Numerical Methods and Its Solution - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 220

Fig. 7.32 Example of some possible boundary condition settings
domain, inflow and outflow boundary conditions are suitable. The flow domain may
also be bounded by open boundaries. Appropriate boundary conditions are also re-
quired to be assigned for external stationary solid wall boundaries that bound the flow
geometry and the surrounding walls of possible internal obstacles within the flow
domain. The flow domain may also be bounded by open boundaries. Although the
intricacies of open boundary conditions are still subject to much theoretical debate,
this boundary condition remains the simplest and cheapest form to prescribe. Appro-
priate boundary conditions are also required to be assigned for external stationary
solid wall boundaries that bound the flow geometry and the surrounding walls of
possible internal obstacles within the flow domain.
General purpose CFD codes also allow the prescription of inflow and outflow
pressure or mass flow rate as boundary conditions. By setting fixed pressures values,
sources and sinks of mass at the boundaries are calculated to ensure the correct mass
flow into and out of the solution zone. It is also feasible to directly allocate sources
and sinks of mass at the boundaries by the mass flow rate instead of pressures to retain
the overall mass balance for the flow domain. To take advantage of special geomet-
rical features that the solution region may possess, symmetric and cyclic boundary
conditions can be employed to reduce the computational domain size Fig. 7.32 ). This
in turn allows the user to place additional cells to the reduced, simplified geometry,
thus enhancing the computational accuracy.