### 11.6.3. Selecting an Appropriate Transient Blade Row Model with Pitch Change

ANSYS CFX provides a variety of transient blade row pitch change models to be used on a diverse range of turbomachinery flow problems:

- Profile Transformation
- Time Transformation
- Fourier Transformation

#### 11.6.3.1. Profile Transformation

The Profile Transformation method should be used to predict performance and improve on steady-state stage simulations. It can be used to capture the strong interactions between components.

The Profile Transformation method can be used for single-stage or multistage machines for all flow physics including liquids and gases, at any range of Mach number.

The Profile Transformation method can be used on small to moderate pitch-ratio configurations. There is no formal limit on the pitch ratio for the Profile Transformation method, but the model error grows proportionally with the pitch ratio between components. For large pitch-ratio modeling, the error can be minimized by adding more than one blade passage per row to reduce the pitch ratio of the ensemble.

The Profile Transformation method can also be mixed freely with stage interfaces, as well as with the Time Transformation method, making it a highly useful and flexible approximation.

#### 11.6.3.2. Time Transformation

The Time Transformation method should be used to predict both performance and blade passing frequency.

For small to moderate pitch ratios (0.75-1.4), usually one blade passage per row is needed. However, this pitch-ratio range can be substantially reduced for a very low rotation machine. If stability of the method is reached, then adding a second blade passage may be necessary to regain stability.

The Time Transformation method usually reaches periodically established flow regimes after just a few blade passing periods.

The Time Transformation method does not make any prior assumptions about the main frequencies involved in the simulation. However, this pitch-change method is only applicable to compressible flows.

For multistage modeling, the Time Transformation TRS interface can be combined with Profile Transformation TRS. Sometimes the accuracy provided by the Time Transformation method can impact performance predictions. For subsonic compressors, the Time Transformation method gives a more accurate resolution of the surge point, but perhaps gives a similar performance prediction as Profile Transformation and even steady-state analysis away from stall. For transonic compressors, the Time Transformation method can give an overall improvement in the accuracy of the performance map due to the strong interactions between components. This is caused by the shocks interacting between components. The choked mass flow and the surge point may both change noticeably for the Time Transformation method compared to Profile Transformation method and steady-state method.

Time Transformation TRS can be combined with stage interfaces to model multistage turbomachines.

#### 11.6.3.3. Fourier Transformation

The Fourier Transformation method is similar to the Time Transformation method however, it can be used on very large pitch ratio configurations (for example, fan inlet distortion problems or fan bypass regions where the pitch ratio between the fan blade and the downstream stator row is very large).

The Fourier Transformation method is capable of modeling both compressible and incompressible flows. For optimum computational efficiency, it is recommended that you use the Fourier Transformation method when the pitch ratio is too large to be modeled sensibly with the Time Transformation method.

The Fourier Transformation method is the best method to use for blade flutter analysis. The Fourier Transformation method allows you to specify the nodal diameter (phase shift) for fluttering blade rows.

A transient blade row simulation with the Fourier Transformation method is more efficient with respect to the reference solution when the machine has a large number of blades per row. Therefore, it may not be useful to perform transient blade row simulation with the Fourier Transformation method on turbomachines with a low blade count.

The Fourier Transformation TRS interface can be combined with stage interfaces to model multistage turbomachines but can not be combined with Profile Transformation TRS or Time Transformation TRS interfaces.

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