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While turbulent drag reduction through the injection of micro-bubbles into a turbulent boundary layer is well established in experiments, there is a lack of corresponding supporting evidence from direct numerical simulations. Here we report on a series of numerical simulations of small bubbles seeded in a turbulent channel flow at average volume fractions of up to 8%. These results show that even for relatively large bubbles, an initial transient drag reduction can occur as bubbles disperse into the flow. Relatively small spherical bubbles will produce a sustained level of drag reduction over time.

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DC-DC converters with high-voltage gain and low-input current ripple have attracted much attention in photovoltaic, fuel cells and other renewable energy system applications. Conventional boost-flyback converter can achieve high-voltage setup ratio; however, its input current is pulsing and the voltage stress across output diode of flyback-cell is high. In this study, by incorporating coupled-inductor into the boost-cell of boost-flyback converter, the voltage stress across the output diode is effectively reduced. Passive snubber circuit is utilised to suppress the voltage spike across power switch, low-voltage-rated metal-oxide semiconductor field effect transistor (MOSFET) with low R ds_on can thus be used to reduce the conduction loss of power MOSFET. In addition, ripple-free input current can be achieved, which makes the design of electromagnetic interference filter easy. Steady-state characteristics of the proposed converter are analysed, and experimental results are given to verify the analysis results.

A simple memristor-based chaotic circuit with an active flux-controlled memristor characterized by a smooth continuous cubic nonlinearity is designed. The proposed chaotic circuit can generate a 2-scroll chaotic attractor on a finite time scale and has an equilibrium set with its stability dependent on the initial state of the memristor. The complex dynamics of the proposed chaotic circuit under different initial state of the memristor are investigated both theoretically and numerically. In particular, some novel transient transition behaviors with different time scales are found in the memristor circuit. Experimental observations based on a universal circuit implementation platform are conducted to partially verify the numerical simulation results.

Bao Bo-Cheng( ) a) † , Xu Jian-Ping( ) b) , Zhou Guo-Hua( ) b) , Ma Zheng-Hua( ) a) , and Zou Ling( ) a)

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