1,721,010 research outputs found
Simulation of permeable surfaces using the pressure–velocity jump approach: A lamellar screen upstream of a ground-mounted obstacle
Full text linkPermeable surfaces are nowadays widely adopted in the construction industry, with applications ranging from wind shields for bridge decks to the external layer of permeable double skin facades. However, due to the large scale separation between the overall structure dimension and the size of the pores, their modelling in CFD simulations is still extremely challenging and over -simplified homogenized models are often used in practice. Inspired by previous studies, the authors recently proposed a generalization of the well-known pressure -jump approach which accounts for flow deflections, denoted as pressure-velocity jump, PVJ . In this study, the derivation of the PVJ approach is briefly recalled and contextualized in the existing literature. Then, we use PVJ to study the influence of a lamellar screen positioned upstream of a ground -mounted obstacle using 2D URANS. In particular, simulations are performed using the proposed PVJ approach and Explicit Models, EM , in which lamellae are explicitly modelled, for square and a rectangular obstacles. Results show a good agreement between EM and PVJ based models, confirming the high potential of the proposed technique
Windbreak effectiveness of shelterbelts with different characteristic parameters and arrangements by means of CFD simulation
No full textShelterbelts can locally reduce wind speed in open terrains, moderating wind induced damage for agriculture and ameliorating growing conditions for crops. In this paper, Computation Fluid Dynamics (CFD) is adopted to clarify the effects of the characteristic parameters and arrangement of shelterbelts on their windbreak efficiency. The Darcy-Forchheimer canopy model is adopted to represent the vegetation, so modeling it as an isotropic porous medium. The shelterbelts are immersed in wind representative of the one expected in the atmospheric boundary layer and the numerical model is firstly validated against full-scale data. The parameters governing the shelterbelt aerodynamic performance, including shelterbelt width, shelterbelt length and opening length, are systematically varied, so providing a clear overview of the effects induced by their modification. The results show that the sheltering effect is enhanced as the shelterbelt width increases, while the effects of shelterbelt length are less pronounced in comparison. When there is opening inside the shelterbelt, the mean wind speed and turbulence kinetic energy behind the canopy is significantly increased due to the passage of airflow. General considerations regarding efficient shelterbelt arrangements are deduced by a comparison among L-, U- and rectangular-shaped types
Effects of low incoming turbulence on the flow around a 5:1 rectangular cylinder at non-null-attack angle
Full text linkThe incompressible high Reynolds number flow around the rectangular cylinder with aspect ratio 5: 1 has been extensively studied in the recent literature and became a standard benchmark in the field of bluff bodies aerodynamics. The majority of the proposed contributions focus on the simulation of the flow when a smooth inlet condition is adopted. Nevertheless, even when nominally smooth conditions are reproduced in wind tunnel tests, a low turbulence intensity is present together with environmental disturbances and model imperfections. Additionally, many turbulence models are known to be excessively dissipative in laminar-to-turbulent transition zones, generally leading to overestimation of the reattachment length. In this paper, Large Eddy Simulations are performed on a 5: 1 rectangular cylinder at non-null-attack angle aiming at studying the sensitivity of such flow to a low level of incoming disturbances and compare the performance of standard Smagorinsky-Lilly and Kinetic Energy Transport turbulence models
Non-stationary winds effects over large partially-open roofs: A CFD study regarding the role of unsteady aerodynamics
Full text linkNon -stationary winds are increasingly attracting the attention of the scientific community and, despite not being explicitly considered in current design practice, they are known to produce important damages. In particular, the non-stationarity of such events has important consequences with respect to four main aspects which affect the definition of design wind loads: (i) the design wind speed definition, (ii) the wind profiles, (iii) the calculation of the structural response and (iv) the insurgence of unsteady aerodynamics effects. While the first three aspects already received considerable attention, the last one is still largely unclear and its relevance still to be assessed. In particular, for non -stationary winds such as macro- and micro-downbursts, the ramp -up time might become so short to trigger unsteady aerodynamic effects in the overall flow arrangement, which cannot be inferred from results obtained for stationary cases. In this paper, we propose a first investigation of such matter considering a large stadium roof using Large Eddy Simulations. Results show that, for the considered case, noticeable unsteady aerodynamic effects, which leads to an amplification of wind loads, can be triggered when the wind rump -up time is in the order of 30 s, and quickly decrease in importance for higher rump -up times
An efficient approach to the evaluation of wind effects on structures based on recorded pressure fields
No full textA well known peculiarity of the structural response to wind loading is that static, quasi-static and resonant effects are present, in general, without a clearly dominant contribution. In such conditions, the calculation of wind effects would require to consider a large number of modes due to the relevant contribution of high frequency ones. In the present paper, in order to alleviate inaccuracies in the evaluation of static and quasi-static effects, induced by the truncation of the modal base, an efficient approach to the evaluation of static and quasi-static corrections is developed. To this purpose, a new class of pressure modes, called Proper Skin Modes (PSMs), is introduced and the corrections are evaluated based on the structural response to such modes, statically applied. PSMs can be seen as a novel modal version of traditionally adopted influence coefficients and naturally arise from the adopted pressure interpolation technique. The obtained approach is compact and efficient leading to a reduced data exchange between structural and wind engineers and to an optimal organization of the software used to perform the analyses. The proposed procedure is tested on a low-rise and a high-rise building showing very good performances
