**1. Introduction**

UVs can be classified in a variety of ways. Submarines are manned underwater vehicles. Their operating speed varies from 8 to 20 m/s and depth of operation varies from 200 to 600 m. The length of submarines in existence varies from 57.3 m (Dolphin 1 class) to 175 m (Typhoon class). Submarines are used in underwater warfare, covert operations, and coastal defense. AUVs are underwater robots with operational speeds varying from 0.5 to 2 m/s, and a depth of operation varying from 200 to 6000 m. The length of AUVs varies from 1.42 m (AUV Cormoran) to 10 m (AUV Urashima). They mostly have torpedo shaped hull forms. AUVs can be employed to collect data samples of physical characteristics of water such as temperature, salinity, density, depth and conductivity and map out hydrothermal vents, tsunamis etc. AUGs are underwater robots which can hold their positions by gliding against the current or waves

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

by making themselves neutrally buoyant and drift with the currents and waves or rest on the sea bed. They do not require thrusters or propellers for propulsion. AUGs are capable of carrying out a mission economically in comparison to AUVs and have much larger endurance. Few existing AUGs even have the capability to derive their propulsive energy from the ocean itself. One concept is to use the temperature differences in the ocean thermocline (principle of thermal stratification to convert heat into mechanical energy). In general, these vehicles are very small in size. Their operating speed varies from 0.1 to 0.5 m/s.

Towed fish are torpedo shaped bodies without any active propulsion. They are towed by a ship by a mooring line and have limitations on the depth of operation. Their depth of operation is usually limited to about 200 m. Typically, they carry instruments such as acoustic Doppler current profiler, DRAKE (Depth and Roll Adjustable Kite for Energy flux measurements) etc. During their deployment, the ship speed is typically in the range of 2.5–5.5 m/s. ROVs are tethered vehicles whose mission plan is executed from onboard a ship. They are often linked to the ship by either a neutrally buoyant tether or a load carrying umbilical cable. Their depth of operation varies from 200 to 11,000 m. They can be employed in exploration and mapping of seabed minerals.

Unlike ROVs, deep submergence vehicles (DSV), autonomous surface vehicles (ASV) and AUGs are unmanned submersibles without tethers or umbilical cables and follow a predefined path without operator intervention. They usually carry the power source onboard in the form of batteries and have a payload in accordance with their mission. The power supply unit and the speed details of few existing AUVs are given in **Table 1**.


Even though the initial interest in AUVs was developed for oceanographic research in the late 1960s by the University of Washington using Special Purpose Underwater Research Vehicle


**Table 1.** General characteristics of AUVs around the world (from Thomas, 2003).

(SPURV) (see **Figure 1(a)**), it is only recently AUVs are being actually used in oceanographic research. Few recent instances where AUVs and ROVs are employed are: the Autonomous Benthic Explorer (ABE) (see **Figure 1(b)**) deployed in 1996-1997 in the Juan de Fuca region off the coast of Oregon to map the magnetic characteristics of a new lava flow; Odyssey IIB (see **Figure 1(c)**) class vehicles from MIT employed to study the convective overturning associated with the mixing of fresh and salt water in the Haro Strait on the US-Canadian northwest border; Woods Hole Oceanographic Institute's Argo ROV (see **Figure 1(d)**) played a major role in the 1985 discovery of the wreck of the RMS Titanic and German battleship Bismarck; and OKPO-6000 (see **Figure 1(e)**) explored successfully 2300 m deep seabed in 1996 near the Dok-Do island in the East Sea of Korea. Few other examples of the present day AUVs are Japan's AUV Urashima (see **Figure 1(f)**), which is a torpedo-shaped vehicle with depth rating up to 3500 m and whose hull frames are made of titanium; Norway's HUGIN 3000 (see **Figure 1(g)**) is a tail boomed vehicle that can operate in 3000 m depth; Canadian AUV Theseus (see **Figure 1(h)**), which is a torpedo-shaped vehicle of length 10.7 m, diameter 0.127 m and depth of operation of to 1000 m; UK's AUV Autosub (see **Figure 1(i)**), which is also a torpedo-shaped vehicle with depth rating of 6000 m, endurance of 4400 hrs and with a hull made of titanium; Denmark's Atlas Maridan Seaotter Mk II (see **Figure 1(j)**), which is a modular flat fish design with a length of 3.65 m and depth of operation 600 m; AUG Seaglider (see **Figure 1(k)**) which is a tear drop shaped vehicle of length 1.8–2 m depending on the configuration and Indian AUV Maya (see **Figure 1(l)**) which is a torpedo-shaped vehicle that is 1.72 m long and 0.234 m in diameter.

