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Figure 7.17: Process Flow for a Prestressed Full Harmonic Cyclic Symmetry Analysis

7.3.5.2. Solving a Mode-Superposition Harmonic Cyclic Symmetry Analysis

The mode-superposition method sums factored mode shapes (obtained from a modal analysis) to calculate the harmonic response. The procedure (in the most general case) consists of these steps:

1.Build the cyclic model

2.Perform a static cyclic symmetry analysis to obtain the prestressed state

3.Perform a linear perturbation modal cyclic symmetry analysis

4.Restart the modal analysis to create the desired load vector from any element loads (for example, pressures)

5.Obtain the mode-superposition harmonic cyclic symmetry solution

6.Review the results

The following flowchart illustrates the process involved in a prestressed mode-superposition harmonic cyclic symmetry analysis. For a non-prestressed solution, you may skip step 2, so that step 3 becomes a stress-free modal analysis.

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Solving a Cyclic Symmetry Analysis

The first step, building the cyclic model, is described in Cyclic Modeling (p. 164). The remaining steps are described below.

Figure 7.18: Process Flow for a Pre-Stressed Mode-Superposition Harmonic Cyclic Symmetry Analysis

7.3.5.2.1. Perform a Static Cyclic Symmetry Analysis to Obtain the Prestressed State

Solving a Static Cyclic Symmetry Analysis (p. 176) describes how to obtain the static solution that will compute the prestressing. In the static analysis:

•Only cyclic loading is permitted

•You must use the cyclic option (CYCOPT,MSUP,ON)

•If you wish to apply real or imaginary pressure loading in the downstream harmonic solution, you must define the SURF154 elements at this stage to facilitate the load application. These elements must be defined before the CYCLIC command is issued.

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7.3.5.2.2. Perform a Linear Perturbation Modal Cyclic Symmetry Analysis

This step is outlined in Solving a Large-Deflection Prestressed Modal Analysis (p. 181), and has the following restrictions and guidelines:

•Only Block Lanczos is supported (MODOPT,LANB).

•The VT Accelerator method is not supported (CYCOPT,VTSOL).

•You must use MXPAND to write the modes to the results file.

•The frequency range on MXPAND is ignored. Specify the frequency range using the MODOPT command instead.

•Be sure to extract all the modes that may contribute to the harmonic response. As a general guideline, modes contributing to the harmonic response will fall in the range ½ Ω to 2 Ω, where Ω is the harmonic frequency (HARFRQ) used in the subsequent harmonic solution.

•Residual vectors (RESVEC) are not supported.

•Enforced motion (MODCONT,,ON) is not supported.

•Mode selection is not supported.

•Stress modes (MXPAND,ALL,,,YES,,YES) are not supported.

An even number of modes (per harmonic index) is always computed. If you specify an odd number of modes on the MODOPT command, it is increased by 1. Only the base sector solution is stored on the MODE and RST files, so they are significantly smaller than the non-mode-superposition modal files (CYCOPT,MSUP,OFF).

If you are performing a stress-free modal analysis, make sure you use the cyclic option (CYCOPT,MSUP,ON).

7.3.5.2.3. Restart the Modal Analysis to Create the Desired Load Vector from Element Loads

If you need to apply harmonically varying elements loads (for example, pressures), specify them in the modal analysis. The program ignores the loads for the modal solution, but calculates a load vector and writes it to the mode shape file (Jobname.MODE). You can also generate multiple load vectors. The load vectors created can then be scaled and used in the harmonic solution. For more information, see Modal Analysis Restart in the Basic Analysis Guide. The following limitations apply:

•You may not introduce additional elements (such as SURF154) to facilitate application of the loads. You must apply the loads directly to the existing base elements and nodes. Specify any load elements in the static step (or prior to the modal solution if there is no prestressing step required) before the CYCLIC command is issued.

•Nodal loads (F) can be applied directly in the harmonic analysis if desired, rather than creating a modal load vector.

It is possible to skip this step and add the loads directly in the modal analysis step. If you choose to skip this step, only one modal load vector can be applied.

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Solving a Cyclic Symmetry Analysis

You may use the /MAP processor to map pressure loads from a CFD analysis, and from a CFX Transient Blade Row analysis in particular, to use in the harmonic analysis. See Unidirectional Pressure Mapping: CFD to Mechanical APDL in the Coupled-Field Analysis Guide.

7.3.5.2.4. Obtain the Mode-Superposition Harmonic Cyclic Symmetry Solution

In this step, the program uses mode shapes extracted by the modal solution to calculate the harmonic response. The following requirements apply:

•The mode shape file (Jobname.MODE) must be available.

•The full file (Jobname.FULL) must be available.

•The database must contain the same model from which the modal solution was obtained. Additionally, the following limitations apply:

•The range of modes on the HROPT command is ignored. All the modes from the modal analysis are considered.

•The cluster option on the HROPT command is not supported.

•Tabular loading with respect to frequency is not supported in the cyclic symmetry mode-superposition harmonic solution. Only tabular loading with respect to location is supported.

•When using the NSUBST command, the NSBMX, NSBMN, and Carry arguments are not supported. The following inputs must also be provided:

•Apply the required load on the base sector. Only nodal forces and the load vector created in the modal analysis are valid. Use the LVSCALE command to apply the load vector from the modal solution. Note that all loads from the modal analysis are scaled, including forces and pressures. To avoid load duplication, delete any loads that were applied in the modal analysis.

•Specify the engine order of the excitation (CYCFREQ,EO). Typically, the engine order is simply a count of the number of stators, combustion nozzles, etc., that cause the disturbance. All loads from the modal solution and nodal loads that are applied during a given load step will be applied as engine order loads. The program computes the “aliased” engine order (including its sign) internally. An engine order excitation typically occurs due to circumferential disturbances in the flow field, for instance from upstream stators

or values.

•Specify the number of harmonic solutions to be calculated (NSUBST). The solutions (or substeps) will be evenly spaced within the specified frequency range (HARFRQ). For example, if you specify 10 solutions

in the range 30 to 40 Hz, the program will calculate the response at 31, 32, 33, ..., 39, and 40 Hz. No response is calculated at the lower end of the frequency range.

•Damping in some form should be specified; otherwise, the response will be infinity at the resonant frequencies. ALPHAD and BETAD result in a frequency-dependent damping ratio, whereas DMPRAT specifies a constant damping ratio to be used at all frequencies. DMPSTR specifies a constant structural damping coefficient. MDAMP cannot be used to specify a modal damping ratio. See Damping in the Structural Analysis Guide for further details.

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