Snowflakes in the atmospheric surface layer: Observation of particle-turbulence dynamics

Andras Nemes, Teja Dasari, Jiarong Hong, Michele Guala, Filippo Coletti

Research output: Contribution to journalArticlepeer-review

14 Scopus citations

Abstract

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 analysed in both Eulerian and Lagrangian frameworks 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 St = 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.

Original languageEnglish (US)
Pages (from-to)592-613
Number of pages22
JournalJournal of Fluid Mechanics
Volume814
DOIs
StatePublished - Mar 10 2017

Keywords

  • atmospheric flows
  • homogeneous turbulence
  • particle/fluid flow

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