### 6.8.6. Case 6: Transient Rotor Stator Cases with Asymmetric Flow

The Fourier Transformation Transient Rotor Stator (FT-TRS) interface can handle flow cases in which an impeller or rotor is attached to a full 360° domain, where the impeller or rotor may experience extremely large inflow variation, such as a once-per-revolution or three-per-revolution inflow disturbance.

Note: An FT-TRS interface enables the modeling of just two impeller or rotor blades connected to the full 360° domain. There is little or no benefit in using the FT-TRS interface in this way for cases that involve a small number of impeller or rotor blades.

The following cases are discussed:

- Modeling an Impeller in a Vaneless Volute
- Modeling an Impeller in a Vaned Volute

#### 6.8.6.1. Modeling an Impeller in a Vaneless Volute

In this case, the impeller experiences a long-period disturbance due to the asymmetry of the volute.

To set up an impeller in a vaneless volute:

1. Specify the FT-TRS interface as the interface between the volute and the blade passages.

2. Create a disturbance of type `Rotor Stator`:

   1. On the impeller side, select `Phase Corrected` and then select the phase-corrected and sampling interfaces (which are between impeller blades).

   2. On the volute side, select `None`.

      The side of the FT-TRS interface that is attached to a 360° sector will have neither a sampling domain interface nor a phase corrected interface.

   The disturbance accounts for the signal from the volute.

Note that a similar procedure could be used to set up a case involving a fan in a crosswind.

#### 6.8.6.2. Modeling an Impeller in a Vaned Volute

In this case, the impeller experiences two flow disturbances: one short-period disturbance due to the vanes, and one long-period disturbance due to the asymmetry of the volute.

To set up an impeller in a vaned volute:

1. Follow the steps for a vaneless volute.

   Because of the presence of the vanes, the disturbance is automatically configured to account for the signal from the vanes.

2. To account for the signal from the volute, create a (second) disturbance of type `Rotational Flow Boundary Disturbance`:

   1. On the impeller side, select `Phase Corrected` and then select the phase corrected and sampling interfaces (which are between impeller blades).

   2. Specify that the number of passages in 360° (under the external passage definition) is one, to capture the once-per-revolution disturbance of the volute.

   3. Set the Signal Motion option to `Stationary` to indicate that the signal comes from the volute side of the Rotor Stator interface.

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