: Over the last decades, the increased desire to use autonomous underwater vehicles (AUVs) for commercial and military applications has led to a great deal of research in this field, for the purpose of not risking human life in dangerous operations. Extensive use of autonomous underwater vehicles (AUV) in oceanographic applications necessitates investigation into the hydrodynamic forces acting over an AUV hull form, operating under submerge condition. When a completely submerged body moves under water, a resisting force, including a frictional and a pressure drag (form drag) force applies into the body of the vehicle. When an AUV moves near free surface, it creates a wave at the free surface which changes its hydrodynamic coefficient, including the drag and lift coefficients of the AUV, comparing with those when the AUV moves in an infinite medium. Indeed, drag and lift coefficients near the surface are influenced by the wave making resistance created near the free surface, which is due to changing the pressure distribution on the body of the vehicle. The aim of the current investigation is to study the effect of fr ee surface on the drag and lift coefficients of an autonomous underwater vehicle. The AUV designed and constructed in subsea research and development center of Isfahan University of Technology used for this numerical study. The two phase flow Reynolds Averaged Navier–Stokes solver (ANSYS CFX) is used to simulate the motion of the axisymmetric geometry of the AUV. The current numerical investigations were carried out to calculate the hydrodynamic coefficients for a velocity range of 0.5 to 3 m/s (corresponding to a Reynolds number range of 97000 to 579000) and a submergence depth range of 0.87D to 4.83D, where D refers to AUV hull diameter. In addition the drag coefficients are calculated for angle of attacks varied from -12 to +12 degree, in steps of 3 degrees. Validation of numerical results is performed by comparing the experimental results obtained in subsea R D towing tank of IUT. The obtained numerical results showed that, at all AUV speeds investigated, as the vehicle approaches the free surface the drag coefficient increases. The results showed that the pressure drag varies sharply near the free surface, while the friction drag is almost independent of the submergence depth and remains almost constant. This can be explained by the fact that pressure around the body is strongly influenced by waves formed by moving body. Also, the numerical results showed that the depth at which the free surface effect becomes negligible depends on the speed of the AUV. For example at a velocity of 1.5 m/s (Re= 280000), free surface effects on hydrodynamics coefficient eliminate in submergence depth of 2.6D ,while in velocity of 3 m/s free surface effects eliminate in submergence depth of 4.8D. It was also shown that in the absence of free surface, the drag coefficient decreases constantly with velocity, while, when the AUV moves near the free surface at some velocity in which the surface wave length becomes equal to the AUV length, a sudden increase in the drag coefficient can be seen. Key worlds: Free surface effect, hydrodynamic coefficient, underwater vehicle, CFD
