# Optimal Attitude Determination for the CR200 Underwater Walking Robot
## 1 Introduction
Some studies specifically focus on biomimetic underwater robots, applying CFD to predict hydrodynamic forces. [PERSON] et al. [9] utilized CFD techniques to analyze an inviscid flow field and measure the drag force acting on the robot. [PERSON] et al. [10] conducted simulations to analyze the fluid forces on the robot and derived a swimming mode using two bio-inspired undulating fins. [PERSON] et al. [11] performed a conceptual design of a bio-inspired underwater robot through CFD simulation, analyzing the swimming mechanism based on hydrodynamic forces in an infinite fluid field. [PERSON] et al. [12] provided a detailed review of the undulating propulsion mechanism of bio-mimetic underwater robots, highlighting the efficiency and maneuverability advantages in harsh underwater environments. One common interest in the design of underwater robots is achieving an optimal design for resistance reduction. CFD simulation is used to determine drag-reducing shapes [13; 14; 15; 16; 17], suction-induced cavitation reduction [18], and surface structures aimed specifically at reducing skin friction [19]. A more comprehensive survey of CFD-driven underwater vehicle development can be found in the review articles [20; 21]. In addition, CFD studies that consider environmental effects have also been conducted. [PERSON] et al. [22] investigated wall-induced hydrodynamic forces on a nuclear reaction pool to enhance control precision in confined spaces. [PERSON] et al. [23] conducted research on underwater vehicles in contact with the seabed, including studies that predict the behavior of underwater drilling platforms by considering interactions between seabed mud and the platform. Notably, among the reported studies, CFD simulations involving underwater walking robots are rare, and the hydrodynamic performance related to the seafloor effect on underwater robots is not well understood. In the case of the CR200 underwater walking robot, determining the posture that maximizes resistance against hydrodynamic drag is essential for remaining anchored to the seabed in strong currents. The reactive force from the seabed along the flow direction essentially acts as friction, which is directly proportional to the downforce generated by the current around the robot. The main contributions of this study can be asserted from the following three perspectives: First, it defines the factors that determine the optimal posture for generating downforce essential for maintaining the underwater robot's stability. Second, the optimal posture is determined through a series of CFD analyses. Lastly, it explains the hydrodynamic principles underlying this phenomenon. To achieve these goals, a series of CFD analyses were conducted with different robot body postures to identify the posture that maximizes the downforce acting perpendicularly to the seabed, a critical force preventing the robot from being swept away by currents. The computational domain includes the rigid seafloor rather than an unbounded flow domain. To identify the optimal attitude of the robot's main body, a reasonable simplification is proposed by neglecting the side legs. Since most of the blockage effect is caused by the robot's main body rather than its legs, the analysis focuses on the hydrodynamic forces acting on the body alone, excluding the legs from this investigation into optimal posture. The robot's posture was defined using two variables: the angle of attack formed by the flow parallel to the body and the distance between the bottom of the robot's body and the seabed (seafloor clearance). A series of flow analyses combining these two variables were performed, and the methodology for identifying the angle and distance that generates the maximum downforce is presented. This paper is organized as follows. In Section 2, we describe the specific specifications of the CR200 underwater robot. Details of the CFD simulation setup, including the computational domain, mesh generation, and boundary conditions, are described.