Non-Reflecting Boundary Conditions Option
One of the options that may be used at pressure outlets is non-reflecting boundary conditions (NRBC). This option is only available when the density-based solver and ideal gas law are used. The NRBC option is used when waves are made to pass through the boundaries while avoiding false reflections. Details of non-reflecting boundary conditions can be found in Section 7.23.2 of this chapter.
Target Mass Flow Rate Option
Two methods (Method 1 and Method 2) are available for adjusting the pressure at a pressure-outlet zone in order to meet the desired mass flow rate. Both methods are based on the simple Bernoulli's equation. However, they differ in the internal iteration strategy for obtaining the change in pressure on a pressure-outlet zone. In general, the target mass flow rate is achieved by adjusting the pressure value at the pressure-outlet zone up and down at every iteration. This is done in accordance with one of the two available methods until the desired target mass flow rate is obtained.
The change in pressure based on Bernoulli's equation is given by the following equation:
where is the change in pressure, is the current computed mass flow rate at the pressure-outlet boundary, is the required mass flow rate, is the computed average density at the pressure-outlet boundary, and is the area of the pressure-outlet boundary.
The default method, Method 1, should suffice in obtaining a converged solution on the targeted mass flow rate. However, if convergence difficulties are encountered while using the default method, then the user may want to select the alternate method, Method 2. There are other solution strategies that may be used if convergence difficulties are encountered, which will be discussed at the end of this section.
The target mass flow rate option can be activated from the Pressure Outlet boundary panel by selecting the target mass flow rate button. This option will allow you to specify either a constant value or attach a UDF to set the target mass flow rate.
| Note that the target mass flow rate option is not available with multiphase flows or when any of the non-reflecting boundary conditions models are used. Moreover, if the pressure-outlet zone is used in the mixing-plane model, the target mass flow rate option will not be available for that particular zone.
Target Mass Flow Rate Settings
The settings for the target mass flow rate option can be accessed from the target-mass-flow-rate-settings text command:
define boundary-conditions target-mass-flow-rate-settings
There are two options under this menu:
Solution Strategies When Using the Target Mass Flow Rate Option
If convergence difficulties are encountered or if the solution is not converging at the desired mass flow rate, then try to lower the under-relaxation factor from the default value. Otherwise, you can use the alternate method to converge at the required mass flow rate. Also, in some cases you may want to limit the pressure values in the zone by adjusting the minimum or maximum pressure limits (note that this limit is applied to all pressure-outlet zones when the target mass flow rate option is selected). All of the above parameters can be accessed from the target-mass-flow-rate-settings text command.
In some cases, you may want to switch off the target mass flow rate option initially, then guess an exit pressure that will bring the solution closer to the target mass flow rate. After the solution stabilizes, you can turn on the target mass flow rate option and iterate to convergence. For many complex flow problems, this strategy is usually very successful.
The use of Full Multigrid Initialization is also very helpful in obtaining a good starting solution and in general will reduce the time required to get a converged solution on a target mass flow rate. For further information on Full Multigrid Initialization, see Section 25.15.
Setting Target Mass Flow Rates Using UDFs
For some unsteady problems it is desirable that the target mass flow rate be a function of the physical flow time. This enforcement of boundary condition can be done by attaching a UDF with
DEFINE_PROFILE functions to the target mass flow rate field.
| Note that the mass flow rate profile is a function of time and only one constant value should be applied to all zone faces at a given time.
An example of a simple UDF using a DEFINE_PROFILE that will adjust the mass flow rate can be found here in the UDF Manual.