The dimple occurs by sudden pressure inversion at the droplet’s bottom interface when a droplet collides with the same liquid-phase or different solid-phase. The air film entrapped inside the dimple is a critical factor affecting the sequential dynamics after coalescence and causing defects like the pinhole. Meanwhile, in the coalescence dynamics of an electrified droplet, the droplet’s bottom interfaces change to a conical shape, and droplet contact the substrate directly without dimple formation. In this work, the mechanism for the dimple’s suppression (interfacial change to conical shape) was studied investigating the effect of electric pressure. The electric stress acting on a droplet interface shows the nonlinear electric pressure adding to the uniform droplet pressure. This electric stress locally deforms the droplet’s bottom interface to a conical shape and consequentially enables it to overcome the air pressure beneath the droplet. The electric pressure, calculated from numerical tracking for interface and electrostatic simulation, was at least ${10}^8$ times bigger than the air pressure at the center of the coalescence. This work helps toward understanding the effect of electric stress on droplet coalescence and in the optimization of conditions in solution-based techniques like printing and coating.

In this study, a particle image velocimetry (PIV) system was used in a large circulating water tank to investigate the wake of a horizontal-axis tidal turbine model, focusing on minor blockage effects and scale influence. A wake map of the turbine was constructed based on PIV measurements, using velocity deficit, turbulence intensity (TI), and turbulence kinetic energy (TKE) as key indicators. The results showed that TKE developed later than TI, forming a plateau-like shape. This plateau was considered the decay region, with the transition and far-wake regions located before and after it, respectively. Additionally, the power law exponent of TI decreased from −0.731 in the decay region to −0.765 in the far wake, indicating a steeper decay further downstream. Overall, the wake map of the tidal stream turbine model exhibited similarities to that of a previously reported wind turbine model.

This paper conducts a comprehensive analysis of undulating and oscillatory movements in fish, utilizing numerical simulations to explore correlations among fin thrust and swimming speed. The study distinguishes itself through a unique approach by employing kinematic equations of motion control, specifically in oscillation and undulation, for computational fluid dynamics. Despite increasing energy loss with undulation, the study reveals a reduction in power demand with oscillation, underscoring its effectiveness in achieving desired speeds. The dynamics of undulating fins in aquatic and aerial locomotion remain insufficiently understood. The trade-off between more energy-consuming but highly propulsive movements or simpler and faster movements requires sophisticated design techniques to reduce volume. The geometry, developed using Rhino 6 software, incorporates precise fluid resistance calculations conducted with Ansys Fluent 19. Spanning flow velocities from 1 to 4 m/s were used for the simulation condition. Critical factors such as flexibility, viscosity, and shape change were meticulously examined for their impact on efficiency enhancement.

Understanding wake interactions between multiple tidal stream turbines is essential for optimizing the performance and layout of tidal energy farms. This study investigates the hydrodynamic behavior of two horizontal-axis tidal turbines arranged in tandem under simplified inflow conditions, where the incoming flow was dominated by the streamwise velocity component without imposed external disturbances. Wake measurements were conducted in a large circulating water tunnel using a mobile, submerged particle image velocimetry (PIV) system capable of long-range, high-resolution measurements. Performance tests showed that the downstream turbine experienced a decrease of approximately 9% in maximum power coefficient compared to the upstream turbine due to reduced inflow velocity and increased turbulence generated by the upstream wake. Phase-averaged PIV results revealed the detailed evolution of velocity deficit, turbulence intensity, turbulent kinetic energy, and tip vortex structures. The tip vortices shed from the upstream turbine persisted over a long downstream distance, remaining coherent up to 10D and merging with those generated by the downstream turbine. These merged vortex structures produced elevated turbulence and complex flow patterns that significantly influenced the downstream turbine’s operating conditions. The results provide experimentally validated insight into turbine-to-turbine wake interactions and highlight the need for high-fidelity wake data to support array optimization and numerical model development for tidal stream turbine array.

In this study, a previously developed fully passive hydrokinetic turbine with two flapping foils was used to directly drive a reciprocating pump, and the performance of this system was investigated at an offshore site in Republic of Korea. The fully passive operation of the turbine worked effectively due to its coupling mechanism, and pumping was successfully carried out during flood tides when the pumping height was consistently maintained using a water level gauge and winch system. Pumping occurred at a height of approximately 9 m when the flow velocity reached 1.8 m/s, at which point the corresponding Reynolds number exceeded one million. In one case where a high pumping flow rate was achieved during offshore trials conducted over a period of time, the pumping efficiency reached up to 34% when the reduced frequency of the turbine was 0.126, falling within the known optimum range. The pump driven by the flapping-foil hydrokinetic turbine, which can be positioned near the shore or in shallow water, could provide a viable solution for off-grid communities needing to pump seawater or generate hydroelectric power.

This paper proposes a velocity-type fuzzy proportional–integral–derivative (PID) control system with integral line-of-sight (ILOS) guidance to precisely follow the path of a biomimetic underwater robot equipped with a caudal fin propulsion mechanism, as well as a fuzzy logic algorithm for efficient path generation. This study involves two distinct key parts. First, a precise path-following control system that incorporates the dorsal fin motion characteristics is proposed. The proposed velocity-type fuzzy PID controller is designed to achieve superior transient response and tracking precision compared to conventional PD controllers. In addition, an ILOS guidance algorithm is integrated to provide compensation for cross-track errors caused by ocean currents and external disturbances. Second, a fuzzy theory–based underwater path generation algorithm is proposed. Through a comparative analysis with the conventional A^* and deep Q-network algorithms, the superiority of the proposed method is demonstrated in terms of computational efficiency, path smoothness, and obstacle-handling capability. The findings of this study demonstrate that precise path-following for a dorsal fin–based biomimetic robot is achievable even in environments prone to disturbances, while validating the performance of fuzzy theory–based path generation.