FREQ3

Frequency List, Alternate Form 3

Description: Defines a set of excitation frequencies for modal frequency response solutions by specifying the number of solution frequencies between two modal frequencies.

Format:

Example:

Field Definition Type Default
SID Set identification number. Integer > 0 Required
F1 Lower bound of modal frequency range in cycles per unit time. Real > 0.0 Required
F2 Upper bound of modal frequency range in cycles per unit time. Real > 0.0, F2 ≥ F1 F1
TYPE Specifies the interpolation type between frequencies, one of the following character variables, LINEAR or LOG:
LINEAR Linear interpolation between frequencies.
LOG Logarithmic interpolation between frequencies.
Character LINEAR
NEF Number of solution frequencies within each subrange including the endpoints. The first subrange is between F1 and the first modal frequency within the bounds. The second subrange is between first and second modal frequencies between the bounds. The last subrange is between the last modal frequency within the bounds and F2. Integer > 0.0 10
CLUSTER Specifies clustering of the solution frequency near the endpoints of the range. See Remark 6. Real > 0.0 1.0

Remarks:

  1. FREQi entries must be selected with the Case Control command FREQUENCY = SID.
  2. In the example above, there will be 10 frequencies in the interval between each set of modes within the bounds 10 and 1000, plus 10 frequencies between 10 and the lowest mode in the range, plus 10 frequencies between the highest mode in the range and 1000.
  3. Since the forcing frequencies are near structural resonance, it is important that some amount of damping be specified.
  4. The DFREQ model parameter specifies the threshold for the elimination of duplicate frequencies. Duplicate frequencies will be ignored if

    where DFREQ is defaulted to 1.0x10-5 and and are the maximum and minimum solution frequencies of the combined FREQi entries. (See Parameters, for more information on DFREQ.)

  5. CLUSTER is used to obtain better resolution near the modal frequencies where the response varies the most. CLUSTER > 1.0 provides closer spacing of solution frequency spacing towards the ends of the frequency range, while values of less than 1.0 provide closer spacing towards the center of the frequency range. For example, if TYPE is LINEAR, then

    and,

    where is a parametric coordinate between -1 and 1 and i varies from 1 to NEF (i=1,2, …, NEF) and,

    f1 is the lower limit of the frequency subrange

    f2 is the upper limit of the frequency subrange

    fi is the i-th solution frequency

Parent topic: Section 4: Bulk Data