A fluid simulation was conducted on the flow states of water and drainage gas from the drainage cover in underwater local dry laser welding. The results of the simulation of gas volume fraction are obtained. Drainage cover was developed based on the drainage cover model. Experiments of drainage in this machined drainage cover was carried out, and were compared with the numerical simulation. The results show high similarity, which verifies the feasibility. Then, innovative structure featuring an uneven buffer around the exit of drainage cover are proposed to improve the performance of drainage. A group of 3D models of fluid domain related to differently characterized drainage covers was designed and the flow states were simulated. They are N-model with a narrow outlet, S-model with a smooth buffer added additionally based on N-model, U-model with an uneven buffer compared with S-model, and W-model with a wide outlet and the diameter of outlet twice than N-model. The gas volume fracture distributions were recorded and in comparison with each other. The results indicate that U-model with the uneven buffer realized the deepest drainage depth.

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