# Boundary Condition Types

All of the problems in fluid dynamics are governed by the same standard conservation equations. What makes the solution of each problem unique is the set of boundary conditions.

Hence, boundary conditions have great impact on the accuracy of the simulation. This section discusses the variety of boundary conditions generally used in a CFD simulation.

## Inflow Boundary Conditions

This type of boundary condition is applied at the boundaries where the flow enters into the domain. There are different ways of specifying inflow conditions.
Velocity Type
A user prescribed velocity is imposed on all of the nodes of the surface. Mass flux through the surface depends on the variation of the prescribed velocity profile.
Pressure Type
A user prescribed pressure values are imposed on all the nodes of the surface. This type of condition is useful when velocity or flow rate is not known, such as buoyancy driven flows.
Mass Flux
A user prescribed mass flux rate is imposed on the surface. A boundary layer profile for velocity and turbulence fields is calculated based on the distance from no-slip walls and estimated Reynolds number. In AcuSolve the profile is recomputed at each time step such that the deforming meshes are properly accounted for in the calculation.
Flow Rate
A user prescribed flow rate is imposed on the surface. A boundary layer profile for velocity and turbulence fields is calculated based on the distance from no-slip walls and estimated Reynolds number. In AcuSolve the profile is recomputed at each time step such that the deforming meshes are properly accounted for in the calculation.
Average Velocity
An average velocity is imposed at the surface. A velocity profile similar to the mass flux boundary condition is used in this case.

## Outflow Boundary Conditions

This type of boundary condition is applied at the boundaries where the flow exits the domain. There are primarily two types of outflow conditions.
Pressure Outflow
A user prescribed pressure values are imposed on all the nodes of the surface. This type of boundary condition is useful when the pressure at the outlet is known, for example the flow leaving into atmosphere.

## Wall Boundary Conditions

This type of boundary condition is used for the physical walls in the simulation domain. It specifies the velocity of the fluid at the surface of the wall. For example, for a stationary wall in viscous flow a wall boundary condition would impose all of the components of the velocity to zero. For a simulation in which a wall itself is moving at a certain velocity a wall boundary condition would impose that velocity to the fluid at the surface of the wall. In short a wall boundary condition can be expressed as ${V}_{normal}=0$ and ${V}_{tangential}={V}_{wall}$ .

## Slip Boundary Conditions

This type of boundary condition is applied to the surface through which there is no normal velocity but has zero shear stress, which means no change in the tangential velocity.(1)
${V}_{normal}=0$

where ${x}_{i}$ is the tangential direction of surface.

## Symmetry Boundary Conditions

For the problems that have a plane of symmetry, the fact that solution is symmetry can be exploited by modeling only one half of the problem and defining the boundary type as Symmetry at the symmetry plane. Mathematically Symmetry type is equivalent to the type Slip, that is, zero flux across the plane and zero shear stress along the plane.

## Far-Field Boundary Conditions

This type of boundary conditions is used to represent conditions far away from the source of disturbance. The flow at these boundaries are typically uniform. Far-field is generally used for external flows, in which variables free stream conditions at far upstream are none.

## Free Surface Boundary Conditions

As the name suggests, free surface is surface of the fluid that is not constrained by any physical boundary, like the top surface of a sloshing box problem. This type of boundary condition imposes normal component of mesh velocity to the flow velocity.