Research on the phase-frequency relationship of the field effect amplifier front-end of shipborne USB system

Normally, most researches on phase calibration of shipborne USB system focus on the means of phase calibration. This article starts with the research on the channel of the system. The composition of the channel is introduced, and the characteristics of the channel is analyzed. Taking the channel of the field effect amplifier front-end as the research object, a mathematical fitting algorithm is used to derive the functional relationship between the phase of the field effect amplifier front-end and the working frequency. The actual calibration data is used for simulation analysis to obtain the fitting order of the function. Combining the phase-frequency relationship of the field effect amplifier front-end and the microwave self-checking phase correction of the field effect amplifier back-end, a new phase calibration method is proposed.

Depth–Duration–Frequency Relationship Model of Extreme Precipitation in Flood Risk Assessment in the Upper Vistula Basin

The Upper Vistula Basin is a flood-prone region in the summer season (May–October) due to intensive rainfall. From the point of view of water management, it is particularly important to assess the variability in this main factor of flood risk, as well as to establish the depth–duration–frequency (DDF) relationship for maximum precipitation, this having not yet been derived for the region. The analysis of a 68-year (1951–2018) data series of summer maximum precipitation collected by 11 meteorological stations showed the series’ stationarity, which supports the conclusion that there is no increase in the risk of rainfall floods due to the intensification of extreme precipitation. A new approach is proposed for the determination of the DDF relationship, where the best-fitted distribution for each station is selected from among the set of candidate distributions, instead of adopting one fixed distribution for all stations. This approach increases the accuracy of the DDF relationships for individual stations as compared to the commonly used approach. In particular, the traditionally used Gumbel distribution turns out to be not well fitted to the investigated data series, and the advantage of the recently popular GEV distribution is not significant.

A fast insight into the nonlinear oscillation of nano-electro mechanical resonators considering the size effect and the van der Waals force

Nano/micro actuators are widely used in micro/ nano electro mechanical systems (NEMS/MEMS) and the study on its nonlinear oscillation is of great significance. This paper begins with a wrong variational principle ([19] Appl Nanosci, 2016, 6: 309-317) of the reduced governing partial differential equation of the resonator which is used to describe the nonlinear oscillation of nano-electro mechanical resonators that takes into account the size effect and the van der Waals force. By using the Semi-inverse method, we establish the genuine variational principle. Then a simple method so called He’s frequency formulation is applied to solve the problem, where it only needs one-step to get the approximate amplitude-frequency relationship. Comparing with the existing method, it shows that the proposed method is simple but effective, which is helpful to be of significance to the study of the nonlinear oscillation in micro/nano electro mechanical systems.

Exploiting internal resonance to improve flow energy harvesting from vortex-induced vibrations

Vortex-induced vibrations have been recently employed to capture scalable flow energy harvesters, which can attain the maximum power when the wind speed is in the lock-in region where the vortex-shedding frequency is close to the structural frequency. Nevertheless, the dynamical mechanism of the synchronization phenomenon has not been reported. To solve this critical problem, in this paper we explore a novel internal resonance to scavenge flow energy from vortex-induced vibrations, the mechanism of synchronous oscillations is introduced by the amplitude-frequency relationship and confirmed by the energy function. To show the capturing performance, an electromagnetic energy harvester with one-to-one internal resonance is proposed. Based on the harmonic balance method, the electromechanical coupling equations are decoupled, and the first order approximate harmonic responses of displacement and current are established. The modulation equations are derived, the amplitude-frequency curves of displacement and current are plotted with different detuning parameters. The advantage of the proposed one-to-one internal resonance is compared to the noninternal resonance case, the results express that the internal resonance scheme can enhance flow energy capture. The effects of physical parameters on the scavenged power are discussed. The accuracy and efficiency of the approximate analytical results are checked by numerical simulations.

Maximal muscular power: lessons from sprint cycling

Maximal muscular power production is of fundamental importance to human functional capacity and feats of performance. Here, we present a synthesis of literature pertaining to physiological systems that limit maximal muscular power during cyclic actions characteristic of locomotor behaviours, and how they adapt to training. Maximal, cyclic muscular power is known to be the main determinant of sprint cycling performance, and therefore we present this synthesis in the context of sprint cycling. Cyclical power is interactively constrained by force-velocity properties (i.e. maximum force and maximum shortening velocity), activation-relaxation kinetics and muscle coordination across the continuum of cycle frequencies, with the relative influence of each factor being frequency dependent. Muscle cross-sectional area and fibre composition appear to be the most prominent properties influencing maximal muscular power and the power-frequency relationship. Due to the role of muscle fibre composition in determining maximum shortening velocity and activation-relaxation kinetics, it remains unclear how improvable these properties are with training. Increases in maximal muscular power may therefore arise primarily from improvements in maximum force production and neuromuscular coordination via appropriate training. Because maximal efforts may need to be sustained for ~15-60 s within sprint cycling competition, the ability to attenuate fatigue-related power loss is also critical to performance. Within this context, the fatigued state is characterised by impairments in force-velocity properties and activation-relaxation kinetics. A suppression and leftward shift of the power-frequency relationship is subsequently observed. It is not clear if rates of power loss can be improved with training, even in the presence adaptations associated with fatigue-resistance. Increasing maximum power may be most efficacious for improving sustained power during brief maximal efforts, although the inclusion of sprint interval training likely remains beneficial. Therefore, evidence from sprint cycling indicates that brief maximal muscular power production under cyclical conditions can be readily improved via appropriate training, with direct implications for sprint cycling as well as other athletic and health-related pursuits.

Piezoelectric rod sensors for scour detection and vortex-induced vibration monitoring

As extreme events increase in frequency, flow-disrupting large-scale structures become ever more susceptible to collapse due to local scour effects. The objective of this study was to validate the functionality of passive, flow-excited scour sensors that can continue to operate during an extreme event. The scour sensors, or piezo-rods, feature continuous piezoelectric polymer strips embedded within and along the length of slender cylindrical rods, which could then be driven into the soil where scour is expected. When scour erodes away foundation material to reveal a portion of the piezo-rod, ambient fluid flow excitations would cause the piezoelectric element to output a voltage response corresponding to the dynamic bending strains of the sensor. The voltage response is dependent on both the structural dynamic properties of the sensor and the excitation fluid’s velocity. By monitoring both shedding frequency and flow velocity, the exposed length of the piezo-rod (or scour depth) can be calculated. Two series of experimental tests were conducted in this work: (1) the piezo-rod was driven into sediment around a mock pier to collect scour data, and (2) the piezo-rod was used to monitor its own structural response by collecting vortex-shedding frequency data in response to varied flow velocities to establish a velocity–frequency relationship. The results showed that the piezo-rod successfully captured structural vortex-shedding frequency comparable to state-of-practice testing. A one-dimensional numerical model was developed using the velocity–frequency relationship to increase the accuracy of voltage-based length prediction of the piezo-rod. Two-dimensional flow modeling was also performed for predicting localized velocities within a complex flow field. These velocities, in conjunction with the velocity–frequency relationship, were used to greatly improve length-predictive capabilities.