The influence of the low frequency turbulence components on the buffeting response of long span bridges was studied through experimental tests performed on a full bridge aeroelastic model in the wind ...tunnel of the Politecnico di Milano, using an active turbulence generator producing a correlated deterministic harmonic turbulence. The experimental evidence underlined the nonlinear effect of the low frequency incoming turbulence on the dynamic resonant response of the structure at higher frequencies.
Numerical simulations are used to explain the bridge behavior considering the variation of the aeroelastic properties of the bridge with the instantaneous angle of attack and reduced velocity. Even though wind tunnel experiment uses an oversimplified wind spectrum with an intentionally high correlation along the main span, it helps to understand the nonlinear interaction between the low frequency and the high frequency buffeting response on a full bridge.
•An experimental case of the nonlinear low-frequency turbulence effects on the aeroelastic response of a full-bridge model.•A simplified bi-harmonic wind spectrum is considered to better highlight the nonlinear effects.•The low-frequency variation of the angle of attack makes the response switching between stable and unstable conditions.•A band superposition model is used to explain the experimental behavior, relying on standard aerodynamic coefficients.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The recent development of high-speed trains over the last decade led to a growing interest in their aerodynamics. A train at full operational speed generates a strong induced airflow that may damage ...infrastructures near the trackside and endanger people near the rails. Nowadays the tests required for the train homologation are based on full scale measurements of the airspeed taken in specified positions close to the railway line. An important part of the work was initially dedicated to the data analysis of an extensive experimental campaign performed on the Italian high-speed line, in order to obtain reliable results to be used for comparison with CFD results. These ones have been performed on the full-scale train geometry using an URANS approach with a wall treatment based on wall functions, checking mesh and turbulence models dependencies. The full-scale simulations highlighted the great complexity of the problem being the comparison with the experimental measurements performed close to the shear layer of the train boundary layer. Encouraging results are obtained from the numerical analysis on the full-scale train geometry indicating the ability of the URANS coupled with the SST turbulence model in the prediction of both the train induced flow and the turbulent structures around it.
•Flow statistics strongly influenced by crosswind.•CFD provide good estimation of the 3D flow around a full scale high speed train in operational conditions.•k-omega SST turbulence model showed a strong stability and versatility in reproducing high turbulence flow.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
We present a unified model of compound semiconductor growth based on kinetic Monte Carlo simulations in tandem with experimental results that can describe and predict the mechanisms for the formation ...of various types of nanostructures observed during droplet epitaxy. The crucial features of the model include the explicit and independent representation of atoms with different species and the ability to treat solid and liquid phases independently. Using this model, we examine nanostructural evolution in droplet epitaxy. The model faithfully captures several of the experimentally observed structures, including compact islands and nanorings. Moreover, simulations show the presence of Ga/GaAs core-shell structures that we validate experimentally. A fully analytical model of droplet epitaxy that explains the relationship between growth conditions and the resulting nanostructures is presented, yielding key insight into the mechanisms of droplet epitaxy.
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We present a novel two-step growth approach for the Ga-assisted growth of GaAs nanowires (NWs) by molecular beam epitaxy on Si. In the first step only Ga is deposited for the controlled formation of ...Ga droplets and in the second step NWs are grown from these droplets at lower Ga flux. This variation of the Ga flux leads to a decoupling of the formation of Ga droplets and the formation of NWs. Thus, the total density of crystal objects can be varied by only changing the parameters of the first step. Also, the NW length distribution of such an ensemble is more homogeneous. Annealing of the droplets can improve the homogeneity even further. The resulting GaAs NW ensembles are ideally suited for the subsequent growth of shell structures. Finally, our new approach enabled us to explore the nucleation of different crystalline objects and analyze the impact of the droplet size on the vertical yield of NWs in detail.
•Variation of the Ga flux leads to a decoupling of the formation of Ga droplets and the formation of NWs.•NW length distribution of samples grown with the novel growth procedure is very homogeneous.•Exploration of the nucleation of different crystalline objects.•Analysis of the impact of the droplet size on the vertical yield of NWs.
