In inlet boundary conditions, the distribution of all flow variables needs to be specified at inlet boundaries mainly flow velocity. This type of boundary conditions are common and specified mostly where inlet flow velocity is known.
In outlet boundary conditions, the distribution of all flow variables needs to be specified, mainly flow velocity. This can be thought as a conjunction to inlet boundary condition. This type of boundary conditions is common and specified mostly where outlet velocity is known. The flow attains a fully developed state where no change occurs in the flow direction when the outlet is selected far away from the geometrical disturbances. In such region, an outlet could be outlined and the gradient of all variables could be equated to zero in the flow direction except pressure.
This type of boundary condition is used where boundary values of pressure are known and the exact details of the flow distribution are unknown. This includes pressure inlet and outlet conditions mainly. Typical examples that utilize this boundary condition include buoyancy driven flows, internal flows with multiple outlets, free surface flows and external flows around objects. An example is flow outlet into atmosphere where pressure is atmospheric.
Axisymmetric boundary conditions
In this boundary condition, the model is axisymmetric with respect to the main axis such that at a particular r = R, all θs and each z = Z-slice, each flow variable has the same value. A good example is the flow in a circular pipe where the flow and pipe axes coincide.
Symmetric boundary condition
In this boundary condition, it is assumed that on the two sides of the boundary, same physical processes exist. All the variables have same value and gradients at the same distance from the boundary. It acts as a mirror that reflects all the flow distribution to the other side. The conditions at symmetric boundary are no flow across boundary and no scalar flux across boundary. A good example is of a pipe flow with a symmetric obstacle in the flow. The obstacle divides the upper flow and lower flow as mirrored flow.
Periodic or cyclic boundary condition
A periodic or cyclic boundary condition arises from a different type of symmetry in a problem. If a component has a repeated pattern in flow distribution more than twice, thus violating the mirror image requirements required for symmetric boundary condition. A good example would be swept vane pump, where the marked area is repeated four times in r-theta coordinates. The cyclic-symmetric areas should have the same flow variables and distribution and should satisfy that in every Z-slice.