Setting Up Inflow Boundaries

You specify inflow conditions on boundaries where the fluid enters the simulation domain.

For each inflow boundary:
  1. Select the Regions > [region] > Boundaries > [boundary] node and set Type to one of the following:
    • Velocity Inlet—set this boundary type for incompressible flow when you know the velocity at the boundary. You can specify the components of the velocity vector or the velocity magnitude together with a flow direction. You are advised to not use this boundary type for compressible flows as it does not prescribe the total energy of the system.
    • Mass Flow Inlet—set this boundary type when you know the mass flow rate or the mass flux (mass flow rate per unit area) at the boundary.

      The mass flow inlet differs from a velocity inlet in the way that density is used. For constant density flows, the two approaches are identical. For variable density flows, if the mass flow is specified, the velocity changes when the density is changed. If the velocity is specified, the mass flow changes when the density is changed.

      The mass flow inlet is designed primarily for specifying mass flow inward across the boundary, but it also works for specifying mass flowing outward. However, do not use it as an outlet if the outflow exceeds inflow, the flow near the boundary is much more than Mach 0.2, or the flow is choked.

    • Stagnation Inlet—set this boundary type when you know the total values for pressure and temperature at the boundary. The stagnation conditions refer to the conditions in an imaginary plenum, far upstream, in which the flow is completely at rest.

    For more information, see the specific topics in the Theory Guide under Boundary Conditions.

  2. Select the Physics Conditions > Flow Direction Specification node and set the flow direction of the fluid that enters through the inflow boundary using one of the following Methods:
    • Boundary-Normal—this is the default. The flow direction is normal to each boundary face.
    • Components—select this option to specify the inflow direction as individual angle components using the Physcs Values > Flow Direction node.

      This method is particularly useful for specifying the swirling flow for the inlet of an Axisymmetric Swirl simulation. In an axisymmetric space, all three velocity components are functions of only the axial and radial coordinates, z and r. With this method, you can define the swirling flow from the inlet by specifying the appropriate z and r components of the velocity field.

    • Angles—select this option to set the flow direction angles directly using the Physcs Values > Flow Angles node.
  3. Expand the [boundary] node and, depending on the selected boundary type, follow the appropriate steps:
    Boundary Type Steps
    Velocity Inlet To set the fluid velocity:
    • If you know all the components of the velocity vector:
      1. Select the Physics Conditions > Velocity Specification node and set Method to Components.

        Choosing this method removes the Physics Conditions > Flow Direction Specification node.

      2. Select the Physics Values > Velocity node and set the inlet velocity vector.
    • If you know the velocity magnitude based on a specified flow direction:
      1. Select the Physics Conditions > Velocity Specification node and set Method to Magnitude + Direction.
      2. Select the Physics Values > Velocity Magnitude node and set the magnitude of the velocity vector based on the Physics Conditions > Flow Direction Specification.

    For both methods, if you want to specify a velocity that varies spatially across the boundary or that depends on iteration or time step, you can do so using field functions or a table. For an example, see Example of Creating a Table for a Profile.

    For more information, see Flow Boundaries Reference—Velocity Inlet.
    Mass Flow Inlet
    1. Depending on the available flow data at the inlet, do one of the following:
      • If you know the mass flow rate per unit area:
        1. Select the Physics Conditions > Mass Flow Option node and set Specification Option to Mass Flux.
        2. Select the Physics Values > Mass Flux node and set the inlet mass flux.

          As for a Velocity Inlet, you can use a field function or a table to describe a dependence on space, iteration, or time step.

      • If you know the mass flow rate for the whole boundary:
        1. Select the Physics Conditions > Mass Flow Option node and set Specification Option to Mass Flow Rate.
        2. Select the Physics Values > Mass Flow Rate node and set the total mass per unit time (kg/s) for the whole boundary.

          You can use a field function and a table to describe a dependence on iteration or time step, but the mass flow rate cannot vary spatially across the boundary.

    2. For compressible flows and when supersonic conditions apply, select the Physics Values > Supersonic Static Pressure node and set the static pressure upstream of the inlet boundary.

    For more information, see Flow Boundaries Reference—Mass Flow Inlet.
    Stagnation Inlet
    1. Select the Physics Values > Total Pressure node and set the total pressure upstream of the boundary.

      As for a Velocity Inlet, you can use a field function or a table to describe a dependence on space, iteration, or time step.

    2. For compressible fluid flows and when supersonic conditions apply, select the Physics Values > Supersonic Static Pressure node and set the static pressure upstream of the boundary.
    3. For steady, compressible, and non-isothermal flows, you can prevent spurious numerical reflection of the solution into the solution domain:
      1. Select the Physics Conditions > Stagnation Inlet Option node and set Option to Non-Reflecting. Note that this option is only available when you use the Coupled Flow model.
      2. Select the Physics Values > Non-Reflection Mode Specification and set the Number of modes to retain. Specify a number of modes that is less than the number of cells in the circumferential direction.

    For more information, see Flow Boundaries Reference—Stagnation Inlet.