1,721,056 research outputs found
Thunderstorm Downbursts and Wind Loading of Structures: Progress and Prospect
No full textIn 1961, Davenport published a paper, considered by most to be a critical work in the study of wind engineering, in which meteorology, micrometeorology, climatology, aerodynamics, and structural dynamics were embedded in a homogeneous framework of the wind loading of structures. This framework, known as Davenport chain and based on a wind model coherent with synoptic-scale extra-tropical cyclones, is so limpid as to become a sort of axiom. In 1977, Gomes and Vickery separated thunderstorm from non-thunderstorm winds, evaluated their extreme wind speed marginal distributions, and from them obtained a mixed statistical model later generalized to other wind types. This viewpoint, dealt with as a milestone in the emerging issue of mixed climatology, pointed out the difficulty of labeling a heterogeneous set of phenomena endowed with different velocity fields, frequencies, durations, and sizes by the generic term “wind.” Many wind types, in particular tropical cyclones, tornadoes, and downslope winds are typical of limited and well-known areas. Extra-tropical depressions and thunderstorms are natural hazards that affect the whole planet. This paper provides a state-of-the-art discussion of thunderstorm downburst, one of the most spectacular and damaging events caused by nature, and its wind loading of structures. Also, in light of the planet's climatology evolution, this topic is a key issue of structural safety and sustainability
Education and dissemination in wind science and engineering
No full textAn adequate education in a discipline as vast and articulated as wind science and engineering cannot fail to recognize its origins, developments, fundamentals, advancements and prospects. This paper analyses and discusses the actual state of education by identifying its limits, gaps and strengths, as well as the hard core of knowledge that should constitute the common basis for a mature and modern discipline capable of generating figures endowed with broad views and specialized skills. Pursuing this vision in a rational and homogeneous framework, it also points out the role of new strategies aiming to exploit technological evolution and communication systems, to rationalize co-operative educational programs on broad territorial scales, to involve different competences as in-depth as they are varied in the wind science and engineering community. Meanwhile, it highlights the necessity of reaching those who need to establish and consolidate this culture, but do not have the opportunity to personally experience the manifold initiatives in the discipline, to seize the interest of people towards wind science and engineering, to disseminate its culture and know-how into everyday life and society
An evolutionary power spectral density model of thunderstorm outflows consistent with real-scale time-history records
No full textThis paper presents a new approach for modelling the evolutionary power spectral density (EPSD) of thunderstorm outflows based on a database of 129 real-scale thunderstorm time-histories. Two equivalent methods of decomposition of the wind velocity are investigated, outlining the derivation of the EPSD. The analyses are based on the hypothesis that the residual turbulent fluctuations can be treated as a uniformly modulated process, whose reliability is verified studying the steadiness of its up-crossing rate. The assumption of considering a constant turbulence intensity is checked through the evaluation of the dynamic response of a set of SDOF systems in time-domain and through the response spectrum technique. Two models for the time-modulating function of the EPSD are proposed that include the parameters characteristic of both the thunderstorm event and the background wind. Finally, the steps for the derivation of the EPSD are outlined pointing out the limits of the representation of the PSD of the reduced turbulent fluctuations through spectral models commonly adopted in wind engineering. The final aim of this research is finalising a triad of complementary methods for evaluating the dynamic response of structures to thunderstorm outflows - EPSD, time-domain and response spectrum - consistent with real-scale time-history records
Gust Buffeting and Aerodynamic Admittance of Structures with Arbitrary Mode Shapes. I: Enhanced Equivalent Spectrum Technique
No full textThe loading and response of structures due to gust buffeting is a dominant topic of wind engineering. One of its crucial aspects is the shape of vibration modes. Although numeric solutions are available for any mode, conceptual interpretations and closed-form solutions mainly are limited to the case in which the sign of the mode does not change along the structural axis. For modes that change sign, it is difficult, if not impossible, to recognize the physical role of the parameters that govern the problem and judge analysis results in qualitative form. This paper addressed this issue in the framework of quasi-steady theory by clarifying the relationship linking aerodynamic admittance with mode shape, showed that any mode may be brought back to a piecewise ensemble of regular modes with constant sign, and used this concept to obtain a closed-form expression for any aerodynamic admittance. This solution is simple for modes with few changes of sign, but becomes laborious as mode complexity increases. In addition, it provides a partial conceptual interpretation. Both of these limits were overcome in the companion paper, in which the use of proper orthogonal decomposition led to a full conceptual interpretation of aerodynamic admittance and a simple and general closed-form solution
