## Boundary Conditions Boundary Conditions in CFD

Boundary conditions are crucial in Computational Fluid Dynamics (CFD) simulations. They define fluid behavior at the boundaries of the computational domain, providing necessary information about the outside world, which the governing equations (Navier-Stokes) do not cover.

These conditions act as constraints on the fluid flow equations, specifying values like velocity, pressure, and temperature (Fixed Value), or enforcing derivative constraints (Fixed Gradient). Properly chosen boundary conditions ensure accurate and stable simulations, matching real-world interactions. Incorrect boundary conditions can lead to unstable and divergent results.

The number of solution variables (solution fields) depends on the problem and determines how many variables should be constrained on each domain’s boundary. For incompressible flow, the solution field includes velocity components and pressure. For compressible flow, it includes velocity components, pressure, and temperature. Additional fields like turbulence quantities and species concentration can also be included.

Boundary conditions must be set for all solution fields in the simulation. However, the combination of boundary condition types is often limited. Selecting a type for velocity may require a specific type for pressure. For example, a velocity inlet typically uses a Fixed Value for velocity and a Zero Gradient for pressure. This means that while we think about boundary conditions, we should always focus on the whole set of solution fields, not only on a particular one. In SimFlow, the concept of boundary conditions set is introduced as a Boundary Condition Character, and by selecting the character, the appropriate boundary conditions for all solution fields are chosen automatically.

## Boundary Conditions Boundary Conditions Types

There are several types of boundary conditions that can be applied in CFD simulations. From the mathematical point of view, the boundary conditions can be divided into three main types:

- Dirichlet - the value of the solution field is fixed at the boundary.
- Neumann - the derivative of the solution field is fixed at the boundary.
- Robin - a linear combination of the dirichlet and neumann boundary conditions.

In CFD, above types like Dirichlet (Fixed Value in SimFlow) are sometimes used directly. However, typically users rely on derived types that implement Dirichlet, Neumann, Robin, etc., with coefficients, enforced values, and blending factors calculated based on the physical context and flow conditions. For instance, a Total Pressure boundary condition uses the Dirichlet condition applied to static pressure, while the enforced value is calculated based on the user-provided total pressure and flow velocity at the boundary.

## Boundary Conditions Boundary Conditions Categories

To facilitate the selection process, we have categorized boundary conditions into distinct groups based on their application and the physical quantities they influence. This structured approach allows users to easily identify and apply the appropriate boundary conditions for various aspects of their CFD simulations. For example, boundary conditions related to velocity, such as flow rate and velocity slip, are grouped separately from those related to pressure, like total pressure and fan pressure. By organizing boundary conditions into specific categories, we provide a clear and systematic way to address the different solution fields or physical contexts encountered in simulations. Each category focuses on a particular type of boundary condition, ensuring that users can efficiently choose the most suitable options for accurate and stable simulation results.