# FREQ3

Bulk Data Entry Defines a set of frequencies for the modal method of frequency response analysis by specifying the number of frequencies between modal frequencies.

## Format

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
FREQ3 SID F1 F2 TYPE NEF CLUSTER

## Example

Define a set of frequencies such that there will be 10 frequencies between each mode, within the frequency range 20 to 200, plus 10 frequencies between 20 and the lowest mode in the range, plus 10 frequencies between the highest mode in the range and 200.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
FREQ3 6 20.0 200.0 LINEAR 10 2.0

## 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.

No default (Real ≥ 0.0 for TYPE = LINEAR; Real > 0.0 for TYPE = LOG)

F2 Upper bound of modal frequency range in cycles per unit time.

Default = F1 (Real > 0.0, F2F1)

TYPE Specifies linear or logarithmic interpolation between frequencies.
LINEAR (Default)
LOG

NEF Number of excitation frequencies within each sub range including the end points. The first sub range is between F1 and the first modal frequency within the bounds. Intermediate sub ranges exist between each mode calculated within the bounds. The last sub range is between the last modal frequency within the bounds and F2.

Default = 10 (Integer > 1)

CLUSTER Specifies cluster of the excitation frequency near the end points of the range. 5

Default = 1.0 (Real > 0.0)

1. FREQ3 applies only to the modal method of frequency response analysis.
2. FREQ3 entries must be selected in the Subcase Information section with FREQUENCY = SID.
3. Since the forcing frequencies are near structural resonances, it is important that some amount of damping be specified.
4. 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:(1)
$|{f}_{N}-{f}_{N-1}|<\mathit{DFREQ}*|{f}_{\mathit{MAX}}-{f}_{\mathit{MIN}}|$
Where,
DFREQ
User parameter with a default of 10-5 * ${f}_{MAX}$
${f}_{MIN}$
The maximum and minimum excitation frequencies of the combined FREQi entries
5. CLUSTER is used to obtain better resolution near the modal frequencies where the response variation is highest, in accordance with:
(2)
${\stackrel{^}{f}}_{k}=\frac{1}{2}\left({\stackrel{^}{f}}_{1}+{\stackrel{^}{f}}_{2}\right)+\frac{1}{2}\left({\stackrel{^}{f}}_{2}-{\stackrel{^}{f}}_{1}\right){|\xi |}^{1/\mathit{CLUSTER}}*\mathit{SIGN}\left(\xi \right)$
Where,
$\xi$
-1 + 2(k - 1)/(NEF - 1) is a parametric coordinate between -1 and 1.
k
Excitation frequency number in the subrange (1,2,3,...,NEF)
${\stackrel{^}{f}}_{1}$
Frequency at the lower limit of the sub range. (If TYPE is LOG, then this is the logarithm of the frequency.)
${\stackrel{^}{f}}_{2}$
Frequency at the upper limit of the sub range. (If TYPE is LOG, then this is the logarithm of the frequency.)
${\stackrel{^}{f}}_{k}$
The k-th excitation frequency. (If TYPE is LOG, then this is the logarithm of the frequency.)

CLUSTER > 1.0 provides closer spacing of excitation frequency 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 the frequency range is between 10 and 20, NEF = 11, TYPE = "LINEAR"; then, the excitation frequencies for various values of CLUSTER would be as shown in the table below.
Excitation Frequency Number $\xi$ CLUSTER
0.25 0.50 1.0 2.0 4.0
Excitation Frequencies in Hertz
1 -1.0 10.00 10.0 10.0 10.0 10.0
2 -0.8 12.95 11.8 11.0 10.53 10.27
3 -0.6 14.35 13.2 12.0 11.13 10.60
4 -0.4 14.87 14.2 13.0 11.84 11.02
5 -0.2 14.99 14.8 14.0 12.76 11.66
6 0.0 15.00 15.0 15.0 15.00 15.00
7 0.2 15.01 15.2 16.0 17.24 18.34
8 0.4 15.13 15.8 17.0 18.16 18.98
9 0.6 15.65 16.8 18.0 18.87 19.40
10 0.8 17.05 18.2 19.0 19.47 19.73
11 1.0 20.00 20.0 20.0 20.00 20.00
6. In design optimization, the excitation frequencies are derived from the modal frequencies computed at each design iteration.
7. 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.
8. This card is represented as a load collector in HyperMesh.