# A CFD Study of the Hydrodynamic Characteristics of an Autonomous Underwater Helicopter
## 1 Introduction  
In recent years, the study of motion stability in autonomous underwater vehicles (AUVs) with a disk-shaped hull has gained increasing importance within deep-sea mobile observation networks [1; 2; 3]. Motion stability analysis is crucial for improving the performance of autonomous underwater helicopters in complex underwater tasks, such as pipeline maintenance, mobile observation networks, resource exploration, and environmental sampling for hydrological research [4; 5; 6]. The challenging task of AUV control depends on the mass, payload, buoyancy, and working region. Notably, successful tests of disk-shaped AUVs with a new method using passive inverted ultra-short baseline (piUSBL) positioning in the South China Sea have marked advancements in the underwater navigation of AUV technology [7]. To ensure data collection, maintaining stability in horizontal motion and precise hovering in vertical motion is essential. Therefore, a new AUV model, the autonomous underwater helicopter (AUH), is proposed. During the operation of the AUH, it will be influenced by multiple factors: wave dynamics, water ingress, thruster performance,buoyancy mechanisms, and others. Furthermore, the growing number of pipelines currently transport oil, natural gas, and fiber optic cables around the world. These systems encounter difficulties from the coastal environment, including seawater corrosion and unstable geological formations, which may result in damage, oil leaks, and disruptions to the global network. [PERSON] et al. [8] compared a multi-beam echo sounder (MBES) with forward-looking sonar (FLS) for the automated inspection of undersea pipelines. The Otsu algorithm is selected to reduce the recalculation time while preserving excellent computational efficiency. [PERSON] et al. [9] reviewed numerous studies demonstrating that various factors influence pipeline monitoring and oil leakage detection in marine environments. Consequently, each approach is only appropriate for specific qualities and localities. Calculating energy is vital for AUVs to acquire more information during operational periods. Three critical factors include motion parameters, hydrodynamic coefficients, and control parameters. The aforementioned components denote the first-order sensitivity indices for energy consumption as proposed by [PERSON] et al. [10]. Two methodologies employed to assess energy management strategies are fuzzy logic and finite-state machines. [11]. [PERSON] et al. [12] investigated the relationship between the Magnus force and controlling AUHs to characterize the disturbance area. In the literature about the seakeeping study of underwater vehicles, [PERSON] et al. [13] presented research on the effects of forces experienced during the water-entering procedure with actual AUVs. The axial loads of the AUV have the greatest value when the AUV is launched and then become stable after a rapid decline. The radial force depends on the entry angle. The effects coming from the seabed are also carefully examined, focusing on understanding how these impact the overall dynamics and performance of underwater vehicles [14; 15]. [PERSON] et al. [16] developed a hybrid dynamic mesh method to simulate the AUH's hydrodynamic behavior. The hydrodynamic interference between the AUH and a cone-shaped dock depend on various docking parameters and ocean conditions. Reducing drag and decreasing power consumption is a critical problem in improving the operation of AUHs. So, [PERSON] et al. [17] used just three vertical thrusters instead of four thrusters following the previous methods. In particular engineering applications, the drag and lift forces are important for operating AUHs. Scientists worldwide continuously find solutions to this problem, so the geometry of the AUH is necessary to produce a smoother and more accurate isogeometric analysis (IGA). Computer-aided design (CAD) methodologies and software tools create the irregular profile of complex shapes. [PERSON] et al.