The perforated shrouds have been proposed to control cylinder flows, while the effects and mechanisms at different Reynolds numbers (Res) remain unclear. Three-dimensional numerical simulations are conducted in this paper to compare the aerodynamic performance of the flow around a smooth cylinder and a shrouded cylinder at Re of 3900 and 1.4×105. The results indicate that the drag of the perforated shrouded cylinder is reduced by 30.8% at the high Re, while increased by 26% at Re of 3900 compared with the smooth cylinder. Differently, the lift oscillations of the cylinder are greatly weakened by 83.3% at the Re of 3900 and 98.5% at the Re of 1.4×105, which implies the wake oscillations are nearly eliminated at the Re of 1.4×105. Further analysis exhibits the near wake region is elongated along the mainstream, with significantly recovered pressure. Especially, the greater pressure loss owing to outer shrouds even leads to the negative drag of the inner cylinder at the high Re. In addition, the incoming flow is broken up by outer shrouds, and different flow patterns appear in the gap. The discernible vortex pairs occur in the gap at the Re of 3900, while as Re increases to 1.4×105, the quantities of small-scale vortex weaken the impact on the inner cylinder. The shear layer characteristics are elucidated by Lamb vector curl and Kelvin–Helmholtz instabilities. The vorticity stretching and tilting in the shear layer of the shrouded cylinder is much weaker at the high Re. Generally, the energy for shear layer instabilities at low frequencies is diminished with the presence of perforated shrouds. However, the energy at higher frequencies is strengthened at the low Re.

The speed increase in high-speed trains is a critical procedure in the promotion of high-speed railway technology. As an indispensable and complex structure of high-speed trains, the pantograph’s aerodynamic drag and noise is a significant limitation in the speed increase process of high-speed trains. In the present study, the hybrid method of large eddy simulation (LES) and Ffowcs Williams-Hawkings (FW-H) acoustic analogy is applied to analyze the aerodynamic and aeroacoustic performances of pantograph installed in different ways, i.e., sinking platform and fairing. The results of simulation show that the application of pantograph fairing can reduce the aerodynamic drag greatly. In addition, compared with the pantographs installed alone on the train roof, the installation of the sinking platform brings about 2 dBA reduction in sound pressure level (SPL). Meanwhile, the utilization of the pantograph fairing mainly decreases the noise in the frequency band above 1000 Hz and the largest SPL reduction is up to 3 dBA among the monitoring points. Further analysis shows that the influence of different diversion strategies on the spectral characteristics actually attenuates the dominant frequency of the panhead. In the horizontal plane, the noise directivity of the pantograph installed with a fairing is similar to the pantograph installed alone on the train roof.

The unconventional physical property variations of supercritical carbon dioxide (S-CO2) near the pseudocritical point present challenges in understanding the heat transfer mechanisms and heat transfer influencing factors when applying S-CO2 to jet impingement cooling. Therefore, a numerical model of CO2 jet impingement cooling in different phase states is established, considering the non-equilibrium phase change of CO2 and actual thermophysical properties while accounting for fluid-solid coupling heat transfer. This study examines the influence of high heat flux and various jet inlet conditions on the cooling performance of S-CO2 jets. In addition, the cooling effectiveness of sub-critical CO2 jet impingement is compared with that of S-CO2 jet impingement. This study found that S-CO2 exhibits significantly superior cooling performance compared to sub-critical CO2, with an increase in the average heat transfer coefficient of approximately 45%–50%. When the jet inlet temperature of S-CO2 is below the pseudo-critical temperature, the heat transfer coefficient is approximately 15%–20% higher than when it exceeds the pseudo-critical temperature. This enhancement is attributed to increased heat transfer capacity due to the large specific heat layer at the chip surface. Furthermore, S-CO2 jet cooling does not experience a critical heat flux (CHF) phenomenon, providing an advantage in high heat flux density cooling surfaces. In addition, it is found that once the jet inlet pressure increases to supercritical levels, the effect of further pressure increases on heat transfer becomes minimal. The results can provide references for the design of cooling systems in high-power electronic devices using S-CO2.