# A CFD Study of the Hydrodynamic Characteristics of an Autonomous Underwater Helicopter
## 3 Results and Discussion
Figure 11 shows the fluid dynamics surrounding the AUH at a velocity of U = 0.8 m/s, examining several angles of attack (\(-15^{\circ}\), \(-10^{\circ}\), \(-5^{\circ}\), \(0^{\circ}\), \(5^{\circ}\), \(10^{\circ}\), \(15^{\circ}\)) in this analysis. The velocity contours around the AUH's body indicate favorable hydrodynamic performance. The current results indicate that the velocity distribution is significantly influenced by the angle of attack. The fluid traverses the surface of AUH, revealing the emergence of a wake zone at the rear of AUH. In the wake region, there is a combination of high pressure and low pressure, so it is easy to promote vortices. The larger the AOA, the larger the vortex area behind the vehicle. Figure 11: _Cont._  
Figure 10: The pressure distribution in the xoz plane (top) of the AUH with different AOAs. Figure 12 shows the relationship between drag and lift forces with angles of attack and varying speeds. The graphic indicates that at an angle of attack of 0\({}^{\circ}\), the drag forces are minimized, which elucidates that the fluid flowing over the AUH does not generate a vortex. The drag resistance will gradually increase: as the AOA increases, the higher the speed and the higher the resistance. The lift force and AOA increase at a linear rate, which makes the AUH unbalanced and makes it difficult to adjust the buoyancy system. Additionally, the high speed also contributes to a greater lift force. Most notably, when analyzing the AUH's operation at angles of attack (AOAs) of \(-20\), \(-15\), \(15\), and \(20\) degrees, the simulation results reveal substantial variations in both drag and lift forces. The drag force increases by two to four times compared to when the AOA = 0\({}^{\circ}\). Similarly, the lift force undergoes significant changes, increasing from single-digit values (AOA = 0\({}^{\circ}\)) to several hundred. This gives us a suggestion that in controlling the AUH, we should operate the AUH with an angle of attack between 10\({}^{\circ}\) and \(-10^{\circ}\). In this range, drag and lift are small and there is no need to use high-power engines. Figure 11: The velocity contours surrounding the AUH with a velocity of U = 0.8 m/s at various angles of attack. This study examines the impact of AUH length on hydrodynamic coefficients. Figure 13 illustrates the modification of the AUH's profile. The height of the AUH re  
Figure 12: The lift and drag forces in relation to the angle of attack at various velocities of the AUH. mains at 400 mm. Conversely, the length varies between 600 mm and 1200 mm. At a velocity of U = 0.8 m/s, Figure 14 illustrates the fluid flow distribution surrounding the disk-shaped hull with varying length/height (L/H) ratios. Across all cases, high-pressure zones are concentrated at the bow of the AUH, whereas low-pressure zones are developed at the rear. The simulation results present increasing L/H ratios, and the high-pressure regions tend to decline. Figure 15 illustrates the hydrodynamic coefficient at various L/H ratios. The numerical results indicate that an increased (L/H) ratio correlates with elevated drag and lift coefficients. This is because in the horizontal direction (\(X\)-axis), the wet cross-sectional area of the AUH will increase as the L/H ratio increases. According to the simulation results, the black solid line (C\({}_{\rm d}\)) has a speed greater than the raised speed of the red solid line (C\({}_{\rm l}\)). Therefore, the lift coefficient/drag coefficient ratio decreases as the L/H ratio becomes larger. However, drag and lift should be restricted to ensure the AUH's ability to operate. By analyzing the dimensionless hydrodynamic coefficient, we propose that the origin AUH model (Figure 13c) approaches suitable hydrodynamic characteristics and enough space to contain the necessary equipment. Figure 14: The pressure contours around the AUH with different length/height. Figure 13: The modified models of the AUH (**a**) 600 mm (**b**) 800 mm (**c**) 1000 mm and (**d**) 1200 mm.