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Hydrodynamic Effects of a Shroud Design For a Hybrid-Driven Underwater Glider
CFD Analysis Helps Determine the Impact of a Propeller Shroud on a Hybrid-Driven Underwater Gliderís Hydrodynamic Performance

By Jianguo Wu
Ph.D. Student
Chaoying Chen
Professor
and
Shuxin Wang
Professor
School of Mechanical Engineering
Tianjin University
Tianjin, China



Proper hydrodynamic design is important for the performance of an underwater vehicle. In order to combine the capabilities of autonomous gliders with those of propulsion-driven autonomous underwater vehicles (AUVs), a winged, hybrid-driven underwater glider (HUG), named PETREL, was developed.

This article focuses on the hydrodynamic effects of one aspect of the vehicleís design, the propellerís shroud, by describing the results of two computational fluid dynamics (CFD) simulations that have been performed, one with the shroud and one without. The characteristics of drag, glide efficiency and stability will be discussed, along with suggestions for altering the HUGís design to improve its hydrodynamic performance.

The PETRELís performance was also tested in Fuxian Lake, one of the deepest lakes in China, from April to September of last year, and the vehicle met its design requirements. The tests focused on long-range glide mode, maneuverability in propeller mode and changing from glide mode to propeller mode.
Benefits of a Hybrid Design
The PETREL glider initially evolved from a float that was part of a coupled ocean observation and modeling system. Since Henry Stommel first presented the concept in 1989, gliders such as Spray, Seaglider and Slocum have been successfully developed by several organizations.

As a special type of AUV, underwater gliders have many advantages, such as long endurance, low noise and low energy cost. A glider can periodically change its net buoyancy using a hydraulic pump, and it utilizes the lift from its wings to generate forward motion. Thus, the glider travels in a sawtooth pattern that can be easily modified by shifting the interior mass forward or backward along the path.

Other AUVs are generally driven by a propeller, which makes them easier to control. However, it also means they can only have a relatively short range, on the order of tens or hundreds of kilometers at most, due to the high power required for propulsion. They are restricted by the amount of energy that can be carried on board. Considering the respective advantages of the glider and the propeller-driven AUV, the team decided to develop a winged HUG.

The PETREL is designed mainly for finding and observing hydrothermal vents. It operates in buoyancy-driven mode, like a legacy glider, when its mission is to collect data in a wide area. When more exact measurements of a smaller area and level flight are needed, it operates using the propeller-driven system.


CFD Simulation Analysis
For underwater vehicles, hydrodynamic performance is an important design consideration. As computer technology develops, some accurate simulation analysis has been achieved by CFD software over the last few years, which in the past could only be accomplished though costly experiments.

For the PETREL, the propeller plays a significant role in the vehicleís hydrodynamic performance, so analysis of the hydrodynamic effect of a propeller with a shroud on a winged HUG was performed with Fluent Inc.ís (Lebanon, New Hampshire) CFD software FLUENT 6.2. To analyze the effects of the shroud, two simulations were run, one with the shroud and the other without.


Vehicle Description
The PETRELís design is based on a body shape from Webb Research Corp. (East Falmouth, Massachusetts), which was chosen for its low drag and simplicity.

The vehicle was designed to work in two modes: glide and propeller. The speed in the glide mode is approximately 0.5 meters per second. In propeller mode, its cruise speed is about 1.5 meters per second, and the maximum velocity is two meters per second.

The paramount importance of drag to performance makes the propeller and its shroud critical for the hybrid in glide mode compared with legacy gliders.


Computational Details
Four vehicle velocities of 0.5 meters per second, one meter per second, 1.5 meters per second and two meters per second were investigated. Computations for drag, lift and moment, and flow field were performed for both models over a range of angles of attack using FLUENT 6.2. Error was calculated at less than 9.35 percent, so the results of the CFD calculation can be considered conclusive.

