Zhen Liu

Oceans, which cover more than 70% of Earth’s surface, are home to vast biological and mineral resources. Deep-sea exploration encounters significant challenges due to harsh environmental factors, including low temperatures, high pressure, and complex hydrodynamic forces. These constraints have led to the widespread use of underwater robots as essential tools for deep-sea resource exploration and exploitation. Conventional underwater robots, whether rigid with fixed stiffness or fully flexible, fail to achieve the propulsion efficiency observed in biological fish. To overcome this limitation, researchers have developed adjustable stiffness mechanisms for robotic fish designs. This innovation strikes a balance between structural rigidity for stability and flexible adaptability to dynamic environments. By dynamically adjusting localized stiffness, these bio-inspired robots can alter their mechanical properties in real time. This capability improves propulsion efficiency, energy utilization, and resilience to external disturbances during operation. This paper begins by reviewing the evolution of underwater robots, from fixed-stiffness systems to adjustable-stiffness designs. Next, existing methods for stiffness adjustment are categorized into two approaches: offline component replacement and online real-time adaptation. The principles, implementation strategies, and comparative advantages of each approach are then analyzed. Finally, we identify the current challenges in adjustable-stiffness underwater robotics and propose future directions, such as advancements in intelligent sensing, autonomous stiffness adaptation, and enhanced performance in extreme environments.

Friction resistance is the primary factor influencing the energy consumption and speed of underwater vehicles. Active air layer drag reduction is an active boundary layer control technique that reduces wall friction drag by injecting gas into the solid-liquid boundary layer. Compared with other drag reduction methods, which are often difficult to scale due to high costs and potential environmental concerns, this technology utilizes a simple auxiliary device. By employing inexpensive and environmentally friendly compressed air or combustion exhaust gases, it effectively lowers fluid resistance. Therefore, active drag reduction technology plays a crucial role in minimizing friction and enhancing overall performance. In this study, molecular dynamics simulations are used to construct a Couette flow shear model, with gas injected at the boundaries of a nanochannel. The flow characteristics and boundary drag reduction of Couette flow in a nanochannel is investigated in this work. The influence of gas injection on these characteristics is examined, along with the effects of surface wettability, shear velocity, and gas injection rate on boundary slip velocity and drag reduction. The results indicate that the gas adsorption on the solid surface in the form of discrete bubbles hinders liquid flowing and slipping near the wall, leading to the increase of drag. However, increasing surface hydrophobicity, shear rate, and gas injection rate facilitates the transverse spreading of bubbles, reduces flow obstruction, and enhances slip. Additionally, these factors promote the formation of a continuous gas layer from discrete bubbles, further improving drag reduction. Once the gas layer forms, shear stress decreases significantly, and slip velocity varies with surface wettability, shear velocity, and gas injection rate. These findings provide a theoretical foundation for developing the active gas layer drag reduction technology and optimizing the surface structures in ships and underwater vehicles.

In this work, the icebreaking performance of the cone structure was investigated using a new numerical model called the common-node DEM developed within LS DYNA. The icebreaking characteristics of a typical conical jacket platform in the Bohai Sea focusing on the JZ20-2NW single-pile-leg platform was studied and the ice load characteristics of the cone structure and the dynamic response of the jacket platform under various ice conditions was investigated. The findings indicate that ice thickness significantly impacts the icebreaking mechanism of the cone structure. Specifically, both the peak ice load and the peak acceleration of ice-induced vibrations are proportional to the square of the ice thickness. Additionally, the upward trend in positive vibration displacement of the jacket platform becomes more pronounced with increasing ice thickness. While both the acceleration and displacement caused by ice-induced vibrations on the jacket increase with rising ice velocity, this effect is less significant compared to the influence of ice thickness. Importantly, the ice load remains below the yield strength of the conical shell plate, demonstrating that traditional conical shell plate structures possess a margin of strength redundancy.

Bubble drag reduction technology is of great significance in improving the propulsion efficiency of underwater vehicle and reducing the comprehensive energy consumption during navigation. Bubble drag reduction is a highly effective method of reducing the frictional resistance encountered by large ships and underwater vehicles during navigation. It exhibits excellent stability in drag reduction, and has advantages such as environmental friendliness, adaptability to various flow environments, and suitability for all underwater components of ships. Therefore, it is greatly significant to conduct in-depth research on bubble drag reduction and its underlying mechanism. In this work, the flow characteristics and the boundary bubble drag reduction mechanism of gas-liquid Couette flow in parallel wall nanochannels are studied by molecular dynamics method, and the influences of surface wettability, wall roughness, and gas concentration on boundary slip velocity and bubble drag reduction effect are analyzed. The results indicate that the bubble drag reduction effect is enhanced with the increase of boundary slip velocity. In the gas-liquid two-phase flow region, with the increase of shear velocity, the lateral deformation of boundary adsorbed bubble and boundary slip velocity increase, thus enhancing the bubble drag reduction effect. The increase of solid-gas interaction strength and gas concentration can lead to the enrichment of gas atoms near the wall, improve the bubble spreading characteristics on the wall, and thus increase the slip velocity of the solid-liquid interface. The wall roughness can change the spreading characteristics of bubble, affect the boundary slip velocity, and then change the drag reduction effect of the fluid-solid interface. As the rib height increases, gas atoms accumulate in the grooves between ribs and the adsorption quantity of gas atoms on the upper surface of the rib decreases, which leads to the decrease of the boundary slip velocity of the solid-liquid interface and ultimately reduces the drag reduction effect. The research results will provide important theoretical guidance for implementing the boundary drag reduction technology in large ships and underwater vehicles.

In this study, the reverse osmosis (RO) of a salt solution was investigated using a molecular dynamics method to explore the performance of a multilayer porous graphene membrane. The effects of the salt solution concentration, pressure, layer separation and pore offset on the RO performance of the membrane were investigated and the influences of the number of layers and the gradient structure were determined. The results show that as the salt solution concentration increases, the energy barrier of the water molecules passing through the bilayer porous graphene membranes changes slightly, indicating that the effect of the water flux on the membrane can be ignored. The salt rejection performance of the membrane improves with an increase in the concentration of the salt solution. When the pressure is increased, the energy barrier decreases, the water flux increases and the salt rejection decreases. When the layer separation of the bilayer porous graphene membrane is the same as the equilibrium spacing of the graphene membrane, the energy barrier is the lowest and the membrane water flux is the largest. The energy barrier of the bilayer porous graphene membrane increases with increasing layer separation, resulting in a decrease in the water flux of the membrane. The salt rejection increases with increasing layer separation. The water flux of the membrane decreases as the energy barrier increases with increasing pore offset and the salt rejection increases. The energy barrier effect is more pronounced for a larger number of graphene layers and the water flux of the membrane decreases because it is more difficult for the water molecules to pass through the porous graphene membrane. However, the salt rejection performance improves with the increase in the number of layers. The gradient pore structure enhances the energy barrier effect of the water molecules permeating through the membrane and the water flux of the membrane decreases. The salt rejection performance is improved by the gradient pore structure. The research results provide theoretical guidance for research on the RO performance of porous graphene membranes and the design of porous graphene membranes.