**3.1. Main parameters and boundary conditions**

**Table 1** shows some equivalencies associated to the general variable.

Dissipation of the turbulent kinetic energy

**Table 1.** Different variables used in the general equation.

322 Numerical Simulation - From Brain Imaging to Turbulent Flows

**Equation Variable (***φ***)** Continuity 1 X momentum u Y momentum v Z momentum w Transport of turbulent kinetic energy k

**Figure 1.** The coupled and segregated solvers comparison diagram, adapted with permission from [8].

The FLUENT solvers work based on a logical sequence to solve the discretized equations in two ways, the segregated one or the coupled one [8]. The segregated mode is recommended

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Although ANSYS FLUENT is a very friendly application, its implementation requires of an adjustment for some predetermined parameters, which will guide the solution process.

Before uploading the mesh associated with the physical domain to be studied, the software requires to define the number of dimensions used to reach the solution; it can be two-dimen‐ sion (2D) or 3D. To analyze uniform cross sections in bodies with infinite length, it is recom‐ mended to use 2D analysis, suppressing the third coordinate because gradients along the depth are not significant. The software offers an option to choose double precision (more signifi‐ cant ciphers). This option is recommended when large gradients in the flow variables are expected, especially at very close nodal position, cells with very large aspect ratio or simply when a better convergence is desirable.

Another important characteristic is the capacity to adjust the number of core processors to be used internally in parallel. This characteristics is especially helpful for problems that require many iterations to be solved (e.g.,, non-stationary models), multiphase and reactive prob‐ lems, very large grid sizes, and very complex geometries such as airplanes and vehicles.

Finally, this computer program allows to scale the grid physical dimensions to required sizes using both English and international systems. It can be used to work on problems such as the flow between building zones located hundreds of meters from each other or problems related to microfluids. It also allows to select constant values to calculate non-dimensional parame‐ ters such as friction coefficient and lift and drag coefficients.

In any CFD model, boundary conditions have to represent faithfully the real conditions at the boundary. According to the mathematical characteristics of the differential equations, boundary conditions can be classified as follows:

**Dirichlet boundary condition:** The value of the flow variable is specified at the boundary. This kind of boundary condition is typically linked to problems involving flow inlets and isothermal walls.

**Neumann boundary condition:** For this kind of boundary condition, the value of the gradient normal to the flow variable is specified. These boundary conditions are often associated with symmetric boundaries and adiabatic walls.

**Mixed boundary conditions:** These are the conditions resulting from a combination of boundary conditions of the kind of Neumann and Dirichlet.

The boundary conditions can be classified by its physical meaning as follows: physical boundaries, such as solid walls, or artificial boundaries, such as outflow. The latter are mathematical approximations to the real behavior of the flow in certain zones. Artificial boundaries can be applied, if the computational domain constitutes part of the total flow field, to research the most interesting region and to reduce computational costs. Artificial boun‐ dary conditions require mathematical formulations that are physically significant to the real flow behavior.

Regarding ANSYS FLUENT, there are different types of boundary conditions that can be used to simulate different flow problems. Next, a general description of the most common boun‐ dary conditions used in most hydrodynamic studies is presented. The authors recommend readers to consult Ref. [8] for more detailed information.
