Effect of Re λ and Rouse numbers on the settling of inertial droplets in homogeneous isotropic turbulence
Résumé
We present an experimental study on the settling velocity of dense sub-Kolmogorov particles in active-grid-generated turbulence in a wind tunnel. Using phase Doppler interferometry, we observe that the modifications of the settling velocity of inertial particles, under homogeneous isotropic
turbulence and dilute conditions (i.e., small liquid fraction φv ≤ O(10)−5 ), is controlled by the Taylor-Reynolds number Reλ of the carrier flow. Meanwhile, we did not find a strong influence of the ratio between the fluid and gravity accelerations on the particle settling behavior. Remarkably,
we find that that the degree of hindering experienced by the particles (i.e., the measured particle settling velocity is smaller in magnitude than its respective one in still fluid conditions) increases with Reλ. This observation is contrary to previous works at intermediate values of Reλ that report
the opposite effect: settling enhancement. Nonetheless, our trend is observed at all particle sizes investigated, and when previous experimental data is included into the analysis, our data suggest that the particle settling behavior may be non-monotonic with Reλ: inducing enhancement at
moderate values of Reλ, and at promoting hindering at higher values of Reλ. Moreover, at the highest Reλ studied, the settling enhancement regime ceases to exist. Finally, we find that the difference between the measured particle settling velocity (Vp) and the particle terminal velocity in still fluid conditions (VT ), normalized by the carrier phase rms fluctuations, (Vp − VT )/u scales linearly with the Rouse number Ro = VT /u (i.e., the ratio between the particles settling velocity and the fluid rms fluctuations). However, such behavior (Vp − VT )/u ∼ −Ro appears only to be valid for moderate values of the Rouse number.
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