Representing the Flow Environment in Simulation CFD
In Autodesk Simulation CFD, the simulation environment interacts with the simulation model based on Boundary Conditions. Consider the entrance and exit of air moving around a building. The simulation model, in this instance, consists of a building surrounded by an air volume. Defining the conditions that occur at the entrance and exit of the air volume are examples of simulation Boundary Conditions.
Left: A plan view of a building (grey) surrounded by moving air (velocity gradients colored with arrows indicating direction).

Simulation CFD provides numerous Boundary Conditions to define a variety of analyses, a small subset of these conditions are all that is necessary to address most AEC applications.
Before proceeding further, reviewing the following online help topics is recommended:
Simulation CFD considers all external surfaces as adiabatic walls (no flow or heat transfer) unless otherwise specified. Defining boundary conditions will enable the interaction of the simulation model with the environment; allowing flow to enter and exit the simulation for example.
Flow Openings
Openings allow flow to enter and exit the simulation. They are defined by Flow Boundary Conditions and typically represent known quantities such as a wind speed or supply air flow rate. The flow openings (inlets and outlets) are the best place to start when setting up Boundary Conditions.
Solving for air movement around this building requires two (2) Boundary Conditions. Each being a flow condition to define the inlet and outlet respectively. In this instance a known velocity is used to define the inlet while an atmospheric pressure (0 gauge) is defined at the outlet. 
Most AEC applications define flow inlets with a Velocity or Volumetric Flow Rate and outlets with a zero (0) gauge pressure (atmospheric pressure). This is the equivalent of having an open door or window.
NOTE:An atmospheric pressure (0 gauge) is a convenient reference for an outlet, it allows the inlet pressure (solved for by Simulation CFD) to represent the system pressure drop (inlet pressure minus the zero pressure of the outlet). Quickly determining pressure drop is convenient for many situations involving equipment sizing such as selecting air handlers or ducting designs.
Flow Boundary Condition Combinations
The known speed of air entering the domain can be directly defined as a velocity boundary condition in simulation. By constraining this inlet with a velocity, an exit must be defined to conserve mass (the laws of physics demand that anything going in must come out).
The inlet being defined as a velocity, over an area of the model, yields a volumetric flow rate. The volumetric flow rate and air density (material property) represent the mass flow rate entering the analysis domain.
Volumetric Flow Rate = Velocity * Area Mass Flow = Density * Volumetric Flow Rate 
Density and Area are given by the simulation model and material properties 
The exit being defined as an atmospheric pressure allows fluid to move across that model boundary. A pressure condition constrains the pressure (value remains constant); this does not constrain the velocity or consequently the mass flow rate out of the system. The analysis can solve for the mass flow rate across this boundary, properly constraining the setup of this problem.
TIP: Remember what goes in must come out when it comes to flow and thermal boundary conditions.
Boundary conditions are typically applied in combinations; for example, an inlet has an outlet, or a hot volume transfers heat to a cool air stream. These are known quantities used to communicate variables to the simulation solver. It is important to understand what combinations of boundary conditions provide the solver with the proper number of known and unknown quantities to solve for.



Properly Constrained  


The unknown variable being solved for is the mass flow rate 

Properly constrained models provide the correct number of unknown quantities to solve for. 
If the return of the small office space was assigned the same velocity boundary condition as the inlet, the mass flow rate in and out of the system would be defined. This means it would not be solved for, overconstraining the setup of this problem.
Another overconstrained setup would consist of both pressure and a velocity boundary condition being defined on the same surface of a simulation model. This is another example of constraining related terms when one should be solved for. In CFD, many terms in the governing equations are interrelated. Below are the most common erroneous boundary condition combinations that either over or underconstrain the solver.
OverConstrained  




Top Row: The mass flow rate is defined at both the inlet and outlet, leaving nothing to solve for. Bottom Row: Pressure and flow rate are defined on the same supply, again leaving the solver with nothing to solve for. Overconstrained models do not provide an unknown quantity to solve for. 
UnderConstrained  


Underconstrained models do not provide enough information to solve for! 
Summary of Flow Boundary Condition Combinations:
Flow boundary conditions provide the entrance and exit inputs to constrain the Simulation CFD solver. They are applied in combinations:
1. An inlet has an outlet
 Inlet > One (1) flow related condition (v, V,m)
 Outlet > Pressure
2. Velocity and Volumetric flow rate applied to the same surface are not a pair.
 V=va > Over=constrained
Properly defining these quantities is critical to converging on a solution in Simulation CFD. Think critically about what inputs are defined and what outputs are solved for. Overconstrained models typically contain conflicting inputs while underconstrained models lack sufficient input to solve properly.
Slip / Symmetry
There is one nonopening flow boundary condition (Slip / Symmetry Boundary Condition) that is commonly used in AEC applications; specifically external air flow analyses (wind around a building).
The Slip / Symmetry boundary condition enables Simulation CFD to neglect friction along walls. It will allow the fluid to slip across the wall as if the fluid domain continued into infinity.
Plan view of velocity moving around a building for an air domain with friction along with side walls (left) vs. no friction along the sidewall (right). Velocity results (in the direction of flow) have been extracted for both analyses along the dashed line and presented below. Left: Velocity results with friction accounted for along the domain side walls Right: Slip / Symmetry boundary condition defined along the air domain left and right sides. 


References for further information relating to external air flow analyses include:
 Wind loading best practices
 Recommended domain size
 Spatial variation  Useful for defining wind velocity as a function of height (linear variation)