Science China Technological Sciences

Accurate contact calculations of real rough surfaces are fundamental but complicated. The model-based methods are convenient and straightforward. But these methods ignore some factors and may lead to less accurate results. This is especially true when considering multi-scale topographic features of engineering rough surfaces. Based on artificially generated rough surfaces, the deterministic contact analysis of two 3D rough surfaces is conducted by the finite element method (FEM). The calculations show that when the separation between surfaces reduces, results of classic model-based methods are quite different from those of this study, especially when the roughness distribution and textures are considered. As friction pairs are working under increasing harsh conditions, the accurate contact calculation in this paper will be of great significance.

The soot formation model based on inverse ethylene diffusion flames was performed to study the sensitivity of the soot formation process to the prediction results. The effects of efficiency parameters such as soot inception, surface growth and coagulation on the simulation results were studied by using the adjustable efficiency model. In addition, the reversible soot model and conjugate heat transfer (CHT) model were also introduced to explore their advantages. Results indicated that, among adjustable efficiency parameters, the nucleation efficiency had the greatest influence on the predicted soot and PAHs distributions, while the H-abstraction-C2H2-addition (HACA) process and PAH adsorption surface growth efficiencies impacted little. The adjustable efficiency parameters had a significant effect on the concentration of soot gaseous precursors and soot particles, but their effects on temperature, gas phase molecules, and intermediate species were not obvious. When the nucleation efficiency increased from 2×10−6 to 1×10−4, the predicted value of the integrated soot was increased by nearly 50%, and the maximum primary particle number density and the number of aggregates were increased by an order of magnitude. The maximum concentration of BAPYR was doubled. However, the peak temperature along the axial direction increased by only 3.5 K. Using the reversible soot model, the approximation results of the adjustable efficiency parameters could be modified, which showed the feasibility of the model. The use of the CHT model promoted pyrolysis of the fuel below the outlet of the fuel tube, with high-temperature zones, soot zones, and PAHs zones moving towards higher flame heights. Besides, when using the reversible model and the CHT model, the maximum soot volume fraction decreased by 39% compared with the basic efficiency parameters, while the concentration of BAPYR increased by 162%, and the concentrations of gas phase species were decreased.

This paper examines the energy and environmental benefits within the whole life cycle shifting from traditional gasoline vehicles to electrified advanced vehicles under regional real-world driving behaviors. The advance vehicles focus on family passenger cars and include battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs), and hybrid electric vehicles (HEVs). The GREET (greenhouse gases, regulated emissions, and energy use in transportation) model is adopted with regional circumstances modifications, especially the UF (utility factors) of PHEVs. The results show that the electrified vehicles offer great benefits concerning energy consumption, greenhouse gas (GHG) emissions as well as urban Particulate Matter 2.5 (PM2.5) emissions. Compared to conventional gasoline vehicles, the life-cycle total energy reduction for advance vehicles is 51% to 57%. There is little difference on energy reduction among the HEVs, PHEVs and BEVs, with the energy mix shifting from petroleum to coal for the stronger electrification. The reductions of GHG emissions are 57% for HEV, 54% to 48% for PHEVs with 10 miles to 40 miles CD range, and 40% for BEV. The life-cycle and local PM2.5 emissions are discussed separately. The life-cycle PM2.5 emissions increase with vehicle electrification and reach a maximum for the BEV which are 5% higher than the conventional vehicle (CV). However, electric vehicles can shift PM2.5 emissions from vehicle operation to upstream operations and help mitigate PM2.5 emissions in urban areas. The local emissions of PHEVs and BEVs can be reduced by 37% to 81% and 100% compared with CVs.

Due to overvoltage produced by inverter output, inverter-fed motor insulation systems often experience fast electrical aging process, especially when partial discharge (PD) is incepted. Before putting into use, the PD detection should be performed on inverter-fed motors at repetitive square voltages to avoid the PD caused insulation deterioration when the motors are collected to inverters having specific characteristics. However, unlike PD tests at AC/DC voltages proposed in IEC 60270, the PD detection at repetitive square voltages is much more complex because of serious interference generated by impulse generator. To solve the problem, ultra-high frequency (UHF) method seems recommendable for its preferable signal-to-noise ratio (SNR). The chief aim of this study is to investigate PD pulse and statistical characteristics of turn-to-turn insulation for inverter-fed traction motors. A square-shaped Archimedes antenna, specially designed for the PD detection at repetitive square voltages of fast rise times, was used to perform PD tests on turn-to-turn insulation models. Time and frequency analysis results indicate that energy component of generator disturbance and PD pulses are mainly distributed in the 0–0.5 GHz and 0.6–1.5 GHz range, respectively. Based on the results, suitable filter was designed for power disturbance suppression. Additionally, resorting to the sensor unit (i.e. antenna and filter) and the PD test system, the PD statistical features at square voltages of different frequencies were obtained. Experimental results show that higher frequency will give rise, statistically, to PD of lower magnitudes distributing at smaller phases. A reasonable interpretation of this phenomenon was presented. Lastly, according to the PD statistical features, some suggestions for the PD detection system design, generator parameter optimization and the PD pulse extracting were given. The results of this work would be beneficial to the increase of the sensitivity when performing the PD detection on insulation systems for inverter-fed motors at repetitive square voltages and thus, improving the reliability of inverter-fed motors.

