelerations are measured. For neutrally buoyant particles (fluid tracers), this technique matches the performance of the silicon strip detector technique introduced at Cornell University (Voth G. A. et al., J. fluid Mech., 469 (2002) 121). This reference dynamics is then compared to that of slightly heavier solid particles (density 1.4) and to air bubbles. We observe that the dynamics of the particles strongly depends on their density. Bubbles have a much faster dynamics and experience much higher accelerations than fluid tracers. Although the particles dynamics are different, we find that the probability distribution functions of accelerations normalized to the variance always remain very close to the one for the fluid tracers. Copyright (C) EPLA, 2008.
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