## Chapter 6: Transient Blade Row Modeling

It can be computationally expensive to predict the unsteady phenomena that occur as a result of the interaction between adjacent blade rows, blade flutter, or boundary disturbances. It can be even more expensive if you need to include more than a few passages per blade row to capture the phenomena more accurately.

In typical turbomachinery applications, it is very common that one or both blade rows have a prime number of blades per wheel. Formerly in such cases, it was necessary to model the whole 360° wheel in order to attain the required level of accuracy. The Transient Blade Row models available in ANSYS CFX make it possible to reduce the size of the computational problem (memory and computational time) by solving the blade row solution for one or two passages per row, while still obtaining reasonably accurate solutions, therefore providing a solution to the unequal pitch problem between the blade passages of neighboring rows.

The Transient Blade Row models available in ANSYS CFX are:

- Profile Transformation (PT)

  The PT method overcomes the unequal pitch problem by scaling the flow profile across the blade row interfaces.

  For modeling information, see Profile Transformation in the CFX-Solver Modeling Guide.

- Time Transformation (TT)

  The TT method overcomes the unequal pitch problem by applying a time transformation to the flow equations. This transformation enables the use of simple periodic boundary conditions on the pitch-wise boundaries.

  For modeling information, see Time Transformation in the CFX-Solver Modeling Guide.

- Fourier Transformation (FT)

  The FT method uses a phase-shifted boundary condition with Fourier data compression to account for the unequal pitch between the blade rows.

  For modeling information, see Fourier Transformation in the CFX-Solver Modeling Guide.

For a description of the settings used in Transient Blade Row modeling, see Transient Blade Row Models in the CFX-Pre User's Guide.

The Fourier Transformation and Time Transformation methods can be used to apply periodic conditions that are phase-shifted in time in order to account for unequal pitches between adjacent blade rows (for transient rotor stator cases) or to account for inequality between signal and domain pitches (for boundary disturbance cases) or to account for the inter-blade phase angle (for blade flutter cases).

Turbomachinery flow is, in general, transient and periodic. You can obtain a faster solution to the final steady-periodic state by using a frequency-based solution method rather than by marching in time. The Harmonic Analysis (HA) solution method in ANSYS CFX is based on the Harmonic Balance / Time-Spectral methods and provides fast solutions for transient periodic turbomachinery flows. A use case involving HA is described in Blade Flutter using Harmonic Analysis in the CFX-Solver Modeling Guide. For details on how HA relates to a transient run with time-marching, see Transient versus Harmonic Solution Method in the CFX Reference Guide.

For more on this topic, and for theoretical information about Profile Transformation, the Time Transformation model, the Fourier Transformation model, and Harmonic Analysis, see Transient Blade Row Modeling Theory in the CFX-Solver Theory Guide.

This chapter describes:

- Transient Blade Row Modeling Terminology
- Setting up a Transient Blade Row Model
- Running and Postprocessing a Simulation that uses a Transient Blade Row Model
- Profile Transformation
- Time Transformation
- Fourier Transformation
- Guidelines for Using Transient Blade Row Features
- Use Cases

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