TY - GEN

T1 - Lagrangian trajectories and settling velocity of snowflakes

T2 - 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017

AU - Nemes, Andras

AU - Dasari, Teja

AU - Guala, Michele

AU - Hong, Jiarong

AU - Coletti, Filippo

PY - 2017

Y1 - 2017

N2 - We report on optical field measurements of snow settling in atmospheric turbulence at Reλ = 940. It is found that the snowflakes exhibit hallmark features of inertial particles in turbulence. The snow motion is analyzed in by large-scale particle imaging, while sonic anemometry is used to characterize the flow field. Additionally, the snowflake size and morphology are assessed by digital in-line holography. The low volume fraction and mass loading imply a one-way interaction with the turbulent air. Acceleration probability density functions show wide exponential tails consistent with laboratory and numerical studies of homogeneous isotropic turbulence. Invoking the assumption that the particle acceleration has a stronger dependence on the Stokes number than on the specific features of the turbulence (e.g. precise Reynolds number and large-scale anisotropy), we make inferences on the snowflakes' aerodynamic response time. In particular, we observe that their acceleration distribution is consistent with that of particles of Stokes number in the range 0.1-0.4 based on the Kolmogorov time scale. The still-air terminal velocities estimated for the resulting range of aerodynamic response times are significantly smaller than the measured snow particle fall speed. This is interpreted as a manifestation of settling enhancement by turbulence, which is observed here for the first time in a natural setting.

AB - We report on optical field measurements of snow settling in atmospheric turbulence at Reλ = 940. It is found that the snowflakes exhibit hallmark features of inertial particles in turbulence. The snow motion is analyzed in by large-scale particle imaging, while sonic anemometry is used to characterize the flow field. Additionally, the snowflake size and morphology are assessed by digital in-line holography. The low volume fraction and mass loading imply a one-way interaction with the turbulent air. Acceleration probability density functions show wide exponential tails consistent with laboratory and numerical studies of homogeneous isotropic turbulence. Invoking the assumption that the particle acceleration has a stronger dependence on the Stokes number than on the specific features of the turbulence (e.g. precise Reynolds number and large-scale anisotropy), we make inferences on the snowflakes' aerodynamic response time. In particular, we observe that their acceleration distribution is consistent with that of particles of Stokes number in the range 0.1-0.4 based on the Kolmogorov time scale. The still-air terminal velocities estimated for the resulting range of aerodynamic response times are significantly smaller than the measured snow particle fall speed. This is interpreted as a manifestation of settling enhancement by turbulence, which is observed here for the first time in a natural setting.

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M3 - Conference contribution

AN - SCOPUS:85033240606

T3 - 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017

BT - 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017

PB - International Symposium on Turbulence and Shear Flow Phenomena, TSFP10

Y2 - 6 July 2017 through 9 July 2017

ER -