High-Fidelity 1D and 2D Models for Static and Dynamic Analyses of Wind Turbine Rotor Blades

Wind energy is an essential renewable source to tackle the most critical environmental problems, such as global warming. Recently, the wind blade size has been increasing to maximize turbine efficiency. However, increased dimensions lead to further design challenges due to severe loadings - inertial and aerodynamic - and unavoidable manufacturing complexities. Therefore, extensive simulation campaigns covering as many operational conditions as possible become crucial for sustainable design and manufacturing. Various numerical tools for this purpose have been proposed to predict the response and damage levels of sizeable composite wind turbine blades. Within this context, this paper presents results based on the Carrera Unified Formulation (CUF) on various blade configurations. The CUF is a hierarchical formulation providing classical and higher-order beam, plate, and shell models using arbitrary kinematic expansions. The one-dimensional (1D) and two-dimensional (2D) CUF-based models can ensure a similar accuracy of three-dimensional (3D) solutions with considerable savings in computational efforts. The principle of virtual work and a finite element approximation is used to formulate both geometrically linear and nonlinear governing equations. The numerical results focus on static, dynamic, and failure analyses performed on composite wind turbine blades. The failure index evaluation uses a global/local approach that combines the CUF models with conventional FE solutions. In addition, future challenges related to health monitoring, damage detection, and developing a digital twin for structural verification will be discussed

Evaluation of local site effects in the city of Sansepolcro (Central Italy): preliminary results obtained by a urban seismic network.

Sansepolcro, one of the most significant city of Tuscan Tiber Valley, is located in an area close to the eastern edge of the Upper Tiber Valley, characterized by a wide local seismicity, related to the Altotiberina Fault (ATF) activity and not far from the main seismogenic areas of Mid-Northern Apennines (Montefeltro, Forlì, Gubbio, etc.). The Upper Tiber Valley has felt in the past several strong earthquakes: in 1789 (VIII MCS in the city of Sansepolcro), in 1917 (> VIII MCS), in 1948 (> VII MCS), in 1964 (VII MCS). Furthermore, many seismic sequences took place in recent years (1987, 1990, 1997), the last one in 2001 (November, 26, 2001, ML = 4.4). From May 2005, some seismic stations have been deployed in the center of Sansepolcro and its vicinities. Five sites were covering different geologic environments, along a direction approximately orthogonal to the valley axis. The seismic array has recorded more than 100 local and regional seismic events. 20 seismic events, with magnitude ranging from 2.0 and 3.8, distributed with a good azimuthal coverage in an area of 150 km radius centered on the city, have been selected. They were recorded by all the seismic stations with a signal to noise ratio higher than 3 in the frequency band of interest (0.5-10 Hz). The selected events have been analyzed with HHSR and HVSR techniques, to obtain the average spectral ratios for each site. Moreover, we analyzed seismic ambient noise in order to evaluate HVNSR, to be compared with the curves obtained by earthquake analysis. The results allow to give a preliminar evaluation of amplification effects due to the surface geology in the urban area. The three average spectral ratios (HHSR, HVSR and HVNSR) provide a good agreement: they show peaks with a raising amplitude centered on frequencies which point to long period moving from east to the center of the valley. The good agreement with 1D theoretical transfer functions, obtained from stratigraphic data and velocity downhole profiles (Vp e Vs), available for the monitored sites, suggests the hypothesis of being in geologic environment characterized by the presence of a stiff layer, identifiable in the well logs and in the velocity profiles obtained by downhole measurements, which seems to be the reason of surface amplification effects. This layer mainly made by firm cobble-stone associated to the sedimentary phases of Afra torrent, a Tiber tributary that flows close to the SE border of the city, is characterized by shear waves velocities comparable with those of a bedrock and it is recognizable in all the locations, at depths that increase from few meters to 100 m, moving from east to west. This layer, that seems to determine the distinct impedance contrast with the fluvial-lacustrine superficial layers, is not bottom of the basin which should be at a depth of more than 1000 m, as shown by the results of recent 3D analysis (Ciaccio e Barchi, 2006)

Time Response Stress Analysis of Solid and Reinforced Thin-Walled Structures by Component-Wise Models