#### **1.1. DESIGN OF UVs**

The design of UV is mission specific and each UV is unique in design because it needs to cater to its unique set of mission requirements. However, the design objectives related to their underwater usage are based on hydrodynamic drag, power, propulsion, maneuvering and buoyancy control. Of these, the hydrodynamic drag is most important because it directly affects the power requirement, range, and endurance. Therefore, minimization of drag is a central objective in AUV design and it is an important problem in the area of marine hydrodynamics. This can be accomplished, in general, by some combination of (i) streamlined shaping of the hull, (ii) controlling boundary-layer, e.g., polymer injection or slot suction, (iii) energysaving propulsion; e.g., a wake adapted propeller or a suction slot with a stern jet, and (iv) efficient maneuvering consistent with hydrodynamic stability. The first two of this list

**Figure 1.** Diverse forms of AUVs. (a) AUV SPURV [2], (b) Autonomous Benthic Explorer [3], (c) AUV Odyssey IIB [4], (d) ROV Argo [5], (e) AUV OKPO 6000 [6], (f) AUV Urashima [7], (g) AUV Hugin 3000 [8], (h) AUV Theseus [9], (i) AUV Autosub [10], (j) AUV Atlas Maridan 300 [11], (k) AUG Seaglider [12], (l) AUV Maya [13].

attempt to reduce skin friction and pressure drag while the third attempts to extract energy lost to the fluid surrounding the vehicle. A complete systems design must simultaneously take into account all these four aspects though the complex nature of the complete problem does not permit analytical systems design approach [1].

For propelled vehicles, a body that has minimum drag need not be the one that requires minimum power because the benefit from the reduction in drag may be lost because of poor wake fraction which implies poor propulsive efficiency of the propeller which is fitted in the vehicle's wake. The efficiency of the propeller located in the wake of the hull is in general inversely proportional to the wake fraction and also to the thrust deduction factor. In order to optimize hull shapes of UVs, both drag (minimization) and wake fraction (minimization) should be considered simultaneously.

At present, the UV design process is mainly dominated by ad-hoc approaches that either use design experience or rely on simple rules of thumb [2] based on empirical formulations of hydrodynamic drag and wake fraction. Although an empirical approach is convenient at the preliminary design stage, it does not consider the local fairing effects on the flow, which play an important role in the estimation of drag and wake fraction. To overcome these limitations, the experimental approach using model testing in towing tank is often used, which, however, is both time consuming and expensive and, as a result, cannot be done for many designs. Typically, a maximum of three designs can be tested in a towing tank, which is certainly not enough for establishing a near optimum design. In this regard, the recent advances in CFD can play an important role in the UV design because with CFD the local fairing effects and the variations of flow can be accurately predicted so that hydrodynamic evaluations of many designs are possible in a short time economically. As a result, a near optimum design can be obtained in principle if CFD approach is integrated into the design process. Use of CFD to analyze the flow field around the hull and to perform computations of viscous drag has found interesting applications in ship design [2–4].

Although CFD has the advantage of reducing the time and cost of each analysis, it is difficult to manually change the design parameters of the UV hull form and conduct each analysis to obtain an optimized shape. Hence, there is a requirement to solve the problem with a process of optimization which is robust and automatic. Such attempts have been made for ships [3, 4] with limited success. In recent years, multidisciplinary design optimization (MDO) methods are being increasingly and effectively used to identify optimal designs [2].

The optimization of hull shape of UVs therefore, will ideally involve simultaneous minimization of drag and wake fraction and maximization of volume subject to constraints on the parameter space. The methodology presented in this thesis seeks to establish such a capability wherein the optimization technique based on genetic algorithm (GA) and CFD solver are seamlessly integrated with the computer aided geometric design (CAGD) tool in a single code that requires no user intervention during the entire optimization process.
