Here I would like to consider specific domain for employing the flow boundary conditions.

A pipe flow - right and left ends are supposed to have boundary conditions.

Prescribing velocity at the inlet:

Usually left end is kind of known to us. Therefore, we can employ Dirichlet boundary condition at this end. Both the temporal and spatial data over the boundary needs to be known. In general people do specify the parabolic flow profile with time varying waveforms at the inlet. This is good in any context. The accuracy primarily depends on the boundary condition at the unknown end.

At the outlet:

We can either do velocity or pressure. If the channel is rigid, it is very simple, we know the flow profile should not change in most cases, and the volume flow rate remains same. So, velocity boundary condition can be employed. But, there is a challenge here. The profile in this case (right end) is out of the domain, and is not in our hands.

Popular boundary conditions:

Common boundary conditions employed are,

1. Constant pressure Dirichlet boundary condition
2. Constant traction boundary condition (Neumann type)

Both of these are most commonly used in early CFD studies. But the flow profile shape is not maintained when these BCs are employed.

In such cases, outflow boundary conditions are employed. But before discussing them, lets evaluate the accuracy of these boundary conditions. The parametric studies are based on Liu et al. (2015).

Boundary condition setups used,

• V-NS0 model: A fully developed velocity profile with pulse waveform was imposed at the inlet and a zero normal stress condition was imposed at the outlet.
• P-V model: A time dependent pressure with waveform Pin was imposed at the inlet. A fully developed velocity profile with pulse waveform was multiplied by a scalar based on the sizes of the inlet/outlet cross sections such that the flow rate at the outlet is equal to that at the inlet. Then this adjusted velocity waveform was imposed at the outlet.
• V-P model: A fully developed velocity profile with pulse waveform was imposed at the inlet and a time dependent pressure with waveform Pout was imposed at the outlet.
• P-P model: A time dependent pressure with waveform Pin and Pout was imposed at the inlet and the outlet boundary, respectively.