Abstract : This paper proposes a unified perceptual–physical underwater motion simulation framework that simultaneously supports immersive user interaction and large-scale crowd behaviors. In underwater environments, various fluid forces, including drag, buoyancy, gravity, and Magnus effects induced by rotation, act on the human body in a coupled manner. However, conventional virtual reality systems have limitations in realistically conveying such forces due to restricted hardware input and the absence of force-feedback devices. To address this problem, we model underwater forces not as direct force feedback but as the temporal evolution of velocity states, and present a unified motion generation framework that can be applied consistently to both user-controlled avatars and non-player characters (NPCs), as well as large-scale crowd simulations. The proposed method explicitly computes buoyancy, drag, and Magnus effects at the level of joints and body segments to generate physically plausible full-body motion, while biomechanically valid joint range constraints prevent unrealistic postures caused by external forces. For user interaction, a controllable perceptual coupling between the simulated physical state and the actual user input intentionally introduces visual discrepancies, allowing users to intuitively perceive underwater resistance without additional haptic devices. The same physical model is further used to naturally produce diverse underwater behaviors in large-scale crowds through variations in initial conditions and body parameters. A real-time implementation in Unity validates the efficiency and scalability of the proposed framework, and experimental results demonstrate stable and consistent underwater motion representation in both immersive interaction scenarios and large-scale crowd simulations. The proposed approach provides a practical solution for integrating physical realism and perceptual experience in underwater environments without requiring additional hardware, and shows potential for applications in virtual reality training, simulation, games, and digital twin environments.

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