(Middle) Lift-to-drag ratio versus angle of attack (solid lines, model with shroud; dashed lines, model without shroud). (Bottom) The static stability coefficient versus the angle of attack for the two models.
Results and Discussion
Vehicle Drag. The drag on the vehicle can be expressed as Equation 1, where D is the force of drag in newtons, ρ is the density of water in kilograms per cubic meter, v is the velocity of the vehicle in meters per second, A is the reference area in square meters and C D is the drag coefficient (dimensionless). The PETRELís reference area is 0.096 square meters.

The overall drag of the two models was calculated by the CFD program and then fitted using Equation 1. The drag coefficient of the two models was 0.32 and 0.26, respectively, and the average relative error of the overall drag between the CFD calculation and semiempirical formula was 4.7 percent.

The CFD results showed that overall drag increased by 21 to 26 percent with the propeller shroud, so the shroud greatly increased the drag on the vehicle in glide mode (in which the propeller does not rotate). The drag on the body, rudders and wings was mainly viscous force, but for the propeller, shroud and global positioning system antenna pole, pressure was the primary force at 0.5 meters per second without an angle of attack. The propeller and its shroud make up more than 30 percent of the total resistance as a result of the shroudís pressure drag in glide mode.

Shroudís Effect on Glider Efficiency. Specific energy consumption can be defined using classical aerodynamics, as seen in Equation 2.

Underwater gliders will have a higher glide efficiency when Ee (specific energy consumption) is lower, so the lift-to-drag ratio, L/D, is a measure of glide efficiency, where bigger values represent higher glide efficiency.

The lift-to-drag ratio of the model with the shroud is lower than the model without it at different angles of attack: This means the vehicle has lower glide efficiency with the shroud than without it. The shroud modelís lift-to-drag ratio ranged from 20 percent to five percent less than the other model as the angle of attack was varied over a range from 2° to 20°.
Shroudís Effect on Glider Stability. Underwater gliders are usually designed for static stability, and the dimensionless hydrodynamic moment arm α is often used to represent the static stability of the underwater vehicleís motion. (See equations 3 and 4, where l is the overall length of the vehicle in meters, M is the hydrodynamic moment in newton meters, and L is the hydrodynamic lift of the vehicle in newtons.)

The underwater vehicles have longitudinal static instability when α>0 and static stability when α<0. When α=0, the vehicles have critical stability.

Stability decreased when the angle of attack was larger than 8°, but increased slightly for the shroud model when the angle of attack was more than 12°. The glide speed has little effect on the vehicleís stability.


Conclusions
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 shroudís resistance is mainly pressure force.

The shroud made the lift-to-drag ratio of the vehicle in glide mode decrease by as much as 20 percent when the angle of attack was 2°. As the angle of attack increased, the shroudís effect was minimized, and the decrease in lift-to-drag ratio ranged down to five percent at an angle of attack of 20°, 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 8° and 10° for velocities lower than one meter per second, and between 10° and 12° for velocities in the range of one to two meters per second.

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

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


Acknowledgments
This project was supported by Grant 50835006 from the State Key Program of the National Natural Science Foundation of China and by the National Science Foundation for Distinguished Young Scholars of China (50925520).



Jianguo Wu received a B.Eng. degree in mechanical engineering from the Inner Mongolia University of Technology in 2003 and an M.E. degree in mechanical engineering from the Hebei University of Technology, in Tianjin, China, in 2006. He joined the School of Mechanical Engineering of Tianjin University in 2006, and since then he has been involved in the research, development and production of various ocean vehicles.

Chaoying Chen received a B.Eng. degree in computer science from the Harbin Institute of Technology in 1982 and an M.E. degree in computer science from the Free University of Brussels, Belgium, in 1988. He is a professor at the School of Mechanical Engineering of Tianjin University in China.

Shuxin Wang received a B.Eng. degree in mechanical engineering from the Hebei University of Technology in 1987 and M.E. and Ph.D. degrees in mechanical engineering from Tianjin University. He is a professor at the School of Mechanical Engineering of Tianjin University and has been active in various aspects of the research and design of ocean vehicles since 2000.



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