Defining Random Vibration Steps

You can define a random vibration step to predict the response of the model when it is subjected to a nondeterministic continuous vibration.

You can specify a random vibration step in the following ways: in terms of the modes extracted in the preceding step; using a frequency curve; using global damping; or with no damping.

This task shows you how to:

See Also
About Random Vibration Steps

Define Random Vibration Steps Using Mode Range

  1. From the Procedures section of the action bar, click Random Vibration Step .
  2. Optional: Enter a descriptive Name.
  3. In the Frequency Range data table, specify the list of frequency ranges in which the step will compute results.

    You can enter one or more rows of frequency data in the table or import data from a file.

    Option Descripton
    Lower Boundary (Hz) Lower limit of the frequency range or a single frequency, in cycles/time.
    Upper Boundary (Hz) Upper limit of the frequency range, in cycles/time. If you do not specify a value, the step computes results only at the frequency of the lower boundary.
    Calculation Points Total number of points in the frequency range for which results are calculated, including the end points.
    Bias Bias parameter. Enter a value greater than 1.0 to provide results points spacing closer to the ends of the frequency interval or less than 1.0 to provide spacing closer to the middle.
    Frequency Scale Select Logarithmic to divide the frequency ranges using a logarithmic scale; or select Linear divide the frequency ranges using a linear scale.

  4. From the Definition list, select Mode range.
  5. From the Damping list, select one of the following damping specifications.

    Option Description
    Critical damping fraction Specifies the fraction of critical damping for one or more mode ranges.
    Structural Specifies damping effects when they are proportional to the internal forces but opposite in direction to the velocity.
    Composite Specifies composite modal damping using the damping coefficients calculated in the preceding frequency step.
    Rayleigh Specifies a general damping technique that provides a convenient way to damp lower (mass-dependent) and higher (stiffness-dependent) frequency range behavior.

  6. For Critical damping fraction damping, specify the following parameters in the data table.

    Option Description
    First Mode Mode number of the lowest mode in a range of modes.
    Last Mode Mode number of the highest mode in a range of modes.
    Critical Damping Fraction (%) Fraction of critical damping to be applied, expressed as a percentage.

  7. For Structural damping, specify the following parameters in the data table.

    Option Description
    First Mode Mode number of the lowest mode in a range of modes.
    Last Mode Mode number of the highest mode in a range of modes.
    Damping Fraction (%) Damping factor to be applied, expressed as a percentage.

  8. For Composite damping, specify the following parameters in the data table.

    Option Description
    First Mode Mode number of the lowest mode in a range of modes.
    Last Mode Mode number of the highest mode in a range of modes.
    Mass Scaling Scaling factor for the mass-weighted fraction of composite critical damping that was calculated in the preceding frequency step. If omitted, the default value is 1.0.
    Stiffness Scaling Scaling factor for the stiffness-weighted fraction of composite critical damping calculated in the frequency step. If omitted, the default value is 1.0.

  9. For Rayleigh damping, specify the following parameters in the data table.

    Option Description
    First Mode Mode number of the lowest mode in a range of modes.
    Last Mode Mode number of the highest mode in a range of modes.
    Mass Damping (_s) Mass proportional Rayleigh damping factor.
    Stiffness Damping (_s) Stiffness proportional Rayleigh damping factor.

  10. To select the eigenmodes to be used for this step, see Selecting Eigenmodes for Mode-Based Procedures.
  11. Click OK.

Define Random Vibration Steps Using Frequency Curve

  1. From the Procedures section of the action bar, click Random Vibration Step .
  2. Optional: Enter a descriptive Name.
  3. In the Frequency Range data table, specify the list of frequency ranges in which the step will compute results.

    You can enter one or more rows of frequency data in the table or import data from a file.

    Option Description
    Lower Boundary (Hz) Lower limit of the frequency range or a single frequency, in cycles/time.
    Upper Boundary (Hz) Upper limit of the frequency range, in cycles/time. If you do not specify a value, the step computes results only at the frequency of the lower boundary.
    Calculation Points Total number of points in the frequency range for which results are calculated, including the end points.
    Bias Bias parameter. Enter a value greater than 1.0 to provide results points spacing closer to the ends of the frequency interval or less than 1.0 to provide spacing closer to the middle.
    Frequency Scale Select Logarithmic to divide the frequency ranges using a logarithmic scale; or select Linear divide the frequency ranges using a linear scale.