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What makes three dimensional semiconductor quantum nanostructures so attractive is the possibility to tune their electronic properties by careful design of their size and composition. These ...parameters set the confinement potential of electrons and holes, thus determining the electronic and optical properties of the nanostructure. An often overlooked parameter, which has an even more relevant effect on the electronic properties of the nanostructure, is shape. Gaining a strong control over the electronic properties via shape tuning is the key to access subtle electronic design possibilities. The Pulsed Dropled Epitaxy is an innovative growth method for the fabrication of quantum nanostructures with highly designable shapes and complex morphologies. With Pulsed Dropled Epitaxy it is possible to combine different nanostructures, namely quantum dots, quantum rings and quantum disks, with tunable sizes and densities, into a single multi-function nanostructure, thus allowing an unprecedented control over electronic properties.
The goal of this work is to apply the idea of the city in 15 min to railway stations that can become the starting point of the analysis as they represent the “gateway”, where users start their last ...mile of travel after getting off the train. Within the research, 11 railway stations located in the Lombardy Region in Italy were identified and analyzed. To perform the analysis, an analytical index was implemented and determined for each station: this index summarizes the main features of the station itself in relation to the territory in which it is located. The adopted approach is comparative: it is not important the absolute value of the index of each station, but the comparison between the different indices. In this way it is possible on the one hand to classify the stations and on the other hand to identify and propose possible interventions to improve the role of a railway station in a territory. The proposed model is expandable and replicable: it is possible to add other useful indicators for the calculation of the index of each station and it is also possible to perform the analysis in different territorial contexts. In fact, it is a decision support tool able to provide indications and information for the planning and programming of the railway system and of the city; among the potential users of the proposed model there are railway station managers and administrations.
Mega-events play an important role in the urban agenda of contemporary metropolises. This importance lies in the fact that such events are seen as possible catalysts for the urban development of a ...metropolis and more generally of a territory, and an opportunity to establish them as global cities.
The planning and programming of a mega-event can be a country's showcase on the international landscape, and the mobility sector plays a significant role in defining its success or failure.
Transport infrastructure generally requires high resources for both construction and maintenance even after the event. Therefore, in order to ensure a good outcome, it needs to be planned strategically at multiple levels; this condition brings a benefit to both the host city and the community. The main purpose of this study is to analyze a mega event like the Olympic Games by highlighting the relationship between existing mobility solutions and sustainable solutions considering also the surrounding area. This is to enable the creation of new intelligent transportation options.
The Olympic Games, in fact, can represent a valuable opportunity to modernize the existing infrastructure of a country by improving the level of service and quality. The study considers, on the one hand, the expected demand for transport during the event and, on the other, the specific features of the different solutions that can be adopted to meet mobility needs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The fabrication and characterization of an infrared photodetector based on GaAs droplet epitaxy quantum dots embedded in Al0.3Ga0.7As barrier is reported. The high control over dot electronic ...properties and the high achievable number density allowed by droplet epitaxy technique permitted us to realize a device using a single dot layer in the active region. Moreover, thanks to the independent control over dot height and width, we were able to obtain a very sharp absorption peak in the thermal infrared region (3-8 m). Low temperature photocurrent spectrum was measured by Fourier spectroscopy, showing a narrow peak at 198 meV (∼6.3 m) with a full width at half maximum of 25 meV. The observed absorption is in agreement with theoretical prediction based on effective mass approximation of the dot electronic transition.
Evaluation of resonances phenomenon in high-speed railway bridges during train passage is a dominant issue in order to confirm the structural safety of the bridge and the stability of the ballast on ...bridge deck. The Sesia viaduct, located in the Turin-Milan high-speed rail line, is one of the most investigated high-speed railway bridges. The maximum accelerations and the resonance train speed were predicted while the bridge vibration modes and the effect of adjacent spans on these were estimated through some detailed experimental and numerical investigations. Nevertheless, some important points remain unclear, such as the actual resonance speed and the influence of deck local vibrations. To verify and clarify these issues, experimental dynamic analyzes based on acceleration measurements of the Sesia viaduct under ETR1000 train passages with speed up to 374km/h were conducted in this study. In the measurements, accelerometers were installed not only on steel box girder but also on concrete deck slab in order to identify local vibration modes of deck members so as to estimate their effect on the evaluation of the accelerations. Comparing the maximum accelerations up to 15Hz with various train speeds, the resonance speed corresponding to the first bending mode of the Sesia viaduct was identified. Furthermore, an analysis based on the identification of the natural frequencies clarified that the high-order resonances between passing train and deck local vibration modes have the largest impact on the maximum acceleration up to 30Hz which is the limit used for the verification of ballast stability in the Eurocode.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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