### 6.7.3. Guidelines for using the Fourier Transformation Feature

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.

However, in general for compressible flow and with small pitch ratios (typically between 0.75 and 1.35) it is recommended that you use the Time Transformation method instead because the calculation will be more efficient than when using the Fourier Transformation method.

- When not using the Turbomachinery wizard in CFX-Pre, an additional step is needed for creating turbo topology; you must create two instances of the blade passage.

  There are two options by which you can create a copy of the original passage:

  - Perform a rotation on the copy of the original geometry (mesh) by its own pitch. The rotation angle must be specified with many decimal digits for minimum round-off error.

  - Perform a turbo rotation and update the number of passages to model and the number of passages in 360° in each domain.

- When running a case in serial, it is highly recommended that you run CFX-Solver using double precision. This recommendation also applies for steady-state cases that are used to initialize Fourier Transformation cases.

- When running a case in parallel:

  - You may run CFX-Solver using single precision, but it is highly recommended that the partitioning is done using double precision. This recommendation also applies for steady-state cases that are used to initialize Fourier Transformation cases.

  - Each pair of sampling/phase shift corrected interfaces must belong to a single domain. This is a condition that should be guaranteed if CFX-Pre Turbo Wizard is used to set up the case.

- When setting up a Fourier Transformation case, any domain interface that is part of the Fourier Transformation Transient Blade Row setup should not span more than one domain.

- It is very important that the boundary condition is periodic in time for the passages being modeled. In the case of blade flutter, this can be achieved by selecting a number of timesteps that is a multiple of .

- In cases where you observe solution instabilities, you should apply frequency filtering. Instabilities are typically seen in elongated domains where the periodic signal becomes very weak at the furthest point away from the source of the disturbance. Frequency filtering removes unwanted frequencies from the solution thereby avoiding the onset of instability. The setting for enabling frequency filtering is described in Frequency Filtering in the CFX-Pre User's Guide.

- Instabilities could also exist related to the mass flow Fourier reconstruction for cases with high speed flow where a shock hits the periodic interface. The recommended workaround is to turn off the mass flow Fourier reconstruction with one of the following expert parameters:

  - For inlet disturbance and flutter cases: `ps reconstr mflow from sfc = f`

  - For cases involving a transient rotor stator Fourier interface: `ps reconstr mflow from sfc trs = f`

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