  4. From the Definition list, select Frequency curve.
  5. From the Damping list, select one of the following damping specifications.

    Option Description
    Critical damping fraction Specifies the fraction of critical damping for one or more mode ranges.
    Structural Specifies damping effects when they are proportional to the internal forces but opposite in direction to the velocity.
    Rayleigh Specifies a general damping technique that provides a convenient way to damp lower (mass-dependent) and higher (stiffness-dependent) frequency range behavior.

  6. For Critical damping fraction damping, specify the following parameters in the data table.

    Option Description
    Frequency (Hz) Mode number of the lowest mode in a range of modes.
    Critical Damping Fraction (%) Fraction of critical damping to be applied, expressed as a percentage.

  7. For Structural damping, specify the following parameters in the data table.

    Option Description
    Frequency (Hz) Mode number of the lowest mode in a range of modes.
    Damping Fraction (%) Fraction of damping to be applied, expressed as a percentage.

  8. For Rayleigh damping, specify the following parameters in the data table.

    Option Description
    First Mode Mode number of the lowest mode in a range of modes.
    Mass Damping (_s) Mass proportional Rayleigh damping factor.
    Stiffness Damping (_s) Stiffness proportional Rayleigh damping factor.

  9. To select the eigenmodes to be used for this step, see Selecting Eigenmodes for Mode-Based Procedures.
  10. Click OK.

Define Random Vibration Steps Using Global Damping

  1. From the Procedures section of the action bar, click Random Vibration Step .
  2. Optional: Enter a descriptive Name.
  3. In the Frequency Range data table, specify the list of frequency ranges in which the step will compute results.

    You can enter one or more rows of frequency data in the table or import data from a file.

    Option Description
    Lower Boundary (Hz) Lower limit of the frequency range or a single frequency, in cycles/time.
    Upper Boundary (Hz) Upper limit of the frequency range, in cycles/time. If you do not specify a value, the step computes results only at the frequency of the lower boundary.
    Calculation Points Total number of points in the frequency range for which results are calculated, including the end points.
    Bias Bias parameter. Enter a value greater than 1.0 to provide results points spacing closer to the ends of the frequency interval or less than 1.0 to provide spacing closer to the middle.
    Frequency Scale Select Logarithmic to divide the frequency ranges using a logarithmic scale; or select Linear divide the frequency ranges using a linear scale.

  4. From the Definition list, select Global damping.
  5. Select whether to apply the damping values to structural or acoustic modes for Modes.
  6. In the Mass Proportional field, enter the alpha factor to create global Rayleigh mass-proportional damping.
  7. In the Stiffness proportional field, enter the beta factor to create global Rayleigh stiffness-proportional damping.
  8. In the Stiffness proportional structural field, enter the s-global factor to create frequency-independent, stiffness-proportional, structural damping.
  9. To select the eigenmodes to be used for this step, see Selecting Eigenmodes for Mode-Based Procedures.
  10. Click OK.

Define Random Vibration Steps Without Damping

  1. From the Procedures section of the action bar, click Random Vibration Step .
  2. Optional: Enter a descriptive Name.
  3. In the Frequency Range data table, specify the list of frequency ranges in which the step will compute results.

    You can enter one or more rows of frequency data in the table or import data from a file.

    Option Description
    Lower Boundary (Hz) Lower limit of the frequency range or a single frequency, in cycles/time.
    Upper Boundary (Hz) Upper limit of the frequency range, in cycles/time. If you do not specify a value, the step computes results only at the frequency of the lower boundary.
    Calculation Points Total number of points in the frequency range for which results are calculated, including the end points.
    Bias Bias parameter. Enter a value greater than 1.0 to provide results points spacing closer to the ends of the frequency interval or less than 1.0 to provide spacing closer to the middle.
    Frequency Scale Select Logarithmic to divide the frequency ranges using a logarithmic scale; or select Linear divide the frequency ranges using a linear scale.

  4. From the Definition list, select No damping.
  5. To select the eigenmodes to be used for this step, see Selecting Eigenmodes for Mode-Based Procedures.
  6. Click OK.