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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.
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Keywords:
- active drag reduction /
- boundary slip /
- Couette flow /
- molecular dynamic
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图 1 (a) Couette流剪切流动及通气系统模型示意图; (b) 模型主要尺寸; (c) 不同边界条件下气体形态图
Figure 1. (a) Schematic diagram of Couette flow shear flow and ventilation system model; (b) main dimensions of the model; (c) gas morphology diagram under different boundary conditions.
图 2 不同固-气耦合强度下, (a)气体形态图, 以及(b), (c)液体原子速度轮廓图, 其中(b)气泡, (c)气膜
Figure 2. (a) Gas morphology and(b), (c) liquid atomic velocity profiles under different solid-gas interaction strengths, where (b) represents bubbles and (c) represents the gas layer.
图 3 不同固-气耦合强度下, (a)边界滑移速度变化图和(b) 剪切应力变化图
Figure 3. Boundary slip velocity (a) and shear stress (b) under different solid-gas interaction strengths.
图 4 不同固-气相互作用强度下的密度分布图 (a)无气体; (b)气泡; (c)气膜
Figure 4. Density distribution under different solid-gas interaction strengths: (a) Without gas; (b) with bubble; (c) with gas layer.
图 5 不同剪切速度下, (a) 气体形态图, (b) 边界滑移速度和剪切应力图, 以及(c) 液体原子密度分布图
Figure 5. (a) Gas morphology, (b) boundary slip velocity and shear stress, and (c) liquid atomic density distribution in the bubbles state Under different shear velocities.
图 6 形成气膜后不同剪切速度下, (a) 气体形态图和(b) 液体原子速度轮廓图(其中半透明的线条为对照的无气体通入时的液体原子速度)
Figure 6. (a) Gas morphology and (b) liquid atomic velocity profile under different shear velocities after the formation of gas layer. In panel (b), the translucent lines represent the liquid atomic velocity without gas injection for comparison.
图 7 不同剪切速度下, (a) 边界滑移速度图、(b) 剪切应力图和(c) 液体原子速度轮廓图(气膜)
Figure 7. (a) Boundary slip velocity, (b)shear stress, and (c) liquid atomic velocity profile in the gas layer state under different shear velocities.
图 8 通入不同气体原子数量时, (a) 气体形态图、(b) 液体原子速度轮廓、(c) 边界滑移速度及剪切应力图和(d) 液体原子密度分布图
Figure 8. (a) Gas morphology, (b) liquid atomic velocity profile, (c) boundary slip velocity and shear stress, and (d) liquid atomic density distribution in the bubbles state under different numbers of injected gas atoms.
图 9 通入不同气体原子数量时, (a)气体形态图、(b)液体原子速度轮廓、(c)边界滑移速度及剪切应力图和(d) 液体原子密度分布图(气膜)
Figure 9. (a) Gas morphology, (b) liquid atomic velocity profile, (c) boundary slip velocity and shear stress, and (d) liquid atomic density distribution in the gas layer state under different numbers of injected gas atoms.
表 1 三相相互作用势能参数
Table 1. Potential energy parameter of three-phase interaction.
两相类型 ε/(kBT)/(kcal·mol–1) σ/Å 固-固 1.2 3.4 液-液 1.2 3.4 气-气 0.4 5.0 固-液 0.7 3.4 固-气 1.0 4.2 液-气 0.4 4.488 
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