**7. Conclusions**

This chapter focuses on the hydrodynamic effects of the main parts of a hybrid-driven underwater glider especially in the glide mode, and conducts analysis of the simulation results of the three main hydrodynamic parts by using the computational fluid dynamics (CFD) ways. The fluent Inc.'s (Lebanon, New Hampshire) CFD software FLUENT 6.2 was adopted by this article. The main conclusions are:

It is found that the glide efficiency is most significantly influenced by the chord length while stability of the vehicle is most remarkably affected by the sweep angle, and the location of the wings mainly affects glide stability but has little influence on glide efficiency. When the vehicle glides at about 6°attack angle it has the maximum ratio of lift to drag. The endurance of the hybrid glider with the same configuration as PETREL will decrease by 10%~35% compared with the legacy gliders.

For the rudder we design, the angle of stall is about 34° for the three dimensional rudders and about 20° for the two-dimensional foil section, so the angle of stall of three dimensional rudder is greater than two-dimension foil section. The area of the rudder of PETREL was calculated using the DNV rules;The hinge moments are little when 0.4 *P c* = for the rudder we design no matter how the angle of attack changes.

It was found that overall drag increased by 21 to 26 percent for the model with the propeller shroud compared with the one without a shroud, but with the same structure and size, the shroud's resistance is mainly pressure force. The shroud made the lift-to-drag of the vehicle in glide mode decrease by as much as 20 percent when the angle of attack was 2º. As the angle of attack increased, the shroud's effect was minimized, and the decrease in lift-to-drag ratio ranged down to five percent at an angle of attack of 20º, meaning glide efficiency decreased due to the propeller shroud. Finally, the shroud decreases the stability of the HUG when the angle of attack is lower than the critical angle, but increases it when the angle of attack is higher than the critical angle. The critical angle is between 8º and 10º for velocities lower than one meter per second, and between 10ºand 12ºfor velocities in the range of one to two meters per second.

These findings indicate that the shroud of the underwater glider will increase drag, decrease the glide efficiency, but it improves the stability when the angle of attack is larger than 8º. Therefore, the shroud is not a successful design element for the HUG in glide mode, but it can increase the thrust of the vehicle in propeller mode.

Using CFD to analyze the shroud's hydrodynamic effects shows that the vehicle should only be equipped with this feature for activities requiring operation in propeller mode.

Finally, the velocity field, pressure distribution of the hybrid glider PETREL were analyzed, which make us understand how those main parts effect on the hydrodynamic characteristic of the vehicle.