Il problema della giustizia e dello stato nell'antichità classica
No full textIl volume ripropone un corso di lezioni di Gioele Solari, illustre filosofo del diritto, dedicato alla filosofia politica nell'antica Grecia, con particolare riguardo a Platone e Aristotele e dei suoi echi nella speculazione di Cicerone e Senec
Gust Buffeting and Aerodynamic Admittance of Structures with Arbitrary Mode Shapes. II: A POD-Based Interpretation
No full textStudies carried out on gust buffeting and aerodynamic admittance show that mode shape represents a sort of watershed. If the mode does not change sign along the structure axis, the problem admits a robust conceptual interpretation based on energy cascade and a simple closed-form solution. If, instead, the mode changes sign, the solution calls for numerical tools, and its physical interpretation definitely is unclear. The companion paper investigated this issue within the novel framework of the enhanced equivalent spectrum technique and derived a closed-form solution of the aerodynamic admittance that can be applied to any mode using quasi-steady theory. This solution is precise and simple for modes with a few changes of sign, but it becomes laborious with increasing mode shape complexity; in addition, it provides a partial conceptual interpretation. Both these limitations were overcome in this paper, in which the application of proper orthogonal decomposition led to a full conceptual interpretation of aerodynamic admittance and to a simple and general closed-form solution. Analyses were limited here to single modes; multiple arbitrary modes are left for future research, as is the generalization of these concepts to arbitrary influence functions
Time varying mean extraction for stationary and nonstationary winds
No full textThis paper discusses different strategies for the extraction of the time-varying mean from wind speed time histories. Due to the advantage of allowing analytical evaluations, the attention is focused on kernel regression techniques, considering different weighting functions, namely a constant, a Gaussian and a cardinal sine weighting function. The problem is firstly treated analytically, and the frequency-domain properties of the filter associated to different kinds of weighting functions in the definition of the slowly varying mean through kernel regression are analysed. Then, different weighting functions are adopted for the analysis of digitally-simulated stationary wind speed time histories and for the time histories of thunderstorm outflows recorded by a tri-axial anemometer. The consequences of the adoption of different weighting functions on the harmonic content and statistical properties of turbulence are studied. The same features are found also for thunderstorm outflow records
Mixed Climatology, Non-synoptic Phenomena and Downburst Wind Loading of Structures
No full textModern wind engineering was born in 1961, when Davenport published a paper in which meteorology, micrometeorology, climatology, bluff-body aerodynamics and structural dynamics were embedded within a homogeneous framework of the wind loading of structures called today “Davenport chain”. Idealizing the wind with a synoptic extra-tropical cyclone, this model was so simple and elegant as to become a sort of axiom. Between 1976 and 1977 Gomes and Vickery separated thunderstorm from non-thunderstorm winds, determined their disjoint extreme distributions and derived a mixed model later extended to other Aeolian phenomena; this study, which represents a milestone in mixed climatology, proved the impossibility of labelling a heterogeneous range of events by the generic term “wind”. This paper provides an overview of this matter, with particular regard to the studies conducted at the University of Genova on thunderstorm downbursts
Aerodynamic admittance functions of streamlined bodies: the indicial approach by CWE
No full textThis paper proposes a method to determine the aerodynamic admittance function of 2-dimensional cylinders by means of a modified indicial approach adapted to a Navier-Stokes solver. The method is based on the simulation of a smoothed-edge gust passing the cylinder and on the evaluation of the transient forces arising on the obstacle. An application to a thin plate is proposed. Firstly, the numerical parameters of the model are optimised by simulating two elementary flow conditions: the gust propagation in a free stream and the steady flow around the plate. Then, both the gust and the plate are introduced in the same computational model. The computed aerodynamic admittance function is compared with the one provided in closed form by Sears in the frame of the thin airfoil theory
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