Active reducing the skin friction drag of vehicles (such as missiles, rockets, aircraft, high-speed train, submarines, etc.) during the navigation process can improve the respond speed effectively, save the energy consumption and increase the endurance time. As lots of active drag reduction methods were proposed for different applications, a review article is urgently needed to guide researchers to choose suitable methods for their pre-research. In this review, the generation mechanisms of skin friction drag under the action of disturbing the turbulent boundary layer are discussed, and the main active drag reduction methods are summarized. According to the actuation modes, we divided the active drag reduction methods into: drag reduction based on wall motion; drag reduction based on volume force control; drag reduction based on wall deformation and drag reduction based on micro vibration generated by piezoelectric actuator. The development status, drag reduction mechanisms, typical structures and drag reduction performance of each active drag reduction method are discussed and summarized in this work, which can provide preliminary research references for those who are engaged in the research of active wall drag reduction.

Moving rapidly and agilely in high-pressure environment is a challenge for existing miniature underwater robots, which limits deeply the ocean exploitation. Here, we present a spherical underwater robot with diameter of 7 cm, and it has merits of miniature structure without bulky and antihydropressure devices, 3-DOFs agile motion, and good adaptability for 25 MPa environment simultaneously. Four designed piezoelectric double-jet actuators are embedded directly in the spherical shell without transmission mechanism, which enables the robot with relatively fast motion speed by compared with the published robots. Six motion modes are implemented by the cooperation of actuators, including rising, hovering, and sinking motions in the vertical direction and straight driving, turning, and in-situ turning motions in the horizontal plane. A robot prototype was fabricated, and its performance was tested. Experimental results indicated that the rising, straight driving, and in-situ turning speed reached up to 95.2 mm/s (1.36 BL/s), 99.6 mm/s (1.42 BL/s), and 5 rad/s, respectively. Besides, the robot could also rapidly turn at 1.3 rad/s with the turning radius of 22.1 mm. The speed deviation within 10% under different water pressures was obtained. The obstacle avoidance was successfully fulfilled by controlling the robot to pass through the narrow gap with width of only 1.5 body length of robot. There excellent characteristics demonstrate agile mobility and give the robot ability to move in the confined area of deep sea.

A novel bearing lubricating device, which is embedded in gyroscope’s bearing system and based on the theory of the piezoelectric micro-jet, was designed for this study. The embedded structure of a bearing lubricating system can make effective use of the limited space of bearing systems without increasing the whole mass of the system. The drop-on-demand (DOD) lubrication can be realized by the piezoelectric micro-jet system to implement the long acting lubrication of the bearing system. A mathematical model of inlet boundary conditions was established to carry on the numerical simulation based on CFD. The motion states of the droplets with different voltage excitations were analyzed via numerical simulations, and the injection performances of the piezoelectric micro-jet lubricating device were tested in accordance with past experiments. The influences of different parameters of voltage excitation on injection performance were obtained, and the methods of adjusting the injection performance to meet different requirements are given according to the analyses of the results. The mathematical model and numerical simulation method were confirmed by comparing the results of past simulations and experiments.

The improvement of sailing speed and endurance of microunderwater vehicle can be achieved by reducing the wall friction drag. We proposed a piezoelectric drag reducer (PDR), which can be embedded into the wall of the underwater vehicle without affecting the original streamline. We discussed the drag reduction characteristics of the PDR with different arrangement angles based on the simulation analyses, and a maximum friction drag reduction rate of 45.3% can be achieved when the arrangement angle between the direction of PDR and the incoming flow is 60°. The influences of key parameters on the turbulence characteristics are obtained with a view to select control parameters for meeting different drag reduction requirements. In order to verify the simulation results, we carried out experimental tests, the experimental data have been analyzed using turbulence statistical method. The relative errors of friction drag reduction results between simulation and experiment are less than 12%, which proved the effectiveness of the proposed arrangement and control parameter selection methods. Then, the feasibility of achieving high-efficiency active drag reduction of microunderwater vehicle, based on the proposed PDR, is verified to be effective according to the test results of total drag experiments.

The driving performance and endurance mileage of microunderwater vehicle can be improved by reducing the wall friction drag. The existing underwater drag reducers can achieve good drag reduction performance, but they are not suitable for microunderwater vehicle due to the large structure size and the high energy consumption. An active drag reducer, which is driven by piezoelectric actuator and can be embedded into the outer wall of microunderwater vehicle hardly changing the streamline structure and structural strength of the wall, is proposed. The active drag reduction on demand and effective energy saving can be realized by combining with the motion monitoring system of the underwater vehicle. The drag reduction mechanism of the proposed drag reducer driven by piezoelectric actuator is discussed. The influences of single parameter and comprehensive effect of parameters on the characteristics of the near-wall turbulent boundary layer are obtained, and the control methods of drag reduction performance for meeting different requirements are proposed. The simulation and experimental results show that the proposed microdrag reducer can effectively reduce the frictional and total drag, and can achieve positive energy saving. The drag reduction rate of 37.82% can be achieved with the excitation amplitude of 150 V and the operating frequency of 1500 Hz. This article can provide an important reference for the research of active drag reduction of microunderwater vehicle.