# Towards a Digital Twin for Open-Frame Underwater Vehicles Using Evolutionary Algorithms
## 2 Related Work  
Developing simulation tools has been a constant effort with the intention of offering a tool for all underwater works, as can be seen in reviews such as [8]. To model dynamic behavior, various aspects such as vehicle dynamics, propulsion, coefficients defining the model, and added masses need to be considered. In a more traditional simulation, all these models stay static while the vehicle evolves and changes; however, when working with a digital twin, both the vehicle and twin need to evolve together. This introduces a limitation into the evolution of the models as the evolution of the model also needs to be considered. Regarding the dynamic model, it has been approached from multiple perspectives, covering a wide range of phenomena that can affect the dynamic behavior of underwater vehicles. Various strategies have been employed to validate the model, including numerical simulations, towing tank experiments, and wind tunnel tests. Early works, such as the dynamic behavior modeling of VideoRay Pro III, assume decoupled movements in the three axes [14]. In [15], low-cost sensors are used in real experiments to validate a simplified dynamic model, applied for AUV control in heading, pitch, and depth. Other cases, like [16], use finite element-based numerical examples to validate the dynamic modeling of umbilical cable effects on AUVs. Dynamic models are also inferred from simulations using computational fluid dynamics (CFD) based on the meshing of the AUV's three-dimensional (3D) model [17]. Additionally, some studies focus on simulating the dynamic behavior of simple solids, as in [18], where models are tested using finite volume methods and validated in a wind tunnel, analyzing the effects of different edge shapes in detail, with results extrapolated to a real ROV. In [19], a numerical model for a BlueROV2 is proposed, using the open source CFD software OpenFOAM to cover the four main freedom axes, and numerically simulated by a Reynolds-averaged Navier-Stokes (RANS) solver. The model is contrasted with hydrodynamic force and moment data measured in real trials, subjecting the ROV to various perturbations caused by different current and flow conditions. Concerning the effects of vehicle propulsion on the dynamic model, different approaches have been considered, often motivated by vehicle control. In [20], the through-body thruster model of a torpedo-type C-SCOUT AUV is generated and numerically simulated, including forces due to hydrodynamic effects on control surfaces. Propeller shaft velocity and ambient flow velocity are used in [21] to estimate axial flow velocity, with the propulsion map divided into zones for analysis of the incoming angle of ambient flow through numerical simulation. Some works address the propulsion model of underwater gliders propelled by buoyancy modulation. For instance, ref. [22] uses computational and semi-empirical methods to analyze the behavior of three different vehicles, ref. [23] examines thruster interactions, and ref. [24] investigates the influences of external currents on the propulsion model, conducting experiments with a BlueROV2 restrained by eight cables in different orientations relative to the incoming flow, and under different propeller operating conditions. With the development of open source simulators like Gazebo, many researchers started developing plugins and extensions to improve the accuracy of the underwater environment simulation such as the UUV Simulator [11], where the added mass, buoyancy, and other hydrodynamics are modeled using the Fossen equations [25]. As for the parameters affecting each model, various works focus on different aspects, and ref. [22] studies hydrodynamic parameters for the equations governing motion, while ref. [26] estimates hydrodynamic coefficients of a ROV using free decay pendulum motion. Real data are obtained in tests with a planar motion mechanism (PMM) for experimental evaluation of hydrodynamic coefficients in [23; 27]. Furthermore, ref.