This paper deals with the evaluation of time response analyses of typical aerospace metallic structures. Attention is focussed on detailed stress state distributions over time by using the Carrera Unified Formulation (CUF) for modeling thin-walled reinforced shell structures. In detail, the already established component-wise (CW) approach is extended to dynamic time response by mode superposition and Newmark direct integration scheme. CW is a CUF-based modeling technique which allows to model multi-component structures by using the same refined finite element for each structural component, e.g. stringers, panels, ribs. Component coupling is realized by imposing displacement continuity without the need of mathematical artifices in the CW approach, so the stress state is consistent in the entire structural domain. The numerical results discussed include thin-walled open and closed section beams, wing boxes and a benchmark wing subjected to gust loading. They show that the proposed modeling technique is effective. In particular, as CW provides reach modal bases, mode superposition can be significantly efficient, even in the case of complex stress states

Vibration analysis of thermally loaded isotropic and composite beam and plate structures

This work proposed the use of the Carrera Unified Formulation (CUF) for the vibration and buckling analysis of structures subjected to thermal loads. In detail, the variation of natural frequencies for progressively large thermal loads is investigated. Here, particular attention is focused on the study of buckling thermal loads as degenerate cases of the vibration analysis and on the mode aberration caused by thermal stresses. From this standpoint, the use of CUF for the development of high-order beam and plate models is fundamental. Indeed, Lagrange-like (LE) polynomials are considered for developing the kinematic expansion and Layerwise (LW) theories are employed to characterize the complex phenomena that may appear in composite structures. A linearized formulation to study the natural frequencies variation as a function of the progressive increasing thermal loadings is adopted. Different isotropic and laminated composite structures have been analyzed and compared with the Abaqus solution to validate the presented methodology and provide some benchmark solutions. In addition, a parametric study was conducted to evaluate the stacking sequence and thickness effect in the vibration modes and thermal buckling loads. The results document the excellent accuracy and reliability of the presented methodology and show the potentialities of this numerical tool able to analyze cases that are difficult to study experimentally

Analysis of vibrations recorded inside the cemetery area of Incisa, central Italy

Seismic stations are usually used to record seismic event and, therefore, they are recommended to be installed far from railways and traffic roads in order to avoid the superposition of ambient noise signals to those provoked by an earthquake. In this paper, instead, seismic stations, placed intentionally in areas near railway and traffic roads, are used to characterize the subsoil spectral properties and to assess the effect of vibrations due to trains and vehicles. A cemetery in the green countryside near Florence is chosen as a reference case study to deal with this topic. Most of the buildings in the cemetery area are affected by an extensive crack pattern. In January 2020 five seismic stations were installed in order to evaluate if the trains running in the tunnels of the regional and high-speed railway lines located below and in the vicinity of the cemetery and the vehicles traveling on the nearby A1 highway and regional road can produce vibrations in the ground that justify the observed damage pattern.Collected data are analyzed using the Nakamura technique in order to estimate the dynamic properties of the ground and compared to the limits provided by the current regulations. Furthermore, the trend of the Root Mean Square average over the entire recording period is computed as well.From the obtained results, it is possible to highlight that the average daily oscillation level increases from early morning until 7 p.m. and then it decreases, and also that the highest amplitudes of transients are concentrated in the late evening and during the night, when the background noise is lower.Furthermore, the computed values of the maximum and average amplitudes are lower than those that can cause damage to buildings as defined by the guidelines, the eigenfrequency of the ground falls in a range far from that ascribable to the cemetery buildings, so that the resonance effects can be excluded. In order to confirm these results, the amplitude of ground shaking due to recorded transients is compared to that produced by two earthquakes (a 3.4 Mw local earthquake at more than 100 km and a Mw 6.6 teleseism from Turkey) which occurred during the monitoring period. One can conclude that it seems unlikely that the shaking produced by nearby vehicles and trains could be responsible for the observed damage

Results from the temporary installation of a small aperture seismic arrayin the Central Apenninesand its merits for local event detectionand location capabilities

