**1. Introduction**

38 Autonomous Underwater Vehicles

Zheng, X. (1992). Layered control of a practical AUV, *Proceedings of the Symposium* 

1992

*on Autonomous Underwater Vehicle Technology*, Washington DC, pp. 142-147,

Autonomous Underwater Vehicle (AUV), Remotely Operated Vehicle (ROV) and Autonomous Underwater glider (AUG) are the main autonomous underwater platforms available currently, which play important role in the marine environmental monitering. The relationships between those three types of vehicles were shown in Figure 1.

Fig. 1. Underwater Vehicles

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 by a hydraulic pump, and utilize the lift from its wings to generate forward motion. The inherent characteristics of a glider can be summarized as buoyancy-driven propulsion, sawtooth pathway, high endurance and slow speed. There exist three legacy gliders named respectively Seaglider, Spray and Slocum [1~6]. In spite that underwater gliders features low level of self noise and high endurance, they also have weaknesses like the lack of maneuverability and the inability to perform a fixed depth or level flight [7].

Driven by a propeller with carried energy source, autonomous underwater vehicles is preprogrammed to carry out an underwater mission without assistance from an operator on the surface. However, they can only cover a relatively short range after each recharge due to the high power consumed for propulsion and generate much more noise than the AUGs because of its propeller and motors [8~10]. The range of AUV's is restricted by the amount of energy carried on board, can was not more than several hundreds kilometers in general [11]. The performances of the underwater vehicle are compared in Figure 2.

Hydrodynamic Characteristics of the Main Parts of a Hybrid-Driven Underwater Glider PETREL 41

number is near <sup>6</sup> 10 for the external flow field, which is called critical Reynolds numbers. It was laminar boundary layer when the <sup>5</sup> Re 5 10 < × , it was seem as turbulent flow while <sup>6</sup> Re 2 10 > × . The Reynolds number of the hybrid underwater glider PETREL at two

steering mode velocity *v* / (m/s) Reynolds number Glider 0.5 1.25×106 AUV 2 5×106

The turbulence model will be adopted because the Reynolds numbers of the PETREL in two steering modes are all above the critical Reynolds numbers. Computations of drag, lift and moment and flow field are performed for both the model over a range of angles of attack by using the commercially available CFD solver FLUENT6.2. The Reynolds averaged Navier– Stokes equation based on SIMPLAC algorithm and the finite volume method were used by our study. In our study RNG k-ε model was adopted and the second-order modified scheme was applied to discrete the control equations to algebra equations. Assuming that the fluids were continuous and incompressible Newtonian fluids. For the incompressible fluid, the

> () ( )*<sup>i</sup> k eff <sup>k</sup> <sup>k</sup> i jj <sup>k</sup> k ku G S*

ρε

 ε

*eff* is effective viscosity, *Gk* is turbulence kinetic energy

2

 ε

<sup>∂</sup> = − <sup>∂</sup> (4)

ρ

 εε

> ε.

(5)

∂∂ ∂ ∂ ⎝ ⎠ (2)

*u C GC RS*

εε

ε

∂∂ ∂ ∂ ⎝ ⎠ (3)

α μ

+ = ⎜ ⎟ +−+

+ = ⎜ ⎟ + − −+

' ' *j*

is respectively the reversible effect Prandtl number for *k* and

3 <sup>ˆ</sup> d 1.72 <sup>d</sup> <sup>ˆ</sup> <sup>ˆ</sup> <sup>1</sup>

ν

⎛ ⎞ ⎜ ⎟ <sup>=</sup> ⎜ ⎟ ⎝ ⎠ − +

turbulence transport variation with the effective Reynolds number can be acquired, which makes the mode having a better ability to deal with low Reynola number and flow near the

 ν

> *C*ν

≈ . Taking the integral of the(5),the exact description of active

ν

*i u*

*x*

*tx x x*

1 2 () ( )*<sup>i</sup> eff <sup>k</sup> i jj*

 αμ

*t x x xk k* ε

*k ij*

ρ

In the *RNG* model,a turbulence viscosity differential equation was generated in the non-

*G uu*

∂∂ ∂∂ ⎛ ⎞

different steering modes is shown in table 1.

*RNG k* −

Here *Sk* and *S*

σ *<sup>k</sup>* andσ ε

here, ˆ

ν

<sup>1</sup> *C* 1.42 ε

ε

Table 1. The Reynolds number at different steering modes

transport equations are [12, 16]:

are source items,

 ρ

∂∂ ∂∂ ⎛ ⎞

μ

2

ρ

*k*

εμ

wall. For the large Reynola number, the equation(5)can be changed into (3-6).

 ε

ρ

ρ ε ρ

ε

 = , <sup>2</sup> *C* 1.68 ε=

dimensional treatment.

μ*eff*

μ = ,*C* 100 ν

induced by mean velocity gradient.

Fig. 2. Performances of three Underwater Vehicles

By combining the advantages of the glider and the propeller-driven AUVs, A hybrid-driven underwater glider PETREL with both buoyancy-driven and propeller-driven systems is developed. Operated in buoyancy-driven mode, the PETREL carries out its mission to collect data in a wide area like a legacy glider. When more exact measurements of a smaller area or level flight are needed, the PETREL will be operated by using the propeller-driven system [5, 7]. This flexible driven glider contributes to have a long range while operated in the buoyancy driven mode like a glider, as well as improve the robust performance to deal with some wicked circumstances by the propeller driven system [7].

Proper hydrodynamic design is important for the improvement of the performance of an underwater vehicle. A bad shape can cause excessive drag, noise, and instability even at low speed. At the initial stage of design, there are two ways to obtain the hydrodynamic data of the underwater vehicle, one is to make model experiment and the other is to use the computational fluid dynamics (CFD). With the development of the computer technology, some accurate simulation analysis of hydrodynamic coefficients have been implemented by using the computational fluid dynamic (CFD) software, instead of by experiments at a much higher cost over the past few years [12-13] . In consideration of the reduced time, lower cost, more flexible and easier optimumal design, the CFD method was used in this article. The fluent Inc.'s (Lebanon,New Hampshire) CFD software FLUENT 6.2 was adopted by this article.

This chapter focuses on the hydrodynamic effects of the main parts of a hybrid-driven underwater glider especially in the glide mode. By analyzing the results of the three main hydrodynamic parts, the wings, the rudders and the propeller, the characteristics of drag, glide efficiency and stability will be discussed, and suggestions for altering the HUG's design to improve its hydrodynamic performance are proposed.
