1.2.7. Some comments on Boundary Conditions

In many real applications, there is a frequent difficulty in defining some of the boundary conditions at the inlet and outlet of a calculation domain in the detail that is needed for an accurate simulation. A typical example is the specification of turbulence properties (turbulence intensity and length scale) at the inlet flow boundary, as these are rarely available for a new design configuration. Other examples are the specification of the boundary layer velocity profile on the walls at an inlet, or the precise distribution of a species concentration at an inlet boundary.

Next a brief list of recommendations about boundary conditions prescription is given.

1.2.7.1. General guidelines on boundary conditions

  • The velocity and turbulence variables have to be prescribed at the inlet boundary. The pressure is sometimes also specified at the inlet boundary. Turbulence variables are automatically prescribed at the inlet boundaries in Tdyn CFD+HT

  • If the conditions at inlet are not well known, examine the possibilities of moving the domain boundaries to a position where boundary conditions are better identified.

  • Check whether upstream or downstream obstacles (such as bends, contractions, diffusers, etc.) outside of the flow domain are present which could significantly affect on the flow distribution. Often, information about components upstream or downstream of the domain is lacking or not available at the beginning of a project.

  • For each class of problem that is of interest, carry out a sensitivity analysis in which the boundary conditions are systematically changed within certain limits to see the variation in results. Should any of these variations prove to have a stronger effect on the simulated results, and lead to large changes in the simulation, it is necessary to obtain more accurate data on the boundary conditions that are specified.

  • Place open boundary conditions (outflow, or pressure prescription) as far away from the region of interest as possible and avoid open boundaries in regions of strong geometrical changes or in regions of re-circulation.

  • Pay an extra attention to the orientation of outlet planes with regard to the mean flow, especially when the boundary condition consists of a constant pressure profile.

  • Select the boundary conditions imposed at the outlet to have only a weak influence on the upstream flow. Extreme care is needed when specifying flow velocities and directions on the outlet plane.

  • Particular care should be taken in strongly swirling flows where the pressure distribution on the outlet boundary is strongly influenced by the swirl. It is therefore not acceptable to specify constant pressure across the outlet.

  • Be aware of the possibility of inlet flow inadvertently occurring at the outflow boundary, during the simulation process. This fact may lead to difficulties in obtaining a stable solution or even to an incorrect solution. If it is not possible to avoid this by relocating the position of the outlet boundary in the domain, try to avoid the problem by restricting the flow area at the outlet, provided so that the outflow boundary is not near the region of interest. If the outflow boundary condition allows the flow to re-enter the domain, the value of all transported variables should be imposed as in an inlet boundary.

  • If there are multiple outlets, impose either pressure boundary conditions or velocity specifications depending on the known quantities.

  • Tdyn CFD+HT allows not making any prescription in the pressure field. If no prescription of the pressure is done, some instabilities may appear.

  • Be aware that Tdyn CFD+HT will impose default boundary conditions for regions of the domain boundaries, where the user has not specified anything. Tdyn CFD+HT also fixes all the turbulence variables in those entities where all components of the velocity have been prescribed. These prescriptions are done due to numerical stability reasons and in most of the cases will not affect the results.

  • If possible, carry out a sensitivity analysis in which the key inlet boundary conditions are systematically changed within certain limits. Depending on the problem, the key parameters that might be examined are: inlet flow direction and magnitude, uniform distribution of a parameter or a profile specification (for example a uniform inlet velocity or an inlet velocity profile), physical parameters and turbulence properties at inlet.