Boundary Conditions

Incorrect or inappropriate specification of boundary conditions is a very common cause of problems. It may lead to the solution of the wrong problem as well as convergence difficulties.

Have the boundary conditions been defined correctly and are they physical?

Have sufficient boundary conditions been defined for a unique solution or is the problem over-constrained because of too many or conflicting boundary conditions?

Particularly problematical areas are:

• in high speed flows, the quantities required change as the flow changes from subsonic to supersonic.
  
  
• multi-phase flows, where one phase may be going out of the domain, and another entering the flow domain.
  
  
• density stratified flows, where the hydrostatic pressure depends on a density that is itself part of the solution procedure.
  
  
• for turbulent problems convergence may be hindered by the incorrect specification of turbulence quantities at the flow inlet.
  
  
• for cases where the pressure drop is specified as the flow boundary condition, the mass flow through the system is not constrained. This can lead to a problem taking a large number of iterations, and much under-relaxation. Consider switching to specifying the flow rates. This should converge much faster, and then the solution can be used as a good guess for determining the flow for a particular pressure drop.