In order to evaluate the detection and localisation improvement of a small aperture array in the Northern Apennines, we installed an irregularly spaced test configuration in the vicinity of Città di Castello (CDC) for a period of two weeks. The experimental array consisted of nine 3-component stations with inter-station distances between 150 m and 2200 m. Seismic data were digitised at 125 Hz and telemetered to a mobile acquisition, processing and storage centre. The data could only be recorded in trigger mode. The peculiarity of the test array installation was the exclusive use of 3-component sensors at all array sites, which also allowed beamforming for S-phases on the horizontal components. Since the altitudes of the single array sites differed considerably among each other, for f-k analysis and beamforming an elevation correction was included. During the two weeks of operation about 20 local earthquakes with magnitudes ML<2.6, 1 regional, and several teleseismic events were recorded. In addition to these events, the array occasionally triggered on coherent noise-signals generated by local industrial activity. The data analysis was performed by means of f-k analysis and beamforming, providing wavenumber characteristics of the incident plane wave. Typical apparent velocities were determined to be 4.8 km/s and 6 km/s for Pg-phases and ~10 km/s for Pn-phases. We observed local seismic events, which occurred just beneath the array. In these cases wavefronts with unusual high apparent velocities, similar to those found for the Pn-phase, were observed. Since no continuously recorded array data were available, we extrapolated the lower detection magnitude threshold as a result of the SNR improvement due to array beamforming. Compared to the actual detection threshold of MT ~1.6 reached by the national seismic network in this area, a nine element array would improve this value up to MT ~ 0.8

Il Monitoraggio sismici della Valtiberina: l'attivita' svolta dall'INGV nell'ambito della convenzione con la Regione Toscana

Nel corso degli ultimi tre anni la Valtiberina Toscana è stata oggetto di una convenzione fra Regione Toscana e INGV riguardante il monitoraggio sismico e la valutazione di pericolosità dei centri urbani maggiormente significativi dell’area. L’attività, svolta principalmente dall’Osservatorio sismologico di Arezzo, si è sviluppata su diverse tematiche: • Ricerca dei siti per l’installazione di tre stazioni sismiche permanenti facenti parte della Rete Sismica Nazionale; • Individuazione del sito e installazione di una stazione sismica in pozzo nell’area urbana di Sansepolcro; • Installazione di una rete sismica urbana per lo studio delle amplificazioni locali nella città di Sansepolcro • Esperimento di caratterizzazione della risposta sismica locale della Valle Verranno presentate le attività svolte e i risultati ottenuti dagli esperimenti realizzati. Le attività descritte sono state realizzate nell’ambito della Convenzione fra Regione Toscana e INGV denominata: “Monitoraggio sismico e valutazione della pericolosità sismica in centri urbani significativi dei comuni della Valtiberina

Caratteristiche del Rumore Sismico nei pressi del Rilevatore di Onde Gravitazionali VIRGO (Cascina, Pisa)

In questo lavoro si presentano i risultati di uno studio sulle caratteristiche del rumore sismico in prossimità del rilevatore di onde gravitazionali VIRGO (Cascina, Pisa), con particolare riferimento alle vibrazioni associate all’azione di un vicino parco eolico. La valutazione delle componenti spettrali del rumore verosimilmente indotte dagli aerogeneratori è stata effettuata mediante (i) Misure dirette alla base di una turbina, (ii) Correlazione fra le ampiezze spettrali del rumore e la velocità del vento; (iii) Determinazione delle proprietà direzionali da misure multicanale, (iv) Misura dell’attenuazione del segnale con la distanza dal parco eolico. Il disturbo provocato dagli aerogeneratori è particolarmente energetico alla frequenza di 1.7 Hz e, in particolari condizioni, è stato osservato fino a distanze di 11 km dal Parco Eolico. Il decadimento spaziale delle ampiezze ha un andamento complesso, che può essere interpretato in termini di una combinazione fra onde superficiali e onde di volume rifratte ad un’interfaccia profonda (~800 m) fra i sedimenti plio-pleistocenici ed i calcari Miocenici. La risposta locale nei dintorni dell’interferometro è stata investigata utilizzando la tecnica dei rapporti spettrali H/V. Si sono così evidenziate due bande di amplificazione imputabili ad effetti di risonanza legati alla geologia a scala locale: il primo intorno alla frequenza di 0.35 Hz, il secondo fra 0.7 e 2.0 Hz. Entrambi i picchi risultano essere essenzialmente omogenei in tutta l’area di studio, a conferma della sostanziale uniformità della struttura geologica