### 6.8.5. Case 5: Transient Rotor Stator Multi-Stage Cases

ANSYS CFX enables you to use Time-Transformation Transient Rotor Stator interfaces to model multistage turbomachinery configurations. In general, the Time Transformation method is used for small-to-moderate pitch-change, multistage turbomachines. Note that the Time Transformation method cannot be used with incompressible cases. The Time Transformation method is not recommended when the rotation rate of the rotor is very low.

The multistage modeling capability of the Time Transformation method in ANSYS CFX is demonstrated by the examples in the following sections.

- Modeling a Single-pitch-ratio Multistage Turbomachine using the Time Transformation Method
- Modeling 1.5 Stages with Two Different Pitch Ratios using the Time Transformation Method
- Modeling a multistage turbomachine with Time Transformation TRS and other interfaces: Profile Transformation & Stage
- Modeling a multistage turbomachine with Time Transformation TRS and single-sided Time Transformation interfaces (STT-TRS)

#### 6.8.5.1. Modeling a Single-pitch-ratio Multistage Turbomachine using the Time Transformation Method

In this special situation, where every other row of the turbomachine has same number of blades, we have technically the configuration of a single disturbance being imposed in each passage. Therefore, there is only one pitch ratio present in the configuration. An example of such a case would be a machine in which all stator rows have the same number of blades and all rotors rows have the same number of blades, but the number of blades differs between the stator rows and the rotor rows. This special case can be modeled accurately by using TT correction on all the TRS interfaces (TT-TRS) between rows. Figure 6.6: 1.5 stage machine with equal number of blades in rows 1 & 3 but different than in row 2. Flow can be accurately modeled via a sequence of TT-corrected TRS interfaces. demonstrates an example of a 1.5 stage turbomachine with an equal number of blades in the first and third rows, but a different blade count in the middle row. TT-TRS interfaces have been applied sequentially to this configuration.

To set up this case:

1. Set the two rotor/stator interfaces (in the example of Figure 6.6: 1.5 stage machine with equal number of blades in rows 1 & 3 but different than in row 2. Flow can be accurately modeled via a sequence of TT-corrected TRS interfaces., the IGV/Rotor and the Rotor/Stator interfaces) as Transient Rotor Stator (TRS).

2. In the Outline tree view, edit the `Transient Blade Row Models` object under the applicable flow analysis.

3. Create two disturbances of the same type (TT) and assign each of them to the respective TRS interfaces created in step 1.

Figure 6.6: 1.5 stage machine with equal number of blades in rows 1 & 3 but different than in row 2. Flow can be accurately modeled via a sequence of TT-corrected TRS interfaces.

This modeling method will allow for capturing accurate blade passing signals in all the rows as well as obtaining accurate machine performance. The limitation on acceptable pitch ratio with use of the TT method still applies to this modeling technique.

#### 6.8.5.2. Modeling 1.5 Stages with Two Different Pitch Ratios using the Time Transformation Method

In this configuration, each row has a different pitch value or two distinct pitch ratios across the machine, as shown in the following figure. The Time Transformation TRS interface can also be applied sequentially between the IGV and Rotor, and between the Rotor and Stator. This extension of the Time Transformation TRS interface allows for capturing both upstream and downstream blade passing frequencies in the rotor passage, making this modeling method useful for forced response analysis.

Figure 6.7: 1.5 stages with differing blade numbers between rows 1, 2 & 3. Flow can be modeled via a sequence of TT-TRS interfaces.

The following procedure is necessary to model a 1.5 stage turbomachine with two different pitch ratios:

1. Set all the rotor/stator interfaces to be of the TRS type.

2. In the Outline tree view, edit the `Transient Blade Row Models` object under the applicable flow analysis.

3. Create Time Transformation disturbances for each of the TRS interfaces that are corrected by Time Transformation.

This modeling strategy will allow for obtaining accurate machine performance as well as capturing blade passing signals in rows 1 and 2. On the other hand, row 3 may contain polluted frequency content due to modeling approximation. The limitation on acceptable pitch ratio with use of the Time Transformation method still applies to this modeling technique.

