Nowadays autonomous underwater vehicles (AUVs) are considered as one of the most important underwater vehicles in marine sciences due to their capability for measuring a specified path and doing operations like sea topography, identifying depth of the seas and so on. In order to correct perception and proper prediction of AUVs maneuvers, it is necessary to study its hydrodynamic forces during accomplishment of operation in depth of water. The main purpose of this work is to determine the drag force of a new AUV and its length variation on the drag forces. This study is performed via experimental and numerical methods. The experimental tests of this study have accomplished in subsea research and development center of Isfahan University of Technology in a towing tank. Experimental tests include towing without angle of attack in two depth of 40 and 60 cm and in velocities of 0.7, 1, 1.5, 2, 2.5 and 3m/s. The towing with angle of attacks of 3, 6, 9 and 12 degree in 60cm depth and similar velocity limits with without angle of attack tests are also performed. In addition, the towing tests with control surfaces are accomplished in 40cm and 60cm depth. In numerical part, all of the cases that are studied in experimental tests except for tests with control surfaces are simulated and their results have compared with experimental ones. In addition, the length variation on AUVs drag force is studied numerically for five different lengths to diameter ratio. The results of this study for new AUV indicate that the experimental and numerical results are suitable for this study. A comparison between two cases, with and without control surfaces, it is observed that the AUV's drag force increases about 14% and 18% for 40cm and 60cm depth, respectively. It is found that an increase in the AUV's length to diameter ratio leads to an increase in the drag force. Meanwhile as the further increase in the length to diameter ratio, the amount of drag force rate decreases either with angle of attack or without angle of attack. Based on this study, the amount of AUV's drag force in a case with length to diameter of 4 increases about 35% in average for all velocities considered. Key words : AUV; drag coefficient; length variation; numerical method, experimental method; towing tank
