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*Spectrum |
 
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*Spectrum
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*Spectrum |
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To request the creation of spectrum plots.
Refer to Database Postprocessing for further information on this feature.
The keyword begins with an optional line defining miscellaneous parameters for all the spectrum plots. This is then followed by blocks of lines repeated as often as necessary to request the actual spectrum plots.
Line defining miscellaneous parameters:
[STEP=Time Step] [, ENSEMBLES=No. of Ensembles] [, START=Start Time] [, CUTOFF=Cut-off Frequency]
Block requesting kinematic spectra. Lines 2 and 3 can be repeated as often as necessary. Line 3 is optional.
TYPE=KINEMATIC
Node (Number or Label), DOF [, Scale] [, PARA=Parameter] [, UNITS=Unit]
[TITLE=Figure Title]
Block requesting force spectra. Lines 2 and 3 can be repeated as often as necessary. Line 3 is optional.
TYPE=FORCE
Element (Number or Label), Local Node, Variable [, Scale] [, LOCATION=Location][, UNITS=Unit]
[TITLE=Figure Title]
Refer to Force Variable Input for further information on acceptable Variable values. Refer to Location Parameter Input for further information on the significance of the Location input. The significance of the Local Node Input used during postprocessing depends on whether element based outputs are stored on a node or integration point basis.
Block requesting reaction spectra. Lines 2 and 3 can be repeated as often as necessary. Line 3 is optional.
TYPE=REACTION
Node (Number or Label), DOF [, Scale] [, UNITS=Unit]
[TITLE=Figure Title]
Block requesting a spectrum of wave elevation. Line 3 is optional.
Scale [, Target Spectrum Flag] [, UNITS=Unit]
[TITLE=Figure Title]
A Time Step value is required in a random sea dynamic analysis with a variable time-step; it is ignored in the case of an analysis with a fixed step. No. of Ensembles defaults to 4. Start Time defaults to the analysis start time. If no Cut-off Frequency is specified, Flexcom calculates a suitable default based on the analysis time variables. If you specify a node/element label rather than a node/element number, it must be enclosed in {} brackets. Scale defaults to 1 where optional. Parameter can be M (for motion, the default), V (for velocity) or A (for acceleration). The latter two are appropriate only if the motion database contains these parameters. Target Spectrum Flag should be set to 1 if the target spectrum is to be created in addition to the realised spectrum. The Unit entry defaults to base units. Figure Title defaults to blank.
To define a number of parameters relating to the calculation of spectra.
Input: |
Description |
Start Time: |
The start time for the calculation of spectra. Flexcom excludes any values before this time. Use this entry to exclude initial transients from spectrum calculations. |
Ensembles: |
The number of ensembles to be used in calculating spectra. See Note (a). |
Time Step: |
The time step to be used when calculating spectra from the results of a random sea dynamic analysis with a variable time step. See Note (b). If the analysis used a fixed time step then this entry is not required, and any value you might specify is ignored. |
Cut-off Frequency: |
The spectrum cut off frequency in Hz. Flexcom truncates all output at this frequency. This entry is optional; by default, Flexcom calculates an appropriate value from the simulation length and the number of ensembles you specify. |
(a)The procedure used by Flexcom in calculating spectra is as follows. Firstly, the output timetrace is divided equally into a number of smaller timetraces or ensembles. A spectrum for each ensemble is then calculated using the Fast Fourier Transform (FFT) algorithm. Finally, the actual spectrum to be output is found as an average of the spectra calculated for each ensemble. The number of ensembles should be sufficiently large to ensure that the effects of the phase associated with each frequency component in the FFT process are minimised. You specify the number of ensembles to be used in this process using the Ensembles entry above. This value should always be greater than 1.
(b)The FFT algorithm requires a record with a fixed time step. When you perform a variable step analysis, obviously such a record is not available, and so Flexcom must synthesise one by interpolating from the variable step record. The Time Step entry tells Flexcom what time step to use in the synthesised record.
To request a spectrum of the motion of a node in one of the global DOFs.
Input: |
Description |
Node: |
The node (number or label) for which the motion spectrum is required. If you specify a node label rather than a node number, it must be enclosed in {} brackets. |
DOF: |
The global degree of freedom (DOF) at this node for which the spectrum is required. Specify a value of 1 for the global X-direction, 2 for the global Y-direction, or 3 for the global Z-direction; 4, 5 or 6 for the components of the rotation vector in the global X, Y and Z axes respectively; or 7 for the magnitude of 4, 5, and 6; or 8 for the magnitude of 1, 2, and 3. |
Parameter: |
This entry allows you to select the response parameter to be plotted against time. The options are Motion (the default), Velocity and Acceleration. See Note (a). |
Title: |
A descriptive title to be associated with the output. This entry is optional. |
Scale: |
A scale factor to apply to the data. This entry is optional, and defaults to a value of 1. |
Unit: |
The units to be used for the vertical axis of the plot. This entry is optional, and defaults to the base units for this output type. See Note (b). |
(a)In order to produce spectra of velocity and acceleration you must have requested extended output to the analysis database. Refer to the *DATABASE CONTENT keyword for more details.
