Mechanical APDL Structural Analysis Guide

4.4.3.3. Apply Loads on the Model

A harmonic analysis, by definition, assumes that any applied load varies harmonically (sinusoidally) with
time. To completely specify a harmonic load, three pieces of information are usually required: the
amplitude, the phase angle, and the forcing frequency range (see Figure 4.2: Relationship Between
Real/Imaginary Components and Amplitude/Phase Angle (p. 82)).

Figure 4.2: Relationship Between Real/Imaginary Components and Amplitude/Phase Angle

Imaginary

Amplitude

Real

Phase Angle

Freal = F 0 cosΨ

Fimag = F 0 sinΨ

F 0

Ψ

F

Ψ = tan-1 (Fmag/F real)

F 0 = F + F^2 real^2 imag

ωt

Amplitude

The amplitude is the maximum value of the load, which you specify using the commands shown in
Table 4.2: Applicable Loads in a Harmonic Analysis (p. 83).

The phase angle is a measure of the time by which the load lags (or leads) a frame of reference. On the
complex plane (see Figure 4.2: Relationship Between Real/Imaginary Components and Amplitude/Phase
Angle (p. 82)), it is the angle measured from the real axis. The phase angle is required only if you have
multiple loads that are out of phase with each other. For example, the unbalanced rotating antenna
shown in Figure 4.3: An Unbalanced Rotating Antenna (p. 83) will produce out-of-phase vertical loads
at its four support points. The phase angle cannot be specified directly; instead, you specify the real
and imaginary components of the out-of-phase loads using the VALUE and VALUE2 fields of the ap-
propriate displacement and force commands. Pressures and other surface and body loads can only be
specified with a phase angle of 0 (no imaginary component) with the following exceptions: nonzero
imaginary components of pressures can be applied via the SURF153,SURF154,SURF156, and SURF159
elements in a full harmonic analysis, or using a mode-superposition harmonic analysis if the mode-ex-
traction method is Block Lanczos, PCG Lanczos, Supernode,or Subspace (see the SF and SFE commands).
Figure 4.2: Relationship Between Real/Imaginary Components and Amplitude/Phase Angle (p. 82) shows
how to calculate the real and imaginary components.

The forcing frequency range is the frequency range of the harmonic load (in cycles/time). It is specified
later as a load step option with the HARFRQ command.

Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information

Harmonic Analysis