A modified snap-through mechanism is used in an electromagnetic energy harvester to improve its effectiveness. It mainly comprises three springs that are configured so that the potential energy of ...the system has two stable equilibrium points. In particular, the small vibration behavior of the harvester around one of the equilibriums is of interest. A multi-scale method (MSM) is used to analyze the frequency response curve. Two snap-through mechanisms are considered. One has both horizontal and vertical springs. The other has only horizontal springs. The frequency response curves of these two classes are compared under the same excitation and electric loading conditions. The latter exhibits more bending of the frequency response curve than the former one. The results are also validated by some numerical work. The averaged power subject to the Gaussian white noise is calculated numerically, and the results demonstrate that bi-stable energy harvesting with only horizontal springs can outperform the mechanism with both horizontal and vertical springs for the same distance between two equilibriums.
Transmissibility is a key parameter to quantify the effectiveness of a vibration isolation system. Under harmonic excitation, the force transmissibility of a linear vibration isolation system is ...defined as the ratio between the amplitude of the force transmitted to the host structure and the excitation force amplitude, and the displacement transmissibility is the ratio between the displacement amplitude of the payload and that of the base. For a nonlinear vibration isolation system, the force or the displacement responses usually have more frequency components than the excitation. For a harmonic excitation, the response may be periodic, quasi-periodic or chaotic. Therefore, the amplitude ratio cannot well define the transmissibility. The root-mean-square ratio of the response to the excitation is suggested to define the transmissibility. The significance of the modified transmissibility is highlighted in a nonlinear two-stage vibration isolation system consisting of two linear spring connected linear vibration isolators with two additional horizontal linear springs. Harmonic balance method (HBM) is applied to determine the responses with the fundamental and third harmonic. Numerical simulations reveal that chaos may occur in the responses. In both cases, the modified transmissibility works while the original definition cannot be applied to chaotic response.
•Two definitions of nonlinear transmissibility are compared.•Two definitions are slightly different for both force and displacement transmission.•Force and displacement transmissibility with modified definition are more agreement.•Transmissibility for aperiodic response is evaluated by the modified definition.
Motivated by the need for improving the isolation performance, many research studies have been performed on isolators with nonlinear characteristics. Based on the shape of their phase portrait, such ...devices can be configured as either a mono- or bi-stable isolator. This paper focuses on investigating the relative performance of these two classes under the same excitations. Force transmissibility is used to measure the isolation performance, which is defined in terms of the RMS of the ratio of the transmitted force to the excitation force. When the system is subjected to harmonic excitation, it is found that the maximum reduction of the force transmissibility in the isolation range using Quasi-Zero stiffness is achieved. When the system is subjected to random excitation, it has the same effect of Quasi-Zero stiffness. Further, optimum damping can be changed with stiffness and has minimum value.
This paper investigates the most desirable configuration of a two-stage nonlinear vibration isolation system, in which the isolators contain hardening geometric stiffness nonlinearity and linear ...viscous damping. The force transmissibility of the system is used as the measure of the effectiveness of the isolation system. The hardening nonlinearity is achieved by placing horizontal springs onto the suspended and intermediate masses, which are supported by vertical springs. It is found that nonlinearity in the upper stage has very little effect and thus serves little purpose. The nonlinearity in the lower stage, however, has a profound effect, and can significantly improve the effectiveness of the isolation system. Further, it is found that it is desirable to have high damping in the upper stage and very low damping in the lower stage.
► A two-stage nonlinear vibration isolation system is modeled. ► Force transmissibility is used as the measure of the isolation performance. ► Nonlinearity in the lower stage can improve the effectiveness of the isolator. ► High damping in the upper stage and low damping in the lower stage are preferable.
In order to treat the kitchen wastes and produce hydrogen, anaerobic fermentation technology was used in this experiment. The results showed that the fermentation type changed from mixed acid ...fermentation to ethanol fermentation in a continuous stirred tank reactor (CSTR) 22 days after start-up. The maximum efficiency of hydrogen bio-production in the CSTR was 4.77 LH₂/(L reactor d) under the following conditions: organic loading rate (OLR) of 32-50 kg COD/(m³ d), oxidation reduction potential (ORP) of -450 to -400 mV, influent pH value of 5.0-6.0, effluent pH value of 4.0-4.5, influent alkalinity of 300-600 mg/l, temperature of 35 ± 1°C and hydraulic retention time (HRT) of 7 h. An artificial neural network (ANN) model was established, and each parameter influencing the performance of the reactor was compared using the method of partitioning connection weights (PCW). The results showed that OLR, pH, ORP and alkalinity could influence the fermentation characteristics and hydrogen yield of the anaerobic activated sludge; with an influence hierarchy: OLR > pH values > ORP > alkalinity. An economic analysis showed that the cost of producing hydrogen in this experiment was less than the cost of electrolysis of water.
