With the first option, all the model openings are classified into “pressure” openings, “flow” openings, and “fans”, depending on the flow boundary conditions which you intend to specify on them.
A “pressure” opening boundary condition, which can be static pressure, or total pressure, or environment pressure is imposed in the general case when the flow direction and/or magnitude at the model opening are not known a priori, so they are to be calculated as part of the solution. Which of these parameters is specified depends on which one of them is known. In most cases the static pressure is not known, whereas if the opening connects the computational domain to an external space with known pressure, the total pressure at the opening is known. The Environment pressure condition is interpreted by Flow Simulation as a total pressure for incoming flows and as a static pressure for outgoing flows. If, during calculation, a vortex crosses an opening with the Environment pressure condition specified at it, this pressure considered as the total pressure at the part of opening through which the flow enters the model and as the static pressure at the part of opening through which the flow leaves the model.
Note that when inlet flow occurs at the “pressure” opening, the temperature, fluid mixture composition and turbulence parameters have to be specified also.
A “flow” opening boundary condition is imposed when dynamic flow properties (i.e., the flow direction and mass/volume flow rate or velocity/Mach number) are known at the opening. If the flow enters the model, then the inlet temperature, fluid mixture composition and turbulence parameters must be specified also. The pressure at the opening will be determined as part of the solution.For supersonic flows the inlet pressure must be specified also.
A “fan” condition simulates a fan installed at a model opening. In this case the dependency of volume flow rate on pressure drop over the fan is prescribed at the opening. These dependencies are commonly provided in the technical documentation for the fans being simulated.
With the second option, you specify the boundary conditions by transferring the results obtained in another Flow Simulation calculation in the same coordinate system. If necessary, the calculation can be performed with another model, the only requirement is the flow regions at the boundaries must coincide. At that, you select the created boundary conditions’ type: either as for external flows (so-called “ambient” conditions, see “External Flow Boundary Conditions”), or as for “pressure” or “flow” openings, see above. If a conjugate heat transfer problem is solved, the temperature at the part of the boundary lying in a solid body is transferred from the other calculation.
Naturally, the flow boundary conditions specified for an internal flow problem with the first and/or second options must be physically consistent with each other, so it is expedient to specify at least one “pressure”-type boundary condition and at least one “flow”-type boundary condition, if not only “ambient” boundary conditions are specified.
If one or several non-intersecting axisymmetric rotating regions (local rotating reference frames) are specified, the flow parameters are transferred from the adjacent fluid regions and circumferentially averaged over rotating regions’ boundaries as boundary conditions.