Computational Fluid Dynamics in AUV/ROV/HOV Hydrodynamics
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Definition
Hydrodynamic performance is one of the most important performances of AUV/ROV/HOV; the economy, practicability, and safety of submersible can be affected directly by its advantages and disadvantages. Computational fluid dynamics (CFD) is an important method to research the hydrodynamic performance of submersible. It is a systematic analysis of physical phenomena including fluid flow through computer numerical computation and image display. The flow can be numerical simulated by the CFD under the control of the basic equation of the flow. The distribution of physical quantities at various positions in the flow field of extremely complex problems and the changes of these physical quantities with time can be obtained through the numerical simulation. The flow field of submersible can be obtained in CFD results, laying a good foundation for the further optimization of submersible designs.
Scientific Fundamentals
The Basic Concept of CFD
The fields of the continuous physical quantities in the spatial domain, such as velocity field and pressure field, are replaced by a series of sets of variable values at finite discrete points. Through a theoretical relation, the algebraic equations of the relation among the variables at these discrete points are established, and then, the approximate values of the variables are obtained by solving the algebraic equations (Renilson 2015).
The Basic Procedure of CFD
Characteristics of CFD Method
 1.Advantages:
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Strong adaptability to physical problems and wide range of application.
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Not subject to conditions, with more flexibility.
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The underwater flow field of the submersible can be visually displayed.
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Disadvantages:
Compared with the other two methods, CFD method has the characteristics of unrestricted conditions and strong adaptability, but it also has a few limitations. 1.
The CFD numerical solution is an approximate calculation method, which can only get the numerical solution of a finite number of discrete points, with certain calculation errors;
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It involves a large number of numerical calculations and requires high computer configuration.
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Key Applications
CFD Software Structure and Common Commercial Software
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Preprocessor
The preprocessor is used to complete the work of preprocessing. In the preprocessing phase, the following tasks should be completed by engineers:
Establish the geometric computational domain of the physical problem.

The computational domain is meshed into nonoverlapping subregions.

Select the control equation.

Define the property parameters of the fluid.

Set boundary conditions for geometric computational domain.

Set the initial conditions for transient physical problems.
At present, preprocessing software which is currently used includes ANSYS ICEM CFD, Pointwise, Gridgen, Hypermesh, GridPro, etc. (Wang 2013).

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The solver
The task of the solver is to read into the grid file (e.g.,“.msh”), boundary conditions, control parameters, etc., by the preprocessor, and then, the results can be outputted and calculated according to the solution algorithm specified by the user. For the convenience of users, preprocessing functions are attached to some software solvers.
At present, CFD solution software which is commonly used includes ANSYS Fluent, ANSYS CFX, STAR CCM+, Phoenics, etc. (Wang 2013).
 3.
Postprocessor
The data obtained from the calculation is presented in an intuitive way through the postprocessing, which is convenient for users to analyze and engineering application. The data is presented in some ways which includes:
Data table: resistance, lift, moment, speed, and other physical quantities

Curve diagram: reflect the changing rule of each physical quantity

Nephogram: reflects the spatial distribution of physical quantities, such as pressure contour, velocity contour, vortex contour, etc.

Vector diagram: reflect the vector spatial distribution, such as the velocity vector diagram

Animation: reflect physical quantity changes with time in the form of animation
At present, the commonly used software of CFD postprocessor includes ANSYS CFDPOST, Tecplot, EnSight, ParaView, etc. (Wang 2013).
In addition to commercial software, users can also compile and modify the code to form their own computing software according to their requirement. Commonly used CFD opensource software (OSS) includes Open Foam, SU2, etc. (Wang 2013).

CFD Applications in Submersible Design
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Prediction of submersible resistance and effective power
Stateoftheart CFD techniques can be used to estimate the resistance of an underwater vehicle, either deeply submerged or close to the surface. In the preliminary design stage of AUV/ROV/HOV, it is necessary to configure the system power unit. Therefore, resistance and effective power prediction are necessary. The advantage in CFD method is short calculation period and low cost. By simplifying the model of the designed submersible firstly and then simulating the flow field around the submersible with CFD method, the resistance of the submersible in different speeds can be obtained. Finally, the effective power of the designed submersible can be predicted through a certain processing of results. In order to ensure the effectiveness of submersible numerical calculation, it is usually necessary to compare the calculated results with the experimental results.
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Shape or molded lines optimization of AUV/ROV/HOV
The shape or molded lines optimization is more conducted for AUVs or HOVs rather than ROVs, since the power of ROVs is supplied from the surface ship through umbilical. However, AUVs or HOVs normally carry the battery themselves to navigate or perform certain operations under the sea. So for AUVs or HOVs, shape or molded lines optimization are usually conducted to achieve as small resistance as possible to better utilize energy. Since CFD can be used effectively to determine the effect of small changes in the hull form, it makes CFD a useful tool in the molded lines optimization.
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Maneuverability analysis of submersible
During the design stage, it is necessary to research the ability and performance of the submersible when changing its motion direction and attitude through the control device. For the maneuverability analysis of submersible, it is necessary to establish six degrees of freedom equation in space to determine the external forces on the submersible (including gravity, buoyancy, thrust, and hydrodynamic force). Among them, the hydrodynamic force on the submersible is generally represented by the hydrodynamic coefficient. Therefore, the CFD method to calculate the hydrodynamic coefficient of submersible is the precondition to research the maneuverability.
CrossReferences
References
 Hu K, Li Z (2015) Explanation of engineering examples in ANSYS ICEM CFD. Posts & Telecom Press, BeijingGoogle Scholar
 Renilson M (2015) Submarine hydrodynamics, Springer Briefs in Applied Sciences and Technology. Springer, ChamGoogle Scholar
 Wang F (2013) Computational fluid dynamic analysis. Tsinghua University Press, BeijingGoogle Scholar