The mixed inlet boundary condition allows both subsonic and supersonic inflow within a single boundary condition. The blending between the two flow regimes is based on the user-specified inflow velocity components—total pressure and total temperature, or static pressure and static temperature. In either case, the flow solver evaluates the local Mach number from the user-specified values.
The Mach number calculation can be based on the velocity component normal to the boundary condition or the full velocity components. By default the flow solver uses the component normal to the boundary condition. You can select which calculation you want via the parameter Blend Mach Number Type.
The following types of mixed inlets are supported. In both cases, the velocity type depends on the Frame Type parameter (Stationary or Rotating, described in Cartesian Velocity Components and Cylindrical Velocity Components):
Velocity, static pressure, and static temperature are specified:
For Ma < 1 (subsonic flow), this would result into the standard 'velocity specified' inlet. The user-specified pressure information is not used, but is computed as part of the iteration process.
For Ma > 1 (supersonic flow), all user-specified data is used as it is done for the supersonic inlet.
Velocity, total pressure, and total temperature/total enthalpy are specified:
For Ma < 1 (subsonic flow) this would result into the standard "total pressure and direction" inlet. The inflow direction is evaluated from the specified velocity components.
For Ma > 1 (supersonic flow) all user-specified data is used as it is done for the supersonic inlet.
The total pressure and total temperature depend on the frame type (that is, total pressure can be in either the stationary or the local frame, and the same applies to total temperature).
When you specify velocity, total pressure, and total temperature, the flow solver evaluates the static pressure and temperature directly from those values. The total enthalpy boundary condition is available only if the total energy is solved. In this case, no transformation of the specified energy is required.
Note: Static and total conditions for pressure and temperature cannot be mixed for real gases or gas mixtures with real gas components. This restriction does not apply to gases and gas mixtures that obey the Ideal Gas law.
Mixed inlets can only be imposed if the Total Energy
model is employed, and if the fluid is compressible. This includes the ideal gas
model, NASA materials, Redlich Kwong model, RGP file materials and materials
with a CEL equation of state, where density is a function of pressure.
Flows that use the wet real gas model (homogeneous binary mixtures) are currently not supported.
At a mixed inlet, all quantities must be specified.
Static or Total Pressure, velocity
components and Static or Total
Temperature or Total Enthalpy are required.
Whenever the Total Pressure option is selected, the total
pressure is in the stationary frame for rotating domains.
The following options are supported:
For the mixed flow regime inlet boundary, the Cartesian components of velocity, together with a value for the Relative Static or Relative Total Pressure.
For the mixed flow regime inlet boundary, the cylindrical components of velocity, together with a value for the Relative Static or Relative Total Pressure (Relative to the domain reference pressure).
The Total Temperature is specified at the mixed inlet
boundary. The static temperature is computed from the specified
Total Temperature. The flow solver will apply
stationary frame total temperature if the domain is rotating.
The Total Enthalpy is specified at the mixed inlet
boundary. The flow solver will apply stationary frame total enthalpy if the
domain is rotating.
The total enthalpy boundary condition is available only if the total energy is solved. The total enthalpy boundary condition is a 'natural' boundary condition for mixed subsonic-supersonic flow as you directly specify the total energy (that is, total enthalpy) value at the inlet. Static temperature and total temperature, on the other hand, have to be converted to total enthalpy before they can be used as a boundary condition.
Special care is required to specify the initial guess for a simulation that
involves a mixed flow regime inlet. It is strongly suggested to use the
Automatic with Value option rather than the
Automatic option, especially if the flow near the mixed
inlet is known to be locally supersonic, which locally imposes a large
difference between the static and total conditions.
Flow solver robustness and convergence will be much better behaved if the flow field near the inlet is initialized as if it would be totally supersonic.