A survey about framing the bases of Impulsive Mechanics of constrained systems into a jet-bundle geometric context

We illustrate how the different kinds of constraints acting on an impulsive mechanical system can be clearly described in the geometric setup given by the configuration space--time bundle $\pi_t:\mathcal{M} \to \mathbb{E}$ and its first jet extension $\pi: J_1 \to \mathcal{M}$ in a way that ensures total compliance with axioms and invariance requirements of Classical Mechanics. We specify the differences between geometric and constitutive characterizations of a constraint. We point out the relevance of the role played by the concept of frame of reference, underlining when the frame independence is mandatorily required and when a choice of a frame is an inescapable need. The thorough rationalization allows the introduction of unusual but meaningful kinds of constraints, such as unilateral kinetic constraints or breakable constraints, and of new theoretical aspects, such as the possible dependence of the impulsive reaction by the active forces acting on the system

Symmetric and Asymmetric Multiple Impulsive Constraints Without Friction and Their Characterization

We present two meaningful and effective non-ideal constitutive characterizations for a multiple impulsive constraints S comprising a finite number of non-ideal frictionless constraints of codimension 1, described in the geometric setup given by the space–time bundleMof a mechanical system in contact/impact with S. Thanks to the geometric structures associated to the elements of S, we introduce a symmetric characterization, that does not distinguish the elements forming S as regards mechanical behavior, and an asymmetric one that makes this distinction. Both the characterizations provide a generalization of the characterization of ideal multiple constraints presented in Pasquero (Q Appl Math 76(3):547–576, 2018). The iterative nature of these characterizations allows the introduction of two algorithms determining the right velocity of the system in case of single or multiple contact/impact with symmetric or asymmetric constraints S, once the elements forming S and the left velocity of the system are known.We show the effectiveness of the two possible choices with explicit implementations of these algorithms in two significant examples: a simplified Newton’s cradle system for the symmetric characterization and a disk in multiple contact/impact with two walls of a corner for the asymmetric one

Analysis and Performance Comparison of DVB-T and DTMB Systems for Terrestrial Digital TV

Orthogonal frequency-division multiplexing (OFDM) is the most popular transmission technology in digital terrestrial broadcasting (DTTB), adopted by many DTTB standards. In this paper, the bit error rate (BER) performance of two DTTB systems, namely cyclic prefix OFDM (CP-OFDM) based DVB-T and time domain synchronous OFDM (TDS-OFDM) based DTMB, is evaluated in different channel conditions. Spectrum utilization and power efficiency are also discussed to demonstrate the transmission overhead of both systems. Simulation results show that the performances of the two systems are much close. Given the same ratio of guard interval (GI), the DVB-T outperforms DTMB in terms of signal to noise ratio (SNR) in Gaussian and Ricean channels, while DTMB behaves better performance in Rayleigh channel in higher code rates and higher orders of constellation thanks to its efficient channel coding and interleaving scheme

Northwestern Pacific typhoon intensity controlled by changes in ocean temperatures.

Dominant climatic factors controlling the lifetime peak intensity of typhoons are determined from six decades of Pacific typhoon data. We find that upper ocean temperatures in the low-latitude northwestern Pacific (LLNWP) and sea surface temperatures in the central equatorial Pacific control the seasonal average lifetime peak intensity by setting the rate and duration of typhoon intensification, respectively. An anomalously strong LLNWP upper ocean warming has favored increased intensification rates and led to unprecedentedly high average typhoon intensity during the recent global warming hiatus period, despite a reduction in intensification duration tied to the central equatorial Pacific surface cooling. Continued LLNWP upper ocean warming as predicted under a moderate [that is, Representative Concentration Pathway (RCP) 4.5] climate change scenario is expected to further increase the average typhoon intensity by an additional 14% by 2100

Assessment of multiaxial fatigue life prediction methodologies for Inconel 718

Abstract Fatigue life prediction methodologies for the assessment of the structural integrity of safety critical components in modern turbine engines require a close integration of advanced multiaxial fatigue life prediction procedures and of specific multiaxial tests, representative of the service conditions of turbine engine components and materials. The objective of the research work presented in this paper is to extend currently employed methodologies for the assessment of fatigue strength of turbine engines disks by integrating suitable multiaxial fatigue criteria and test results of multiaxial fatigue experiments conducted on Inconel 718 material at temperatures similar to those experienced by the disc materials during service. Smooth tubular specimens of Inconel 718 have been employed for conducting tension/torsion strain controlled high temperature fatigue tests. Specimens have been cut out from forged parts utilised for the production of engine discs, thus preserving the typical properties of disc materials (microstructure, basic mechanical properties, etc.). Different models/criteria have been evaluated by comparing fatigue life predictions and multiaxial fatigue experiments. It's well known that agreement of life predictions with experimental life is strongly affected not only by the choice of the multiaxial fatigue criteria but also by the way the reference fatigue data are integrated in the criteria. Therefore, specific multiaxial fatigue tests have been carried out, in order to validate and to improve the assessment capabilities of the lifing procedures. Moreover, multiaxial fatigue tests permit advances in the basic understanding of materials behaviour that might be utilised in the processes of declaring component service lives