A wind tunnel study of the effects of collision processes on the shape and oscillation for moderate-size raindrops

Szakáll, Miklós; Kessler, S; Diehl, K.; Mitra, Subir K.; Borrmann, Stephan

doi:10.1016/j.atmosres.2013.09.005

Cited by 22 publications

(25 citation statements)

References 27 publications

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“…The highly symmetric axis ratio distributions (about the equilibrium value) from the 80 m fall experiment [Thurai and Bringi, 2005] also support, indirectly, the dominance of the axisymmetric mode for D > 2 mm [see also Kubesh and Beard, 1993]. Review articles by Beard et al [2010] and Szakáll et al [2010], and the references contained therein give a fuller description of drop shapes and oscillation modes. Thus, the background oscillation state of drops with D > 2 mm (in the absence of collisions) is the (2,0) mode.…”

Section: Raindrop Oscillationsmentioning

confidence: 77%

“…where r 0 = D/2 is the "mean" radius of the drop, D is the equivolume drop diameter, A is the oscillation amplitude, and P n,m (θ) are the Legendre polynomials. Recently, techniques based on the time variation of the apparent axis ratio and other image characteristics-such as circumscribed box and projected area-have been developed for the wind-tunnel measurements [Szakáll et al, 2014] to identify the three distinct fundamental modes: (i) the axisymmetric (spherical harmonic n = 2, m = 0) mode, (ii) the transverse (2,1) mode, and (iii) the horizontal (2,2) mode. For these modes, Legendre polynomial functions are…”

Section: Raindrop Oscillationsmentioning

confidence: 99%

“…(2) Figure 4 depicts the three fundamental modes at two different times within the oscillation cycle. From a frequency analysis of the time series of these image characteristics [Szakáll et al, 2014], it was found that the dominant mode of oscillation for D > 2.5 mm was the axisymmetric, "oblate-prolate" mode, with the smallamplitude transverse modes also being mixed in [e.g., Foote, 1973;Feng and Beard, 1991]. The highly symmetric axis ratio distributions (about the equilibrium value) from the 80 m fall experiment [Thurai and Bringi, 2005] also support, indirectly, the dominance of the axisymmetric mode for D > 2 mm [see also Kubesh and Beard, 1993].…”

Section: Raindrop Oscillationsmentioning

confidence: 99%

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Investigating raindrop shapes, oscillation modes, and implications for radio wave propagation

et al. 2014

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Studies of raindrop shapes, oscillation modes, and implications for radio wave propagation are presented. Drop shape measurements in natural rain using 2-D video disdrometers (2DVDs) are discussed. As a representative exception to vast majority of the cases where the "most probable" shapes conform to the axisymmetric (2,0) oscillation mode, an event with a highly organized line convection embedded within a larger rain system is studied. Measurements using two collocated 2DVD instruments and a C-band polarimetric radar clearly show the occurrence of mixed-mode drop oscillations within the line, which in turn is attributed to sustained drop collisions. Moreover, the fraction of asymmetric drops determined from the 2DVD camera data increases with the calculated collision probability when examined as time series. Recent wind-tunnel experiments of drop collisions are also discussed. They show mixed-mode oscillations, with (2,1) and (2,2) modes dramatically increasing in oscillation amplitudes, in addition to the (2,0) mode, immediately upon collision. The damping time constant of the perturbation caused by the collision is comparable to the inverse of the collision frequency within the line convection. Scattering calculations using an advanced method of moments numerical technique are performed to accurately and efficiently determine the pertinent parameters of electrically large oscillating raindrops with asymmetric shapes needed for radio wave propagation. The simulations show that the scattering matrix and differential reflectivity of drops are dependent on the particular oscillation modes and different time instants within the oscillation cycle. The technique can be utilized in conjunction with 3-D reconstruction of drop shapes from 2DVD data.

