Transforming time-domain to frequency

Two operations are available as part of the Model and Load Pre-processing solution process to let you convert time-domain data to frequency domain data or time data to waterfall of time data.

• Time to Waterfall of Time

• Time Signal

For example, you can convert time series data from a Motion results file for an engine run-up sweep to a set of frequency spectra (one set of spectra for each RPM) that can be used as loads for the Simcenter Nastran frequency response solutions (SOL 108 and SOL 111).

You add the operations to Model and Load Pre-Processing containers in the Simulation Navigator. The nodes under the Model and Load Pre-Processing containers represent the sequence in which the operations are applied to the input file. You can select and set the parameters for each operation and then solve the solution process to convert the input file to the desired state.

Time to Waterfall of Time operation

In the Time to Waterfall of Time operation, you start with data with time as an independent axis, and select an RPM signal (RPM versus time) to describe the RPM changes. You define the desired RPM range to split the input data into multiple segments, and you link each segment with a particular RPM value.

The software performs the time segmentation to convert time data to waterfall of time in the following ways:

• The computation RPMs are identified from the RPM step definition, which you can set in the Step Definition group of Time to Waterfall of Time dialog box.

• The computation RPM-time pairs are then identified from the RPM function, which you can set in the Select the RPM Function group in Time to Waterfall of Time dialog box.

• For each function in the time-dependent input data from the data source:The time segment is identified for each computation RPM-time pair according to the frame size.If a full frame is not found, that particular time segment is ignored, and the waterfall values for the corresponding rpm value are not generated.If a full frame is found, a new function is created with:The definition from the input data function with an extra RPM variable.Independent axis shifted to start from 0 seconds.The input data value-time pairs from the selected segment.

For more information, see Time to Waterfall of Time dialog box.

Time Signal operation

In the Time Signal operation, you start with the data with time as the independent axis, set the parameters for input signal manipulation, averaging, and windowing, and take the Fast Fourier Transform (FFT) of the time data to convert it into frequency domain. The FFT is single sided and corrected with a scaling factor 2 / N, where N is the number of samples of the original time signal.

The software performs the conversion of time data to frequency data in the following ways:

• The input range for the time data is identified according to the time limits, and the number of times a signal repeats itself.Note: Optionally, you can specify how many times to repeat the input signal before other operations, such as FFT. Typically, you repeat a time signal to increase the frequency resolution of the FFT.

• The Fourier transform options are identified by window type, correction mode, averaging, and pre- and post-padding.Note: With post-padding, you can append zero values (also known as zero-padding) to the time signal until a specified frequency resolution is achieved. This lowers the frequency resolution of the FFT. The software executes the zero-padding after it windows the time signal. Also, you can pre-pad (prefix) the time signal to counteract sampled time signals that start at non-zero time values. This indicates that the signal starts with a certain time delay. You can handle this time delay by prefixing the time signal with an appropriate number of zeros equal to the first time sample value divided by the time step.

• Frequency filtering is applied on the computed values.

Note:

For waterfall of time data, the workflow steps are repeated for each RPM.

For more information, see Time Signal Processing dialog box.

Note:

• Input data from the data source must be time dependent and with a single abscissa.

• The time values must be equidistant with a 25% tolerance relative to the mean time difference between all the samples.

For an example to convert time-domain data to frequency domain, see Transform Motion result files for use in vibro-acoustic response.

How do I

Transform Motion result files for use in vibro-acoustic response

Learn more

Processing Motion results for use in a vibro-acoustic response

Motion translate workflow in Pre/Post for vibro-acoustic loading

Motion MPF transform workflow for vibro-acoustic loading

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Command reference

Pre/Post video examples

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Transforming time-domain to frequency, Simcenter 3D 2021.1 Series

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