**Drag Coefficient **

Computational Fluid Dynamics (CFD) is one of the lines of research in fluid mechanics, which studies the dynamics of fluid flow in natural or modified environments. In addition, it is capable of representing several characteristics present in the medium in which the fluid flows (whether gaseous or liquid), such as velocity, fluid wave propagation, depth, pressure, flow, water column, between others.

The CFD simulation starts from mostly phenomenological and differential equations that, if a numerical integration of such equations is made, it is possible to predict local characteristics and parameters of the flow. With this, the software CFD demonstrates the behavior of such functions in 3D or 2D form, pointing out where possible imbalances that cause losses may be occurring, so that they can be resolved and thus meet all safety and structural integrity standards.

Simulations are usually run in software such as Ansys Fluent, Ansys CFX, CFD Autodesk, Openfoam, CFD ++(solver), Tecplot (viewer). Such software differs by the method of integrating the numerical equation, making the choice of the program to be used based on the choice of the project and the computational capacity.

Its efficiency in showing results close to reality is so good that it is applied to a wide variety of lines of research, engineering and industry, including chemical process development, aerodynamics, aerospace, climate simulations involving environmental engineering and natural sciences, heat transfer, engine and combustion analysis.

At Nautilus, this tool is used to simulate various real-life situations of fluid behavior inside and outside the AUV. Some of our simulations are the analysis of the vehicle at high depth, the generated vorticity, the study of cavitations formed in the propellers, if there is tightness and the drag coefficient. These simulations help us define the materials, shape and size of our projects.

One of the simulations that is important in the hydrodynamic analysis of our vehicles is the simulation of the drag coefficient, which is a dimensionless value used to define how the friction between an object and a fluid is given, which, in our case, is the resistance of the our AUVs in water.

The drag coefficient is defined by the joining of two basic contributors of drag, fluid dynamic which is the surface friction, which is the interaction between the fluid and the surface of the object, and the drag such that it is due to the shape of the object to be analyzed. , showing that the larger the shape of the object, the greater the drag suffered by them.

This simulation helps us visualize the design evolution of our AUV's over the years. In the images presented below, we see the drag coefficient simulation of our last two projects, BRhue and Lua, both located in environments with equal characteristics (velocity, viscosity, material, mesh size). However, as they have a different size and shape, they present a different drag coefficient, 1.25 for BRhue against 1.005 for Lua, thus showing the evolution of the shape and arrangement of the components present in the AUV.

Figures 3 and 4: current lines representing fluid moving on the BRhue and the Lua, respectively.

** Written by Luana Lima**

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