In the present study, the long-term and short-term cumulative risks of pyrethroids exposured for the Chinese general population and children through fruits consumption were evaluated. A total of 1450 ...fruit samples and seven pyrethroids were included based on the pesticide residues monitoring programme of China from 2013 to 2015. The exposure was estimated using both deterministic approach and semi-probabilistic model for comparison. The hazard index approach was used to assess cumulative risk. 26% of samples contained pyrethroid residues with concentrations ranged from 0.0050 mg/kg to 1.2 mg/kg, of which 30% simultaneously with 2–4 mixture residues. Results demonstrated that the cumulative health risks were extremely low for both general population and children (1–6 years old) of China in the long term. Acute risk estimations calculated by deterministic method were several or many times overestimated than the results based on semi-probabilistic method. Acute cumulative exposure of children to pyrethroid compounds in 0.76% samples were exceeded 1 in worst case scenario. More detailed assessments with adequate data in the future use probabilistic method is expected to reduce the uncertainties of cumulative dietary exposure.
Display omitted
•26.1% of 1450 samples contained pyrethroid residues and of which 30.4% were with 2–4 residues.•The cumulative health risks were extremely low in the long term.•Acute cumulative exposures of children to pyrethroid compounds in 0.76% samples were unacceptable.•Acute risk estimation calculated by deterministic method was overestimated than that by semi-probabilistic method.
The interplay between pesticides plays a critical role in ecotoxicology since these chemicals rarely emerge as single substances but rather in mixtures with other chemicals. In the present work, we ...purposed to clarify the combined toxic impacts of pyraclostrobine (PYR) and metiram (MET) on the zebrafish by using numerous indicators. Results exhibited that the 4-day LC
50
value of MET to fish embryos was 0.0025 mg a.i. L
−1
, which was lower compared with PYR (0.019 mg a.i. L
−1
). Combinations of PYR and MET presented a synergetic impact on fish embryos. Contents of POD, CYP450, and VTG were drastically increased in the plurality of the single and joint treatments relative to the baseline value. Three genes, including
vtg1
,
crh
, and
il-8
, related to the endocrine and immune systems, were also surprisingly up-regulated when fish were challenged by the individual and mixture pesticides compared with the baseline value. These results afforded valuable information on the latent toxicity mechanisms of co-exposure for PYR and MET in the early growth stage of fish. Moreover, our data also revealed that frequent application of these two pesticides might exert a potentially ecotoxicological hazard on aquatic ecosystems. Collectively, the present study provided valuable guidance for the risk evaluation of chemical combinations.
This paper concerns the effect that a stochastic resonance can have on a vibration isolation system. Rather than reducing the transmitted force, it is shown that it is possible to significantly mask ...the component of the force transmitted though the isolator, when the system is excited harmonically. This can be achieved by adding a very low intensity of random noise to the harmonic excitation force. The nonlinear mechanical vibration isolation system used in the study consists of a vertical linear spring in parallel with two horizontal springs, which are configured so that the potential energy of the system has a double-well. Prior to the analytical and numerical study, an experiment to demonstrate stochastic resonance in a mechanical system is described.
•A nonlinear mechanical isolation system for stochastic resonance is modeled.•Component of force transmitted though isolator is masked by adding very low noise.•Stochastic anti-resonance of transmitted force occurs before stochastic resonance.•Noise intensity for stochastic resonance is increased as frequency increasing.•Stochastic resonance in mechanical system is validated by experiment work.
The large-scale structure systems in engineering are complex, high dimensional, and variety of physical mechanism couplings; it will be difficult to analyze the dynamic behaviors of complex systems ...quickly and optimize system parameters. Model order reduction (MOR) is an efficient way to address those problems and widely applied in the engineering areas. This paper focuses on the model order reduction of high-dimensional complex systems and reviews basic theories, well-posedness, and limitations of common methods of the model order reduction using the following methods: center manifold, Lyapunov–Schmidt (L-S), Galerkin, modal synthesis, and proper orthogonal decomposition (POD) methods. The POD is a powerful and effective model order reduction method, which aims at obtaining the most important components of a high-dimensional complex system by using a few proper orthogonal modes, and it is widely studied and applied by a large number of researchers in the past few decades. In this paper, the POD method is introduced in detail and the main characteristics and the existing problems of this method are also discussed. POD is classified into two categories in terms of the sampling and the parameter robustness, and the research progresses in the recent years are presented to the domestic researchers for the study and application. Finally, the outlooks of model order reduction of high-dimensional complex systems are provided for future work.
PDF