Cooling water is an important part in a Spallation Neutron Source target cooling system, but the unstable vortexes at the exits of the slits between every two tungsten target slices have a negative impact on the stable running of the target system. We apply the field synergy principle for fluid flow to obtain the optimal flow field, which has a uniform velocity distribution without eddy, and then, optimize the geometrical structure of the cooling water flow channel based on the optimal flow field. The results show that when the cooling water flows in the optimized channel, the eddy sizes decrease, the time fluctuations of velocity and pressure almost vanish, and the volume flow rates of the cooling water in each parallel slit are uniform. Therefore, it effectively improves the running stability of the target system with the premise of satisfying the target heat load.

A stochastic averaging technique is proposed to study the randomly excited single-degree-of-freedom (SDOF) strongly nonlinear systems with delayed feedback fractional-order proportional-derivative (PD) controller. The delayed feedback fractional-order PD control force is approximated by an equivalent non-delay feedback control force combining with a quasi-linear elastic force and a quasi-linear damping force. The averaged Itô stochastic differential equation for amplitude of the equivalent nonlinear system is derived by the generalized harmonic functions. The analytical stationary probability density function (PDF) is obtained with solving the reduced Fokker-Planck-Kolmogorov (FPK) equation. Two examples of van der Pol oscillator and Rayleigh-Duffing oscillator are studied to illustrate the application and effectiveness of the proposed method. Numerical results display that the proposed method can yield to the high precision, and the time delay could ruin the control effectiveness, but also even amplifies the response of the system more than that of uncontrolled system. Furthermore, the study finds that the parameters of fractional-order α and time delay may cause the stochastic P-bifurcation. It is indicated that the delayed feedback fractional-order PD controller can offer a potentially effective tool for anti-control of stochastic bifurcation

The process to achieve time synchronization and ranging for a network of mobile nodes is raising a concern among researchers, and hence a variety of joint time synchronization and ranging algorithms have been proposed in recent years. However, few of them handle the case of all-node motion under unknown positions and velocities. This study addresses the problem of determining ranging and time synchronization for a group of nodes moving within a local area. First, we examined several models of clock discrepancy and synchronous two-way ranging. Based upon these models, we present a solution for time synchronization with known positions and velocities. Next, we propose a functional model that jointly estimates the clock skew, clock offset, and time of flight in the absence of a priori knowledge for a pair of mobile nodes. Then, we extend this model to a network-wide time synchronization scheme by way of a global least square estimator. We also discuss the advantages and disadvantages of our model compared to the existing algorithms, and we provide some applicable scenarios as well. Finally, we show that the simulation results verify the validity of our analysis.

Due to the excellent dynamic performance, the Finite Control Set Model Predictive Control has been widely used in various types of converters. However, when Finite Control Set Model Predictive Control is adopted, the switching frequency of converters varies significantly with system operating conditions. Consequently, constant-frequency predictive control strategy has been proposed. Two active voltage vectors and a zero voltage vector are selected within each sampling period. The action time sequence is then calculated. Due to the unsymmetrical distribution of current variation rates around zero, the calculated value of the voltage-vector action time will turn up negative. According to common sense, the voltage-vector action time is greater than or equal to zero. The action time is normally forced to zero whenever a negative value is predicted, resulting in the control failure and performance deterioration. To solve this problem, this paper proposes modified strategy. The modified strategy examines the action time calculated out. When negative action time comes out, the modified strategy reselects the active voltage vector accordingly, instead of forcing the action time to be zero. Optimized action time sequence is further determined by minimizing the cost function. The effectiveness of the modified strategy is clearly verified by experimental tests, and analytical remarks are all founded in practical results.

Spectral bandpass filters serving as crucial signal processing components are widely applied in various electromagnetic/optical systems. However, the filtering of reflection signal always faces problems of notably accompanied transmission or strict angular limit. Here, we propose novel transmissionless metagratings for specular-reflection bandpass filtering under wide oblique incidence angles. The metagratings are composed of metallic groove gratings and high-refractive-index dielectric particles, which of them respectively provide continuum of localized cavity mode and discrete magnetic dipole resonance under oblique transverse magnetic waves, and produce broadband and narrowband anomalous reflection. Their destructive interference gives rise to a Fano-type narrow bandpass filter in the specular reflection. By integrating CaTiO3 ceramic particles with metallic groove gratings, we experimentally demonstrate a single-bandpass metagrating filter at microwave frequency which exhibits a low insertion loss and narrow bandwidth under wide-angle oblique incidence, and agrees with simulations well. Furthermore, by arranging two different sized CaTiO3 ceramic particles or changing the temperature, the metagrating filter is endowed with dual-bandpass or thermally tunable feature, respectively. Finally, the design is extended to terahertz frequencies by improving the structure to a double-groove configuration. The proposed composite metagratings with novel bandpass filtering function and angular insensitivity not only enrich the category of spatial filter but have promising applications in microwave communication systems and optical devices.