24.2.2. Transient Method

You can select the methodology for obtaining a transient solution: Time Integration, or Harmonic Balance. For additional information, see Transient versus Harmonic Solution Method in the CFX Reference Guide.

Note:  If you restart a case from existing Fourier Transformation results, you do not have to specify the minimum number of periods for the specified method.

The following topics are discussed:

 

24.2.2.1. Time Integration

The Time Period settings are used to establish a value for the time period in which each disturbance of interest cycles an integer number of times. The options for obtaining this number are:

  • Value

    This option requires that you specify a value for the time period directly.

  • Passing Period

    When this option is selected, the time period is calculated automatically as:

    • For inlet disturbance cases:

      The signal pitch divided by the relative velocity between the signal and the domain.

    • For transient rotor stator cases:

      The result of dividing the blade pitch by the absolute value of the angular velocity of the rotor(s), for the specified domain.

      Note that the blade pitch is indirectly specified via the Passages in 360 setting on the Basic Settings tab for the domain.

    This resulting time period is displayed as the Passing Period.

  • Automatic

    When this option is selected, the time period is calculated automatically. The calculation uses the information specified for each disturbance, and the blade pitch of the domain. Note that the blade pitch of a domain is indirectly specified via the Passages in 360 setting on the Basic Settings tab for the domain.

    The time period is given a value such that, during one period, each disturbance undergoes an integer number of cycles. This time period is displayed as the Passing Period.

    The Min. Timesteps / Per. is computed and displayed; it is the minimum number of time steps required to resolve each disturbance cycle period into an integer number of time steps.

In a typical transient simulation, you can specify the time step size directly. However, when using a Transient Blade Row model, the time step size is set indirectly.

The Time Steps settings control the size of a time step indirectly in one of two ways, depending on the option you choose:

  • Number of Timesteps per Period

    The time step size is computed by dividing the Time Period setting by a number, Timesteps/Period, that you specify.

  • Timestep Multiplier (available only when the Time Period option is set to Automatic)

    The time step size is computed by dividing the Time Period setting by the number of time steps per period, where the latter is the product of Min. Timesteps / Per. and Timestep Multiplier.

You must ensure that the number of time steps per period is sufficient to resolve each disturbance.

The Time Duration settings control the length of the simulation directly by controlling the number of periods. This setting also indirectly controls the total number of time steps in the simulation.

CFX-Pre automatically determines, lists, and uses the domains with the longest passing period. For the Fourier Transformation results to be sensible, CFX-Solver requires a minimum of 15 of the longest periods for the transient rotor stator cases, and a minimum of five of the longest periods for the inlet disturbance cases. Alternatively, you can use the extended list to select any other domain. If you pick a domain from the extended list, which does not have the longest passing period, then CFX-Pre will warn you. In this case, you will have to ensure that the Number of Periods per Run is increased to match what is needed for the domain with the longest passing period.

 

24.2.2.2. Harmonic Balance

Specify the number of modes to retain in the approximation.

Specify a Time Period option; the available options are described in Time Integration. Note that if Transient Blade Row Model > Option is set to Profile Transformation then, with Transient Method > Option set to Harmonic Balance, the Time Period > Option of Value can be specified with a single Period value or with a comma-delimited list of two Period values.

Note:

  • You cannot set up a case with Harmonic Balance and a Stage (Mixing-Plane) interface.

  • The harmonic balance transient method does not support 1:1 mesh connection for domain interfaces. Any interface will use GGI connection even if the mesh is conformal.

For guidelines on Harmonic Analysis, see Harmonic Analysis in the CFX-Solver Theory Guide.