FREQ5

Bulk Data Entry Defines a set of frequencies for the modal method of frequency response analysis by specification of a frequency range and fractions of the natural frequencies within that range.

Format

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
FREQ5 SID F1 F2 FR1 FR2 FR3 FR4 FR5
FR6 FR7 etc.

Example

Define a set of frequencies such that the list of frequencies will be 0.6, 0.8, 0.9, 0.95, 1.0, 1.05, 1.1, and 1.2 times each modal frequency between 20 and 200.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
FREQ5 6 20.0 200.0 1.0 0.6 0.8 0.9 0.95
1.05 1.1 1.2

Definitions

Field Contents SI Unit Example
SID Set identification number.

No default (Integer > 0)
F1 Lower bound of modal frequency range in cycles per unit time.

Default = 0.0 (Real ≥ 0.0)
F2 Upper bound of frequency range in cycles per unit time.

Default = 1.0E20 (Real > 0.0, F2 > F1)
FRi Fractions of the natural frequencies in the range F1 to F2.

No default (Real > 0.0)

Comments

  1. FREQ5 applies only to the modal method frequency response analysis.
  2. FREQ5 entries must be selected in the Subcase Information section with FREQUENCY = SID.
  3. The frequencies defined by this entry are given by fi = FRi * f N .

    Where, f N are the modal frequencies in the range F1 through F2.

    If this computation results in excitation frequencies less than F1 and greater than F2, those computed excitation frequencies are ignored.

  4. Excitation frequencies may be based on natural frequencies that are not within the range (F1 and F2) as long as the calculated excitation frequencies are within the range.
  5. Since the forcing frequencies are near structural resonances, it is important that some amount of damping be specified.
  6. All FREQi entries with the same set identification numbers will be used. Duplicate frequencies will be ignored. f N and f N − 1 are considered duplicated if:
    | f N − f N − 1 | < DFREQ * | f MAX − f MIN |
    Where,
    DFREQ
    User parameter with a default of 10-5 * f M A X
    f M I N
    The maximum and minimum excitation frequencies of the combined FREQi entries
  7. In design optimization, the excitation frequencies are derived from the modal frequencies computed at each design iteration.
  8. In modal analysis, solutions for modal degrees-of-freedom from rigid body modes at zero excitation frequencies may be discarded. Solutions for non-zero modes are retained.
  9. This card is represented as a load collector in HyperMesh.