24.2.1. Transient Blade Row Model Settings

The Transient Blade Row Model > Option setting can have one of the following values:

None
This option disables Transient Blade Row modeling. It is useful when running transient simulations without any transformations.
Profile Transformation
The Transient Method settings (Time Period, Time Steps, and Time Duration) must be configured. These settings are described in Transient Method.
Time Transformation
To help the solver apply the Time Transformation method to the applicable domains, you must accurately characterize a disturbance. For details, see Time Transformation Disturbance Settings.
The Transient Method settings (Time Period, Time Steps, and Time Duration) must be configured. These settings are described in Transient Method.
Fourier Transformation
To help the solver apply the Fourier Transformation method to the applicable domains, you must accurately characterize a disturbance. For details, see Fourier Transformation Disturbance Settings.
The Transient Method settings (Time Period, Time Steps, and Time Duration) must be configured. These settings are described in Transient Method.

24.2.1.1. Time Transformation Disturbance Settings

The Time Transformation method improves solution accuracy over that of the Profile Transformation method when there is unequal pitch between the blade row subsets on each side of a domain interface that uses the Transient Rotor Stator frame change/mixing model.

A given domain can receive a disturbance from:

An upstream or downstream domain interface that uses the Transient Rotor Stator frame change/mixing model. For example, the wake from a blade can pass through a domain interface and disturb the downstream domain(s).
Inlet or outlet (or opening) boundaries. For example, a boundary condition could use a CEL function that depends on space and time in order to mimic the wake of an upstream blade.
The Time Transformation method can be applied to a domain that has an inlet or outlet disturbance.

To characterize a disturbance, create a new item by clicking Add new item , enter a unique name for the disturbance, then specify information about the disturbance in one of the following ways:

Rotor Stator option
This option can be applied only when the disturbance originates from a domain interface that uses the Transient Rotor Stator frame change/mixing model.
After selecting this option:
1. Select a Domain Interface from the existing domain interfaces that use the Transient Rotor Stator frame change/mixing model.
2. For each side of the interface (Side 1 and Side 2, which correspond to the first and second sides of the selected domain interface, respectively) specify the Option setting.
The Option setting for each side is used to determine the domain(s) to which the Time Transformation method is applied. The options for each side are:
- Automatic
  This option causes the solver to identify the domain(s) directly touching the applicable side of the interface. This option is suitable when all the domains that are affected by the disturbance are in contact with the applicable side of the interface.
- Domain List
  This option enables you to manually select the domain(s) that are on the applicable side of the interface and that are affected by the disturbance. You might need to use this option if, for example, a blade row is modeled with two domains with one being downstream of the other, in which case you would select both domains as being on one side of the interface, even though one of those domains does not touch the interface.
- None
  This option prevents the present disturbance (but not any other disturbances) from applying the Time Transformation method to any domain that is on the applicable side of the interface.
For details, see Case 1: Transient Rotor Stator Single Stage in the CFX-Solver Modeling Guide.
Rotational Flow Boundary Disturbance option
Use this option to characterize a disturbance that originates from a boundary condition (for example, an inlet or outlet boundary condition that is specified using one or more CEL expressions that depend on space and time).
After selecting this option:
1. Set Domain Name to the name of the domain(s) that are affected by the disturbance.
2. Set the Signal Motion settings:
  - Setting Option to Stationary causes the signal to be stationary in the absolute (stationary) frame of reference.
  - Setting Option to Rotating enables you to select a coordinate frame as a way of specifying the signal motion. Any boundary using this coordinate frame will be made transient periodic with the period calculated from the pitch and rotating speed of the signal. For details on moving coordinate frames, see Frame Motion Settings.
3. Specify information about the external blade row that creates the disturbance in the External Passage Definition settings:
  - For a case with rotational periodicity, specify Passages in 360: the number of passages in 360° of the external blade row that creates the disturbance. Also specify Pass. in Component: the number of passages in the external blade row that creates the disturbance.
4. For a case with rotational periodicity, the Passages in 360 and Pass. in Component settings on the domain Basic Settings tab apply.
For details, see Case 2: Flow Boundary Disturbance in the CFX-Solver Modeling Guide.

24.2.1.2. Fourier Transformation Disturbance Settings

24.2.1.2.1. Basic Settings Tab

The Fourier Transformation method improves solution accuracy over that of the Profile Transformation method when there is unequal pitch between the blade row subsets on each side of a domain interface that uses the Transient Rotor Stator frame change/mixing model.

The Fourier Transformation method works for compressible as well as incompressible flows and for all ranges of disturbance pitch ratios from small to very large. For example, the Fourier Transformation method can handle a once-per-revolution disturbance on a blade passage, which is a situation that cannot be handled with the Time Transformation method.

