Review of flow characteristics and kinematics of hydrometeors in free fall

Böhm, Johannes Peter

doi:10.1016/0169-8095(91)90023-p

Cited by 10 publications

(5 citation statements)

References 73 publications

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“…Potential variations of q from twig to twig are considered in the optimized parameter D h . When the value of N Re is between about 2000 and 450,000, turbulent fluxes occur and a laminar boundary layer δ exists (Böhm 1991), but when N Re exceeds 450,000, the boundary…”

Section: Modeling Potential Twig Transpirationmentioning

confidence: 99%

Midday stomatal closure in Norway spruce--reactions in the upper and lower crown

Zweifel

Böhm²,

Häsler

2002

Tree Physiology

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We used local microclimatic conditions and twig sap flow rates to interpret midday stomatal closure in the canopies of two 250-year-old Norway spruce (Picea abies (L.) Karst.) trees at a subalpine site in the Swiss Alps (1,650 m a.s.l.). Both trees showed midday stomatal closure on most clear summer days, despite the permanently wet soil. We used a modified Penman-Monteith formula to simulate potential transpiration of single twigs (ET(T)) based on high-resolution temporal and spatial microclimate data obtained both inside and outside the crowns. Comparison of calculated ET(T) values and measured twig sap flow rates enabled us to pinpoint the occurrence of midday stomatal closure and the microclimatic conditions present at that time. We found that vapor pressure deficit (and for upper-crown twigs, ET(T)) largely explained the timing of initial midday stomatal closure but gave no explanation for the different patterns of stomatal behavior after initial closure in upper- and lower-crown twigs. After the initial stomatal closure, upper-crown twigs maintained high transpiration rates by continuously regulating stomatal aperture, whereas stomatal aperture decreased rapidly in lower-crown twigs and did not increase later in the day. Midday stomatal closure in lower-crown twigs occurred on average 1 h later than in upper-crown twigs. However, the microclimate at the time of initial stomatal closure was similar at both crown locations except that lower-crown twigs received significantly less solar radiation than upper-crown twigs both at the time of initial stomatal closure and afterwards. High rates of sap flow in twigs did not always lead to stomatal closure and therefore could not explain the phenomenon. We conclude that stomatal conductance can be modeled accurately only when both local microclimatic conditions and tree water status are known. Further, we hypothesize that both the quantity and quality of light play an important role in the reopening of closed stomata during the day.

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Section: Modeling Potential Twig Transpirationmentioning

confidence: 99%

Midday stomatal closure in Norway spruce--reactions in the upper and lower crown

Zweifel

Böhm²,

Häsler

2002

Tree Physiology

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“…The basic mechanisms for how fluidic forces are redistributed to restore the apex-leading orientation in the face of perturbations and how oscillations are damped also remain to be investigated. Nevertheless, our characterization of flight stability across the family of cones could prove applicable to a variety of problems in which shape effects are important, such as the atmospheric motions of hail and other so-called hydrometeors (15,16) as well as particulate dynamics in chemical engineering applications (51,52).…”

Section: Discussionmentioning

confidence: 99%

“…An everyday example is the fluttering and tumbling of falling paper, where motion under gravity and flow-induced bending deformations arise interactively (11,12). Other examples include the reshaping and ballooning of raindrops (13,14), freezing or sublimation of atmospheric ice (15,16), and the dissolution of particulates in sedimenting or stirred suspensions (17,18).…”

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confidence: 99%

The role of shape-dependent flight stability in the origin of oriented meteorites

Amin

Huang

et al. 2019

Proc. Natl. Acad. Sci. U.S.A.

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The atmospheric ablation of meteoroids is a striking example of the reshaping of a solid object due to its motion through a fluid. Motivated by meteorite samples collected on Earth that suggest fixed orientation during flight—most notably the conical shape of so-called oriented meteorites—we hypothesize that such forms result from an aerodynamic stabilization of posture that may be achieved only by specific shapes. Here, we investigate this issue of flight stability in the parallel context of fluid mechanical erosion of clay bodies in flowing water, which yields shapes resembling oriented meteorites. We conduct laboratory experiments on conical objects freely moving through water and fixed within imposed flows to determine the dependence of orientational stability on shape. During free motion, slender cones undergo postural instabilities, such as inversion and tumbling, and broad or dull forms exhibit oscillatory modes, such as rocking and fluttering. Only intermediate shapes, including the stereotypical form carved by erosion, achieve stable orientation and straight flight with apex leading. We corroborate these findings with systematic measurements of torque and stability potentials across cones of varying apex angle, which furnish a complete map of equilibrium postures and their stability. By showing that the particular conical form carved in unidirectional flows is also posturally stable as a free body in flight, these results suggest a self-consistent picture for the origin of oriented meteorites.

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“…Ice crystal densities were selected to represent the density reported by Ryan et al (1976). The ice crystal orientations are based on the conclusion of Böhm (1991) and the experimental results presented by Bürgesser et al (2016), who reported that, under low Re (Re < 1), columnar ice crystals show a random orientation during free fall. Furthermore, remote-sensing observations of ice crystal orientation had shown a 50% chance of horizontally oriented ice crystals at any height in the cloud (Thomas et al, 1990).…”

Section: Datamentioning

confidence: 99%

“…(). The ice crystal orientations are based on the conclusion of Böhm () and the experimental results presented by Bürgesser et al . (), who reported that, under low Re ( Re <1), columnar ice crystals show a random orientation during free fall.…”

Section: Datamentioning

confidence: 99%

Sedimentation analysis of columnar ice crystals in viscous flow regimes

Bürgesser

Giovacchini

Castellano

2019

Quart J Royal Meteoro Soc

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The sedimentation process of columnar ice crystals was evaluated using data obtained by the lattice Boltzmann method. The data used correspond to columnar ice crystals with maximum dimension less than 100 μm and aspect ratios between 1 and 3. The terminal velocity was computed for different ice‐crystal bulk densities and for three falling orientations. The analysis corresponds to ice crystals falling in viscous flow regimes, where theoretical formulations overestimate the terminal velocity. Different characteristic lengths of columnar ice crystals and different theoretical proposals for the sedimentation process were tested in order to find the best representation of the data. Characteristic lengths reported in the literature do not represent the sedimentation process for all the falling orientations used in this study. Thus, it was not possible to obtain a unique relation between the Best and Reynolds numbers. In particular, columnar ice crystals falling with their longer dimension parallel to the vertical direction show a large dispersion that it does not seem possible to reduce. The theoretical and semi‐empirical formulations of the terminal velocity evaluated show large deviations in the computed velocity, with a strong dependence on ice‐crystal aspect ratio. The dispersion observed seems to be intrinsically related to the dimensionless variables used to parametrize the terminal velocity. To derive a unique scale law that could represent the sedimentation process of ice crystals, geometric, kinematic, and dynamic similarities are required. However, these conditions are not fulfilled in the sedimentation process.

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Review of flow characteristics and kinematics of hydrometeors in free fall

Cited by 10 publications

References 73 publications

Midday stomatal closure in Norway spruce--reactions in the upper and lower crown

Midday stomatal closure in Norway spruce--reactions in the upper and lower crown

The role of shape-dependent flight stability in the origin of oriented meteorites

Sedimentation analysis of columnar ice crystals in viscous flow regimes

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