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Section: Raindrop Oscillationsmentioning

confidence: 77%

Section: Raindrop Oscillationsmentioning

confidence: 99%

Section: Raindrop Oscillationsmentioning

confidence: 99%

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Investigating raindrop shapes, oscillation modes, and implications for radio wave propagation

et al. 2014

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“…Various agents may induce raindrop oscillations in natural rainfall as vortex shedding Beard et al, 2010), raindrop collisions (Johnson and Beard, 1984;Szakáll et al, 2014), and wind shear and turbulence (Tokay and Beard, 1996). Small-amplitude raindrop oscillations are typically analyzed by spherical harmonic perturbations (Raleigh, 1879).…”

Section: Introductionmentioning

confidence: 99%

Free fall of water drops in laboratory rainfall simulations

Chowdhury

Testik

Hornack

et al. 2016

Atmospheric Research

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Motivated by various hydrological and meteorological applications, this paper investigates the free fall of water drops to provide guidance in laboratory simulations of natural rainfall and to elucidate drop morphodynamics. Drop fall velocity and shape parameters such as axis ratio (ratio of the maximum vertical and horizontal chords of the drop), chord ratio [ratio of the two orthogonal chords where one chord (c l ) is the longest chord in the drop and the other one (c s ) is the longest chord that is orthogonal to c l ], canting angle (angle between the longest chord of the drop and the horizontal axis), and relative fluctuation of chords (difference between vertical and horizontal chord fluctuations) were investigated for three selected water drop sizes (2.6, 3.7, and 5.1 mm spherical volume equivalent diameter) using high speed imaging. Based upon experimental observations, three distinct fall zones were identified: Zone I, in which source-induced oscillations and shape adjustment take place; Zone II, in which equilibrium-shaped drops accelerate to achieve terminal velocity; and Zone III, in which equilibrium-shaped drops fall at terminal velocity. Our results revealed that the fall distance values of approximately 6 m and 12 m can be used as conservative reference values for rainfall experiments with oscillationfree fall of drops (i.e. end of Zone I and onset of Zone II) and with equilibrium-shaped drops falling at terminal velocities (i.e. end of Zone II and onset of Zone III), respectively, for the entire raindrop size spectrum in natural rainfall. These required fall distance values are smaller than the distances discussed in the literature. Methodology and results presented here will facilitate optimum experimental laboratory simulations of natural rainfall.

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“…3. Note that the size error is exclusively related to the limitation of the image resolution and does not consider other sources of errors such as the asymmetric modes of large drops due to oscillation and collision (Szakáll et al, 2010(Szakáll et al, , 2014. However, this uncertainty would be relatively minor because the drops captured by the HSC in the indoor experiment is expected to reach terminal velocities with equilibrium-shaped status.…”

Section: Quantitative Comparisonsmentioning

confidence: 99%

Measuring droplet fall speed with a high-speed camera: indoor accuracy and potential outdoor applications

Hsieh

Yuter

et al. 2016

Atmos. Meas. Tech.

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Abstract. Acquisition of accurate raindrop fall speed measurements outdoors in natural rain by means of moderatecost and easy-to-use devices represents a long-standing and challenging issue in the meteorological community. Feasibility experiments were conducted to evaluate the indoor accuracy of fall speed measurements made with a high-speed camera and to evaluate its capability for outdoor applications. An indoor experiment operating in calm conditions showed that the high-speed imaging technique can provide fall speed measurements with a mean error of 4.1-9.7 % compared to Gunn and Kinzer's empirical fall-speed-size relationship for typical sizes of rain and drizzle drops. Results obtained using the same apparatus outside in summer afternoon showers indicated larger positive and negative velocity deviations compared to the indoor measurements. These observed deviations suggest that ambient flow and turbulence play a role in modifying drop fall speeds which can be quantified with future outdoor high-speed camera measurements. Because the fall speed measurements, as presented in this article, are analyzed on the basis of tracking individual, specific raindrops, sampling uncertainties commonly found in the widely adopted optical disdrometers can be significantly mitigated.

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A wind tunnel study of the effects of collision processes on the shape and oscillation for moderate-size raindrops

Cited by 22 publications

References 27 publications

Investigating raindrop shapes, oscillation modes, and implications for radio wave propagation

Investigating raindrop shapes, oscillation modes, and implications for radio wave propagation

Free fall of water drops in laboratory rainfall simulations

Measuring droplet fall speed with a high-speed camera: indoor accuracy and potential outdoor applications

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