Tdyn reference manual
- 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.
- 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
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 will impose
default boundary conditions for regions of the domain boundaries, where
the user has not specified anything. Tdyn 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.