12/28/2023 0 Comments Falling raindrop shapefor turbulent intensity of 10 % ( u ′ / V t see Table 1). Their conclusion was that both sized drops showed a decrease of the mean fall speeds (settling speeds) relative to terminal by 5 %–7 %. The recent article by Ren et al. (2020) uses direct numerical simulation (DNS) to study the drop dynamics of 2 and 3 mm sizes in turbulent flow. Predicted that mean settling velocity of rain drops modelled as rigid spheres in homogeneous isotropic turbulence (HIT) would decrease (from terminal) depending on turbulent intensity and the Reynolds number ( R e) but did not predict the enhanced dispersion of the fall speed distribution (observed by Bringi et al., 2018). The mechanism most likely for (non-transient) sub-terminal fall speeds (that Note, there is another 2DVD (SN72) outside the double fence a few meters away. To put this in perspective, the mean time between collisions for a 2 mm (3 mm) drop with any other sized drop is around 10 s (3 s) at rain rate of 50 mm h −1 (McFarquhar, 2004).įigure 1Close-up pictures of (a) the 100 Hz sonic anemometer (outside the wind-fence, upwind) and (b) 2DVD:SN16 (inside the double fence), together with Pluvio gauge and other instruments. The latter mechanisms depend on the collision frequency, and after a transient period of several 100 ms the drops recover to their terminal fall speeds (Szakáll et al., 2010). “slows” down after coalescence) and the super-terminal velocity afterīreakup (the smaller drop fragments tend to have the same fall speed as the Mechanisms are not clear, but sub-terminal velocity after a collision-coalescence event (tiny drop collides with large drop which A number of articles, for example Thurai et al. (2013), Larsen et al. (2014), Montero-Martinez and Garcia-Garcia (2016), Yu et al. (2016), and Bringi et al. (2018) are largely observationally-based,ĭocumenting conditions that sub- or super-terminal fall speeds occur. InĪddition, the distributions show finite dispersion and (at times) the shapeĬan exhibit skewness. Greater than, respectively, Gunn–Kinzer (Montero-Martinez et al., 2009). Sub-terminal or super-terminal when the mean fall speed is 30 % less or Where the mean fall speed can deviate from Gunn and Kinzer (1949), termed However, under windy or turbulent conditions the fall speed for a given dropĭiameter ( D) will not be unique and has to be treated as a distribution The measurement errors) when the conditions are calm (Bringi et al., 2018). Instruments have been shown to follow the Gunn–Kinzer relation (to within Time and again the raindrop (terminal)įall speed ( V t) versus diameter ( D) relation based on modern Under laboratory conditions with the pressure correction of Beard (1976) hasīeen the “gold” standard since 1949. Measurements of the terminal fall speed of drops by Gunn and Kinzer (1949)
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