Wind loads on heliostat tracker: A LES study on the role of geometrical details and the characteristics of near-ground turbulence
No full textThe accurate evaluation of the operational and survival wind loads for ground-mounted heliostat trackers is vital for reducing the total cost during the life cycle of concentrated solar power plants. Current studies rely on reduced scale Wind Tunnel Tests (WTTs), which makes the investigation of the effects of supporting components and near-ground turbulence not trivial. As a result, there is an urgent need to develop complementary techniques to guide wind-resistant designs of commercial heliostats. In this paper, we investigate the performance of Large Eddy Simulations (LES) to systematically study the mean and peak wind loads over heliostats. The effects of elevation and azimuth angles, supports components, turbulence intensity and length scale are investigated. Results show that the proposed LES model accurately reproduces integral force coefficients and the local pressure distributions. Further, an accurate evaluation of the survival wind loads needs to consider the effects of supports components, as they contribute more than 50% to the mean lift and overturning moment in the stow positions. Finally, the effects of turbulence intensity and integral scale are clarified by considering eight different combinations of parameters to characterize the near-ground turbulence. This study provides useful guidelines for design wind loads and lays a solid foundation for LES of heliostat arrays
Linearly elastic constitutive relations and consistency for GBT-based thin-walled beams
No full textThe present paper focuses on the constitutive assumptions, both for the isotropic and orthotropic cases, and consistency in the framework of the Generalized Beam Theory. In particular, a novel approach based on energetic arguments, able to automatically select appropriate constitutive relations in accordance with the GBT kinematics, is presented. Furthermore, the concept of consistency of a GBT-based model is established and a consistency analysis is presented. This yields a formal rational basis to investigate the effects of the various families of cross-section deformation modes in terms of predictive capabilities of the GBT model. Some numerical examples illustrate the arguments exposed in the paper
Numerical study of a twin box bridge deck with increasing gap ratio by using RANS and LES approaches
No full textTwin box decks are becoming increasingly adopted in long span bridges due to their good aerodynamic performance with respect to the flutter instability. For small gaps between the decks they behave as a whole continuous body while, increasing the gap, strong dynamic interaction between the two occurs: the windward deck causes a turbulent wake that impinges on the leeward girder inducing unsteady aerodynamic forces. This complex behaviour makes the simulation of the aerodynamics of these structures a challenging task from the Computational Fluid Dynamic point of view. In this context, the present paper aims at investigating the capabilities and limitations of RANS and LES based simulations in reproducing the flow field organization around a twin box deck section. Different configurations, with increasing separation gap, are studied and compared with experimental data. Finally, the effects induced by the presence of barriers is investigated for the largest separation gap
Design of a Lightweight Origami Composite Crash Box: Experimental and Numerical Study on the Absorbed Energy in Frontal Impacts
Full text linkOrigami-shaped composite structures are currently being explored for their ability to absorb energy in a progressive and controlled manner. In vehicle passive safety applications, this prevents the occurrence of peak forces that could potentially cause injuries to vehicle passengers. The work presents the design of a carbon fiber-reinforced polymer (CFRP) crash box for a Formula Student race car, using a numerical model validated by experimental tests. An initial characterization of the material is conducted according to the standards. Following, six origami samples are manufactured and subjected to crash tests to gather accurate experimental data. The numerical model is validated on the tests and used for the design of the race car’s impact attenuator. The designed crash box meets the Formula Student requirements while reducing the total mass by 14% and the maximum deceleration of 21% compared with the previous design. The study confirms the potential use of origami structures to improve crashworthiness while reducing vehicle weight
Preliminary Flutter Stability Assessment of the Double-Deck George Washington Bridge
Full text linkWe deal with the flutter analysis of the George Washington bridge, in both the single- and double-deck configurations of 1931 and 1962, respectively. The influence of the additional lower deck on the aerodynamic behavior is investigated. To overcome the lack of aerodynamic data, a simplified approach is followed based on Fung's formulation, in which the flutter derivatives are expressed in terms of the real and imaginary parts of the Theodorsen function and of the steady-state aerodynamic coefficients of the deck cross-section. The latter are obtained by Computational Fluid Dynamics simulations conducted in ANSYS FLUENT, whereas the ANSYS Mechanical APDL finite element package is used to perform the flutter analyses. Two different methods for the application of the aeroelastic forces are employed for the double-deck configuration: (i) self-excited forces, based on flutter derivatives related to the whole cross-section, acting on the upper deck; and (ii) self-excited forces, based on flutter derivatives related to the single deck, simultaneously applied to the upper and lower decks. The obtained results are critically compared with theoretical predictions of simple formulas available from the literature; it is suggested that laboratory tests are needed since no experimental results seem to be available
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