During a lunar module landing, the gas extended from the engine nozzle impinges on the lunar surface. A recirculation bubble could form beneath the surface shock wave, which alters the flow pattern close to the surface, thus affecting the surface soil transportation process. This study conducted numerical simulations to investigate the flow characteristics, formation mechanism, and effects of this recirculation bubble on surface soil erosion using direct simulation Monte Carlo method or solving the Navier–Stokes equations. It is found that during the descent, the recirculation zone under the surface shock wave first disappears and then reappears. The shock wave systems in the plume at different lander heights is analyzed, revealing that the formation of the recirculation bubble can be attributed to the total pressure loss due to gas crossing different wave structures. When the lander descends to a close proximity to the lunar surface, the recirculation bubble can even expand into the nozzle. Furthermore, this study investigated the effects of the recirculation bubble on lunar soil transportation by a gas–solid two-phase solver. It is shown that the recirculation bubble at low landing altitude will entrain the lunar dusts and result in a high ejection angle of the latter, thus aggravating the obstruction of surface observation for the safe landing.

The wake-vortex/boundary-layer interaction is significant for both fundamental and practical aspects of fluid mechanics. The wake vortex triggered the well-known single-secondary vortex over a canonical circular-cylinder/flat-plate configuration. However, recent investigations reported wake-triggered double-secondary vortices over a multi-element airfoil or a simplified circular-cylinder/airfoil configuration. Motivated by this contrast, this paper revisits this topic with a circular-cylinder/flat-plate configuration and finally observes the double-secondary vortices in the boundary layer. The disturbances related to the shedding frequency of the wake vortex and its second harmonic are found to penetrate the boundary layer and be further amplified to form the double-secondary vortices.

Aquatic organisms have evolved exceptional propulsion and even transoceanic migrating capabilities, surpassing artificial vessels significantly in maneuverability and efficiency. Understanding the hydrodynamic mechanisms of aquatic organisms is crucial for developing advanced biomimetic underwater propulsion vehicles. Underwater tetrapods such as sea turtles use fins or flippers for propulsion, which exhibit three rotational degrees of freedom, including flapping, sweeping, and pitching motions. Unlike previous studies that often simplify motion kinematics, this study employs a specially designed experimental device to mimic sea turtle fins’ motion and explore the impact of pitching amplitude, asymmetric pitching kinematics, and pausing time on lift and thrust generation. Force transducers and particle image velocimetry techniques are used to examine the hydrodynamic forces and flow field, respectively. It is found that boosting the fin’s pitching amplitude enhances both its lift and thrust efficiency to a certain extent, with a more pronounced effect on thrust performance. Surprisingly, the asymmetrical nature of the pitching angle’s pausing time within one flapping cycle significantly influences the lift and thrust characteristics during sea turtle swimming; extending the pausing time during the forward and upward flapping process improves lift efficiency; and prolonging the pausing time during the downward flapping process enhances thrust efficiency. Furthermore, the mechanism for high lift and thrust efficiency is revealed by examining the vortices shed from the fin during different motion kinematics. This research contributes to a more comprehensive understanding of the fin’s hydrodynamic characteristic, providing insights that can guide the design of more efficient biomimetic underwater propulsion systems.

The kinematics of vortical structures behind heaving plates with different flexibility has been investigated using particle image velocimetry. The rectangular plates with different thicknesses undergo a trapezoidal velocity profile (acceleration–steady–deceleration). The Reynolds number Rec based on the chord length and the steady velocity is 5330. The formation and evolution processes of the wake vortices are investigated, with the focus on the comparison between the rigid wing and flexible wings. Wake vortices for the rigid wing initiated near the plate edges, while for flexible wings vortices initiated near the quarter-chord from the edges and slid toward the edges as the plates deform and move forward. During the first half cycle (moving forward), both the vortex trajectories and circulation indicate a global two-stage growth of the starting vortices for all the plates, corresponding to the acceleration and steady motion stages, respectively. The flexible plates have larger wake width and attain higher peak circulation initially; however, the circulation for the rigid plate achieves higher peak value during the steady stage. During the second half cycle (moving backward), the vortex trajectories present a two-stage process; however, the circulation is a single growth and decay process. Besides, detailed study reveals that during the acceleration, the circulation grows rapidly and there is a small decay at the end of the acceleration, thus forming the local peak for the first stage. The vortex trajectories for the flexible plates show greater range transversely, as well as the motion reversal in the streamwise direction.

The aim of this study is to detect and visualise the influence of span-wise coherence on propagating sound waves emanating from a flow around circular cylinders with span-wise variations of the local radius. Synchronous particle image velocimetry (PIV) and microphone measurements are performed in a circular wind tunnel with a nozzle size of 0.4 m×0.4 m at a maximum flow speed of U∞ = 43m s−1 . The test section is surrounded by a full anechoic chamber of approximately 9 m×9 m×5 m.