Browsing by Author "Zhang, Yu"

Now showing 1 - 2 of 2
  • Thumbnail Image
    ItemOpen Access
    How wavelength affects hydrodynamic performance of two accelerating mirror-symmetric undulating hydrofoils
    (American Institute of Physics (AIP), 2023-08-02) Lin, Zhonglu; Liang, Dongfang; Bhalla, Amneet Pal Singh; Sheikh Al-Shabab, Ahmed A.; Skote, Martin; Zheng, Wei; Zhang, Yu
    Fish schools are capable of simultaneous linear acceleration. To reveal the underlying hydrodynamic mechanism, we numerically investigate how Reynolds number Re ¼ 1000–2000, Strouhal number St ¼ 0:2–0:7, and wavelength k ¼ 0:5–2 affect the mean net thrust and net propulsive efficiency of two side-by-side hydrofoils undulating in anti-phase. In total, 550 cases are simulated using immersed boundary method. The thrust increases significantly with the wavelength and the Strouhal number, yet only slightly with the Reynolds number. We apply a symbolic regression algorithm to formulate this relationship. Furthermore, we find that mirror-symmetric schooling can achieve a net thrust more than ten times that of a single swimmer, especially at low Reynolds numbers. The highest efficiency is obtained at St ¼ 0:5 and k ¼ 1:2, where St is consistent with that observed in the linear-accelerating natural swimmers, e.g., Crevalle jack. Six distinct flow structures are identified. The highest thrust corresponds to an asymmetric flow pattern, whereas the highest efficiency occurs when the flow is symmetric with converging vortex streets.
  • Thumbnail Image
    ItemOpen Access
    Refining microstructure of medium-thick AA2219 aluminium alloy welded joint by ultrasonic frequency double-pulsed arc
    (Elsevier, 2023-02-14) Wang, Yipeng; Li, Hong; Li, Zhuoxin; Zhang, Yu; Qin, Jian; Chen, Guangyu; Qi, Bojin; Zeng, Caiyou; Cong, Baoqiang
    The increasing demand for achieving high-efficiency and high-quality medium-thick aluminium alloy welded structures, especially for large scale aerospace components, presents an urgent challenge to the conventional TIG arc welding process. This work proposed a novel double-pulsed variable polarity tungsten inert gas (DP-VPTIG) arc, in which the variable polarity square wave current was simultaneously modulated into ultrasonic frequency (20–80 kHz) and low frequency (0.5–10 Hz) pulses. Full penetration welds of 6 mm thick AA2219 aluminum alloy were successfully obtained by using this process. The microstructure and mechanical properties of the weld produced by DP-VPTIG arc were investigated, taking the conventional VPTIG arc as a comparative study. Results show that the microstructure of weld zone by DP-VPTIG arc showed an alternating distribution of fine equiaxed grain band and slightly coarse equiaxed grain band. Compared to VPTIG arc, the grain structure was effectively refined in the weld zone with DP-VPTIG arc, showing a significant reduction of average grain size by 51.2% along transverse section and 61.3% along longitudinal section. The morphology of α-Al+θ-CuAl2 eutectics transformed from continuously distributed netlike shape to separately distributed granular shape, and segregation of Cu solute element was obviously improved. The average microhardness of weld zone was increased by about 8.7% and 5.6% along transverse section and along longitudinal section. The tensile properties of ultimate tensile strength, yield strength and elongation were increased by 6.6%, 10.6% and 20.5%, respectively. The results provide a valuable basis for improving welding efficiency and joint quality through a hybrid pulsed arc.