Important: It is not recommended that you use Time Transformation TRS for every rotor-stator interface beyond a 1.5 stage configuration. For turbomachines with more than 1.5 stages, we recommend the modeling strategy shown in Modeling a multistage turbomachine with Time Transformation TRS and other interfaces: Profile Transformation & Stage.

#### 6.8.5.3. Modeling a multistage turbomachine with Time Transformation TRS and other interfaces: Profile Transformation & Stage

Accurate multistage performance can be predicted by using a combination of Time Transformation TRS and other interface models such as Profile Transformation TRS or Stage interface (mixing-plane). For maximum accuracy, it is recommended that you use Time Transformation TRS at each rotor passage inlet, and a Profile Transformation TRS or Stage interface at the rotor passage outlet. The idea is to minimize modeling error at the TRS interfaces where flow shocks might be crossing by using Time Transformation TRS modeling at those interfaces. The next two figures illustrate two common possible combinations of interfaces for modeling multistage configurations.

Figure 6.8: Multistage modeled with a combination of TT and PT interfaces. For high speed flows, the TT-TRS interfaces should be placed where shocks are crossing between the adjacent passages. This is typically happening at the interface between the exit of the stator row and the inlet of the rotor row.

Figure 6.9: Multistage modeled with a combination of TT, PT and Stage interfaces

The following procedure is necessary to model multistage turbomachines with Time Transformation and Profile Transformation interfaces:

1. Set all the rotor/stator interfaces to be of the TRS type.

2. In the Outline tree view, edit the `Transient Blade Row Models` object under the applicable flow analysis.

3. Create Time Transformation disturbances for the TRS interfaces that are corrected by Time Transformation.

This modeling strategy will allow for obtaining accurate machine performance for turbines and compressors. However, this modeling strategy is not intended to be used for capturing accurate blade passing signals throughout the machine due to the modeling approximation of the Profile Transformation method. The limitation on acceptable pitch ratios using the Time Transformation method still applies to this modeling technique.

#### 6.8.5.4. Modeling a multistage turbomachine with Time Transformation TRS and single-sided Time Transformation interfaces (STT-TRS)

For multistage machines, an alternative to using PT-TRS or TT-TRS models on both sides of TRS interfaces is to apply the so-called single-sided TT interface (STT-TRS). The STT-TRS interface model applies the PT model to one side of the interface (typically the upstream side) and the TT model to the other side (typically the downstream side).

Also, you can reverse this correction if necessary (for example, if you want to capture signals from the downstream component). The advantage of using STT-TRS over PT-TRS is that STT-TRS maintains the upstream blade wake thickness as it convects downstream. This modeling strategy results in more accurate accounting of machine performance and losses.

For maximum accuracy, it is recommended that you use TT-TRS at the rotor passage inlet and STT-TRS on the rotor passage exit. Figure 6.10: Multistage modeled with combination of TT-TRS and STT-TRS interfaces illustrates how the STT-TRS is used with TT-TRS in modeling a multistage machine.

Figure 6.10: Multistage modeled with combination of TT-TRS and STT-TRS interfaces

It is also possible to apply sequentially several STT interfaces, as shown in Figure 6.11: Multistage modeled with several STT-TRS interfaces in sequence.

Figure 6.11: Multistage modeled with several STT-TRS interfaces in sequence

The following procedure is necessary to model multistage turbomachines with TT-TRS and STT-TRS interfaces:

1. Set all the rotor/stator interfaces to be of the TRS type.

2. In the Outline tree view, edit the `Transient Blade Row Models` object under the applicable Flow Analysis object.

3. Create TT disturbances for all of the TRS interfaces that are corrected by either TT or STT.

4. For the STT-TRS interfaces, the TT disturbances have to be modified on one of the domain interface sides. On the side of the interface not modeled by TT (the PT side as explained above), the side option should be changed to `None`. The setting for the other side can be left as it is; the TT model will be applied to that side. Note that the interface side name is displayed in the transient rotor stator domain interface’s Basic Settings tab.

This modeling strategy can improve the accuracy of machine performance predictions for both turbines and compressors. However, this type of modeling is not intended for capturing accurate blade passing signals throughout the machine due to the modeling approximation of the STT-TRS method. The limitation on acceptable pitch ratio with use of the TT method still applies to this modeling technique.

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