A given domain can receive a disturbance from:

Inlet or outlet (or opening) boundaries. For example, a boundary condition could use a CEL function that depends on space and time in order to mimic the wake of an upstream blade.
An upstream or downstream domain interface that uses the Transient Rotor Stator frame change/mixing model. For example, the wake from a blade can pass through a domain interface and disturb the downstream domain(s).

To characterize a disturbance, create a new item by clicking Add new item , enter a unique name for the disturbance, then specify information about the disturbance in one of the following ways:

Rotor Stator option
This option can be applied only when the disturbance originates from a domain interface that uses the Transient Rotor Stator frame change/mixing model.
After selecting this option:
1. Select a Domain Interface from the existing domain interfaces that use the Transient Rotor Stator frame change/mixing model.
2. For each side of the interface (Side 1 and Side 2, which correspond to the first and second sides of the selected domain interface, respectively) select the Phase Corrected option and then specify the Phase Corrected Intf. and Sampling Domain Intf. settings.
  The Phase Corrected Intf. setting specifies the periodic GGI-only interface (having rotational periodicity) to which to apply the phase shift with respect to the sampling domain interface signal.
  The Sampling Domain Intf. setting specifies the non-periodic non-frame-change GGI-only interface on which the Fourier coefficients will be accumulated. This is the interface between a pair of adjacent blades in a given component.
For details, see Case 1: Transient Rotor Stator Single Stage in the CFX-Solver Modeling Guide.
Rotational Flow Boundary Disturbance option
Use this option to characterize a disturbance that originates from a boundary condition (for example, an inlet or outlet boundary condition that is specified using one or more CEL expressions that depend on space and time).
After selecting this option:
1. Specify the Phase Corrected Intf. and Sampling Domain Intf. settings.
  The Phase Corrected Intf. setting specifies the periodic GGI-only interface (having rotational periodicity) to which to apply the phase shift with respect to the sampling domain interface signal.
  The Sampling Domain Intf. setting specifies the non-periodic non-frame-change GGI-only interface on which the Fourier coefficients will be accumulated. This is the interface between a pair of adjacent blades in a given component.
2. Set the Signal Motion settings:
  - Setting Option to Stationary causes the signal to be stationary in the absolute (stationary) frame of reference.
  - Setting Option to Rotating enables you to select a rotating coordinate frame as a way of specifying the signal motion. Any boundary using this coordinate frame will be made transient periodic with the period calculated from the pitch and rotating speed of the signal. For details on moving coordinate frames, see Frame Motion Settings.
3. Specify information about the external blade row that creates the disturbance in the External Passage Definition settings:
  - For a case with rotational periodicity, specify Passages in 360: the number of passages in 360° of the external blade row that creates the disturbance. Also specify Pass. in Component: the number of passages in the external blade row that creates the disturbance.
4. For a case with rotational periodicity, the Passages in 360 and Pass. in Component settings on the domain Basic Settings tab apply.
For details, see Case 2: Flow Boundary Disturbance in the CFX-Solver Modeling Guide.
Blade Flutter option
Use this option to characterize a disturbance that originates from the periodic motion of a wall boundary.
After selecting this option:
1. Specify the Phase Corrected Intf. and Sampling Domain Intf. settings as applicable.
  The Phase Corrected Intf. setting specifies the periodic GGI-only bitmap-intersection-method interface (having rotational periodicity) to which to apply the phase shift with respect to the sampling domain interface signal.
  The Sampling Domain Intf. setting specifies the non-periodic non-frame-change GGI-only interface on which the Fourier coefficients will be collected. This is the interface between a pair of adjacent blades in a given component.
2. Specify the Blade Boundary.
  The Blade Boundary setting specifies the wall boundary with a Periodic Displacement mesh motion boundary condition that creates the disturbance. This boundary contains the information required (frequency, and phase angle) to compute the phase shift to be applied on the rotationally periodic boundaries.
For details, see Case 3: Blade Flutter in the CFX-Solver Modeling Guide.

Note: You must ensure that the disturbance is periodic in time when using the Rotational Flow Boundary Disturbance option.

24.2.1.2.2. Advanced Options Tab

It is possible to specify some further parameters to control the Fourier Transformation model. These parameters are specified on the Advanced Options tab of the Transient Blade Row Models details view. This tab becomes visible once you set Transient Blade Row Model > Option to Fourier Transformation.

On the Advanced Options tab, you can select Fourier Transformation Control > Frequency Filtering.

24.2.1.2.2.1. Frequency Filtering

Frequency filtering is available on the Advanced Options tab under Fourier Transformation Control. Frequency filtering enables filtering of undesired frequencies that might be present in the solution and that could trigger instabilities. The Scaled Domain Damping option requires that you set a value for Damping Scale Factor. This value is a dimensionless positive number that has a lower bound of 0.0, which specifies that there is no filtering. The recommended value for Damping Scale Factor is 1.0.