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Digital Shadowgraphy applied to the measurement of drifting snow

 

In the SLF boundary-layer wind-tunnel a digital shadowgraphy is employed to investigate the characteristics of the snow saltation. The system allows to obtain high-frequency and high-resolution information of mass-flux, velocity and size of the snow particles in the saltation layer, which can deepen the understanding of the uncovered physics of drifting snow.

The measurements of drifting snow in the SLF cold wind tunnel are also performed by employing Digital Shadowgraphy. The technique consists in illuminating the saltating snow crystals from one side of the tunnel and by acquiring images at high frequency from the other side of the tunnel. The obtained images contain little dark spots each one representing a snow particle flying through the field of view at the time of the acquisition.

    Die Darstellung zeigt schematisch den Aufbau der digitalen Shadowgraphie. Die Schneekristalle bewegen sich von rechts durch den Tunnel und werden von der hinteren Tunnelseite beleuchtet. Auf der gegenüberliegenden Seite werden in hoher Frequenz Fotos gemacht, auf denen dunkle Flecken – die Schneepartikel – zu sehen sind.
    Figure 1: Sketch of the digital shadowgraphy setup. (Image: SLF)
     

    The dimension of the particles is obtained by simple image analysis while their displacement in time is determined by applying a Particle Tracking Velocimetry algorithm. Particle size and velocity finally allow the estimation of the mass-flux. From the post-processing of all the acquired images we can obtain the spatial (i.e. mass-flux vertical profiles) and temporal features (i.e. mass-flux time-series) of the drifting process as well as any statistical distribution of particle dimension, velocity or any of their derived quantities.

    Our shadowgraphy system consists of a 90W spotlight LED lamp as a light source and a high-speed CMOS camera with spatial resolution of about 1MP and an acquisition frequency up to 3.6 kHz. The measurement plane is aligned before each experiment in the middle of the test section above the snow cover, as shown in Figure 1.

    Digital Shadowgraphy has several advantages over the more standard point-wise sensors:<br/>

    • the availability of information over a two-dimensional domain, thus allowing the acquisition of profiles without repeating the experiments multiple times along a vertical direction
    • the ability to capture also the vertical component of the mass-flux which can give useful information regarding the snow particle entrainment
    • the possibility to detect not only the dimension but also the shape of the particles
    • the possibility to run the experiment at high frequency

    Nevertheless the employment of a high-speed camera, the need for accurate calibration and alignment of the light source and camera make this technique not easy to deploy in the field at the moment.

    Latest development

    We have compared the results obtained by digital shadowgraphy and by the Snow Particle Counter to determine the reliability of the shadowgraphy technique in a typical drifting snow experiment. The study has resulted in fairly good matching between the two measurements techniques, and allowed to obtain optimal thresholds for the images post-processing in typical drifting snow experiments.

    Project details

    Project duration

    2013 - 2017

     

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