(b)The units entry explicitly specifies what units are to be assigned to the vertical axis of the plot. This entry is optional, and if omitted, Flexcom simply assigns suitable units depending on the base units for the particular output type. If you do explicitly specify a unit, then it must be consistent with the dimension of the output type. For example, force can be specified in Newtons, Kips etc. The program is quite flexible in terms of accepting unit descriptions – e.g. Newtons, Newton, N etc. are all equally valid. For a list of the units accepted by Flexcom, refer to Units Reference Guide.
To request a spectrum of restoring force in a particular element.
Input: |
Description |
Element: |
The element (number or label) for which the force spectrum is required. If you specify an element label rather than an element number, it must be enclosed in {} brackets. |
Local Node: |
This option allows you to choose between three locations on the specified element. The significance of the Local Node Input used during postprocessing depends on whether element based outputs are stored on a node or integration point basis. |
Variable: |
This list allows you to select a relevant output parameter. The entries are largely self-explanatory. See Note (a). |
Title: |
A descriptive title to be associated with the output. This entry is optional. |
Scale: |
A scale factor to apply to the data. This entry is optional, and defaults to a value of 1. |
Location: |
This parameter identifies the circumferential or radial location on the element. See Note (b). |
Unit: |
The units to be used for the vertical axis of the plot. This entry is optional, and defaults to the base units for this output type. See Note (c). |
(a)Refer to Force Variable Input for further information on acceptable Variable values.
(b)Refer to Location Parameter Input for further information on the significance of the Location input.
(c)The units entry explicitly specifies what units are to be assigned to the vertical axis of the plot. This entry is optional, and if omitted, Flexcom simply assigns suitable units depending on the base units for the particular output type. If you do explicitly specify a unit, then it must be consistent with the dimension of the output type. For example, force can be specified in Newtons, Kips etc. The program is quite flexible in terms of accepting unit descriptions – e.g. Newtons, Newton, N etc. are all equally valid. For a list of the units accepted by Flexcom, refer to Units Reference Guide.
To request a spectrum of a reaction at a restrained node.
Input: |
Description |
Node: |
The node (number or label) for which the reaction spectrum is required. If you specify a node label rather than a node number, it must be enclosed in {} brackets. |
DOF: |
The global degree of freedom (DOF) for which the plot is required. Specify a value of 1 for the global X-direction, 2 for the global Y-direction, or 3 for the global Z-direction; 4, 5 or 6 for moments about the global X, Y and Z axes respectively, or 7 for the magnitude of 4, 5, and 6; or 8 for the magnitude of 1, 2, and 3. |
Title: |
A descriptive title to be associated with the output. This entry is optional. |
Scale: |
A scale factor to apply to the data. This entry is optional, and defaults to a value of 1. |
Unit: |
The units to be used for the vertical axis of the plot. This entry is optional, and defaults to the base units for this output type. See Note (a). |
(a)The units entry explicitly specifies what units are to be assigned to the vertical axis of the plot. This entry is optional, and if omitted, Flexcom simply assigns suitable units depending on the base units for the particular output type. If you do explicitly specify a unit, then it must be consistent with the dimension of the output type. For example, force can be specified in Newtons, Kips etc. The program is quite flexible in terms of accepting unit descriptions – e.g. Newtons, Newton, N etc. are all equally valid. For a list of the units accepted by Flexcom, refer to Units Reference Guide.
To request a spectrum of water surface elevation (at Y = Z = 0).
Input: |
Description |
Scale: |
A scale factor to apply to the data. |
Title: |
A descriptive title to be associated with the output. This entry is optional. |
Plot Target Wave Spectrum: |
This option allows you to plot the target wave spectrum, in addition to the realised spectrum, which is useful for comparison purposes. |
Unit: |
The units to be used for the vertical axis of the plot. This entry is optional, and defaults to the base units for this output type. See Note (a). |
(a)The units entry explicitly specifies what units are to be assigned to the vertical axis of the plot. This entry is optional, and if omitted, Flexcom simply assigns suitable units depending on the base units for the particular output type. If you do explicitly specify a unit, then it must be consistent with the dimension of the output type. For example, force can be specified in Newtons, Kips etc. The program is quite flexible in terms of accepting unit descriptions – e.g. Newtons, Newton, N etc. are all equally valid. For a list of the units accepted by Flexcom, refer to Units Reference Guide.