Showing papers by "Zexiang Li published in 2022"

Drag reduction of blowing-based active control in a turbulent boundary layer

Abstract: Direct numerical simulations (DNS) are conducted to gain insight into the blowing-based active control in a spatially developing turbulent boundary layer at a low Reynolds number. The drag reduction properties and mechanisms of different blowing velocity distribution forms under constant wall-normal mass flux are comparatively studied, including uniform blowing and blowing-only opposition control (BOOC). After the application of blowing control, the self-similarity of the Reynolds shear stress is influenced. The property of drag reduction and control gain of the blowing-based active control schemes in the turbulent boundary layer is similar to that in turbulent channel flow, i.e., the BOOC scheme can achieve higher drag reduction than uniform blowing, but the control gain reduces. Due to the coexistence of the opposition effect and the induction effect, the negative wall-normal velocity fluctuations accompanied by the sweep motion are induced to form small-scale flow structures in the near-wall region. The decomposition of the skin-friction drag coefficient shows that the changes of each contribution term are basically the same for different blowing schemes, except that the BOOC scheme has a more substantial influence on mean convection and spatial development. According to the property that the drag reduction of the BOOC scheme with additional threshold limitation is equivalent to that without the restriction, it can be determined that the effect of blowing-based active control is mainly based on the temporal and spatial averaging effects of blowing, including the opposition effect and the induction effect.

The control and evaluation method of the cascaded buck–boost converter for equalization on battery strings

Abstract: With the further implementation of the carbon peak carbon neutral strategy, new energy vehicles will play an important cornerstone in the realization of this policy. However, the power source lithium battery pack has reduced the charge and discharge capacity due to the variation in the energy of the single cells, which largely limits the wide application of new energy vehicles. A cascaded buck–boost converter is proposed to balance the inconsistent energy in the battery pack. This equalization circuit is equipped with two fundamental working modes on which a matrix equalization strategy is developed and optimized accordingly. Then, the systematic evaluation method on average efficiency and time is applied to investigate the performance of the converter. Subsequently, the proposed converter and its control method have shown better performance than the existing equalization circuit via simulation and experimental verification. The design has high scalability and is expected to be applied to large‐scale energy storage systems to serve the construction of the global energy Internet.

Design and Implement of Staggered Parallel Lithium Battery Equalization Converter With Jumper Switches

Abstract: To mitigate the pressure on energy storage and enhance the flexibility of the power system, lithium-ion batteries are widely utilized in large-scale energy storage in smart grids due to their superior charging and discharging performance and energy storage characteristics. Whereas the inconsistency of lithium battery cells has become a key issue that limits the overall performance of the battery pack. A novel flexible equalization converter is proposed to handle this issue. This converter is combined with a staggered parallel topology and jumper switched, which enables an energy transfer path to exist between any cell to the rest pack and any substring to the rest substring. The electromagnetic transient model and its small-signal model of the proposed converter have been developed in MATLAB to prove its effectiveness. Finally, the conclusion could be arrived at that the proposed topology shows a better performance compared to the traditional method.

Project based learning in the context of cutting-edge robotics competition

Abstract: RoboMaster is an annual robotics competition pitting student’s cutting edge robot designs in a Multiplayer Online Battle Arena (MOBA) gamified shoot-out battle. The highly technical-advanced competition rules are designed to encourage innovation in robotics automation and intelligent systems while placing robot battles at centre-stage. It manages to display the beauty of engineering technology and popularize robotics to the wider audience by balancing technical challenge and entertainment value. Hong Kong University of Science and Technology (HKUST) sees RoboMaster as a unique platform for promoting STEM to the wider university student body. Through 4 years of team development and student participations, we have homegrown a group of outstanding scientific and technological engineers who have excelled in both academic research and entrepreneurial ventures. In this paper we review the development, growth and success of the 4 years of the HKUST RoboMaster ENTERPRIZE team and evaluate assessable learning outcomes of the competition as a project course. The degree of engineering design complexity and resulting the comprehensive learning outcomes, inspired the launch of a new bachelor programme in which many foundation engineering courses are replaced by year-long cornerstone project courses mirroring the RoboMaster project course. The outcome is a more individualized high-impact programme allowing students to build their engineering studies around technologies of our time.