Directional decomposition and properties of thunderstorm outflows

Shi, Z.; Solari, Giovanni; Burlando, Massimiliano; Yang, Qingshan

doi:10.1016/j.jweia.2019.03.014

Cited by 37 publications

(18 citation statements)

References 53 publications

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“…Canepa [32] analyzed a large set of experimentally produced downburst outflows and found that the usual hypothesis adopted in literature, i.e., turbulence intensity assumed constant in time I V = I V (where I V is the temporal mean of the slowly-varying turbulence intensity I V ), loses validity in controlled conditions. However, this was also noticed in the full-scale environment by several authors [1,7,43,44], who found an asymmetry of the parameter µ(t) = I V (t)/I V in correspondence to the peak wind speed or, in other words, concurrently with the passage of PV by the measuring instrument. Specifically, µ showed a local maximum and minimum, respectively, before and after the recording of the maximum wind speed.…”

Section: Turbulence Propertiessupporting

confidence: 62%

Experimental Investigation of the Near-Surface Flow Dynamics in Downburst-like Impinging Jets Immersed in ABL-like Winds

et al. 2022

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Downburst winds are strong downdrafts of cold air that embed into the atmospheric boundary layer (ABL) and produce intense horizontal outflow upon impingement on the ground. They are highly transient and three-dimensional extreme wind phenomena with a limited spatiotemporal structure that often makes the anemometric measurements in nature inadequate for reconstructing their complex flow fields. In the framework of the project THUNDERR, an experimental campaign on downburst outflows has been carried out at the WindEEE Dome at Western University, Canada. The present study analyzes the three-dimensional interaction between downburst (DB) outflows produced as large-scale impinging jets and ABL winds. Most experimental, numerical and analytical models in the literature neglect this flow interplay or treat it in an oversimplistic manner through a vector superposition. We found that the generated near-surface outflow is asymmetric, and a high-intensity wind zone develops at the interface between DB and ABL winds. The time variability of the leading edge of the outflow was investigated by synchronizing all wind measurements across the testing chamber. The three-dimensional flow structure was studied using a refined grid of Cobra probes that sampled the flow at high frequencies. The passage of the primary vortex produced a significant decrease in the height of maximum radial wind speed, predominantly in the ABL-streamwise direction. The turbulence intensity was the highest in the region where DB propagates into oppositely directed ABL winds.

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Section: Turbulence Propertiessupporting

confidence: 62%

Experimental Investigation of the Near-Surface Flow Dynamics in Downburst-like Impinging Jets Immersed in ABL-like Winds

et al. 2022

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“…Having two different philosophies of decomposition, though, precludes a parallel treatment and a robust comparison between the structural response to thunderstorm outflows and to synoptic winds. This shortcoming was overcome in [27], where a unitary directional decomposition was introduced (Sect. 3.2).…”

Section: Wind Speed Decompositionmentioning

confidence: 99%

“…The effects of transient aerodynamics [22] are considered by invoking the hypothesis that the passage of the gust front is moderately slow [13,35,43]; accordingly, the aerodynamic coefficients are expressed here as slowly-varying functions of time. This is made possible by assuming that the Reynolds number is associated with the slowly-varying mean wind velocity [44] whereas the angle of attack is associated with the slowly-varying mean wind direction [27].…”

Section: Aerodynamic Wind Loadingmentioning

confidence: 99%

“…To overcome these shortcomings, a new directional decomposition strategy of the wind speed has been recently formulated [27] which opens the doors to robust comparisons and parallel analyses of thunderstorm outflows and synoptic wind in terms of wind speed, of wind loading and of structural response, taking into account the time-variation of the slowlyvarying mean wind speed and direction. Besides the effects of the change of direction of thunderstorm outflows in the wind loading and response of structures, a clear evaluation of this parameter is fundamental to recognize the travelling nature of the downbursts, to reconstruct their evolution and to clarify the crucial role of the intensification of the wind speed due to the translation of the thunderstorm cell.…”

Section: Introductionmentioning

confidence: 99%

“…This aim is pursued by making recourse to the dataset of the records acquired during the ''Wind and Ports'' [28] and ''Wind, Ports and Sea'' [29] European projects and by evaluating the windexcited response of two vertical slender test structures in the time domain [19,20]. The role of the wind direction is considered since the wind speed decomposition stage [27], by means of two families of methods, which are afterward described. Based on them, the time-varying aerodynamic forces are analytically expressed through the classical quasi-steady theory, generalized here by the use of aerodynamic coefficients slowly-varying over time.…”

Section: Introductionmentioning

confidence: 99%

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Directional response of structures to thunderstorm outflows

2019

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Due to the combined effect of the background wind, the translation of the downburst and the radial nature of the downdraft after the touchdown, a peculiar characteristic of a thunderstorm outflow as detected by an anemometer or experienced by a structure is the potential variation of the flow direction. The study of the wind-excited response of structures has habitually neglected this issue because, dealing traditionally with synoptic events, their direction is endowed with a regular pattern. This paper proposes two families of methods to take into account directional changes in the evaluation of the windexcited response of structures. The first one generalizes the method usually applied to downbursts, which by its nature implicitly assumes a non-directional response. The second one is based on a new directional decomposition strategy of the wind speed that represents a generalization to thunderstorm outflows of what is historically done for synoptic events. The conceptual aspects involved in and the results provided by these methods are critically discussed and compared both to each other and with regard to the traditional non-directional structural analysis of thunderstorm outflows.

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Experimental investigation of the near-surface flow dynamics in downburst-like impinging jets

et al. 2022

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Downbursts are strong downdrafts that originate from thunderstorm clouds and create vigorous radial outflows upon hitting the ground. This study is part of the comprehensive experimental research on downburst outflows produced as large-scale impinging jets in the WindEEE Dome simulator at Western University, Canada. The 2800 tests carried out form the largest database of experimental measurements on downburst winds developed thus far, which is made available to the public in its whole and described in detail in a complementary study. Therefore, the current manuscript merely focuses on the data post-processing outcomes and interpretation of results from a selected subset of measurements. Impinging jets are here simulated as transient phenomena in which velocity time series are characterized by a sudden ramp-up of velocity, followed by the velocity peak, a short statistically stationary region, and the final velocity slowdown, as it is expected to occur in the actual downbursts. A dominant velocity peak that was systematically observed in all velocity records is associated with the radial advection of the primary vortex in the outflow. Depending on the radial distance from the downdraft, the primary vortex was sometimes preceded by a secondary, much smaller, vortex close to the surface. Vertical profiles of mean velocity and turbulence intensity are for the first time characterized through the extent of a downburst-like event in the spatiotemporal domain. Particularly, these profiles rapidly change in relation to the passage of the primary vortex and consequent variation of the surface layer thickness. This study lays out a foundation for an experimental model of non-stationary downburst outflows to come.

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Directional decomposition and properties of thunderstorm outflows

Cited by 37 publications

References 53 publications

Experimental Investigation of the Near-Surface Flow Dynamics in Downburst-like Impinging Jets Immersed in ABL-like Winds

Experimental Investigation of the Near-Surface Flow Dynamics in Downburst-like Impinging Jets Immersed in ABL-like Winds

Directional response of structures to thunderstorm outflows

Experimental investigation of the near-surface flow dynamics in downburst-like impinging jets

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