Modal Analysis

1
Modal Analysis

• Appendix Five

2
Basics of Free Vibration Analysis

• A free vibration analysis (a.k.a. modal or normal
  modes analysis) is performed to obtain the
  natural frequencies and mode shapes of a
  structure
• Free Vibration analysis does not consider the
  response of the structure under dynamic loads but
  just solves for the natural frequencies. A free
  vibration analysis is usually the first step
  before solving more complicated dynamic problems.
• A free vibration analysis is a subset of the
  general equation of motion

3
Basics of Free Vibration Analysis

• In free vibration analysis, the structure is
  assumed to be linear, so the response is assumed
  to be harmonic
• where fi is the mode shape (eigenvector) and wi
  is the natural circular frequency for mode i.
• By substituting this value in the earlier
  equation, the following is obtained
• Noting that the solution fi 0 is trivial, wi is
  solved for

4
Requesting Results

• The corresponding ANSYS commands for the
  Frequency Finder branch are as follows
• If Frequency Finder branch is present,
  ANTYPE,MODAL is set
• The number of modes is set with the nmodes
  argument, and the beginning and ending search
  frequencies are specified with freqb and freqe of
  the MODOPT,,nmodes,freqb,freqe command
• All modes are expanded via the MXPAND command.
  To save disk space and calculation times, the
  element solution option of MXPAND is not turned
  on unless stress or strain results are requested.

5
Solution Options

• For a regular modal analysis, none of the
  solution options except for Solver Type have
  much effect
• Large Deflection and Weak Springs are meant
  for static analysis cases and should not be
  changed.
• Solver Type can be set to Direct or
  Iterative
• Program Controlled or Direct result in the
  Block Lanczos eigenvalue extraction method with
  the sparse direct equation solver (MODOPT,LANB
  and EQSLV,SPARSE). This is the most robust
  eigensolver, as it handles small large models
  and beam, shell, or solid meshes, so it is the
  default option.
• Iterative results in the PowerDynamics solution
  method, which is a combination of the subspace
  eigenvalue extraction method with the PCG
  equation solver (MODOPT,SUBSP and EQSLV,PCG).
  The PowerDynamics eigensolver can be efficient
  for large models of solid elements, when
  requesting only a few modes.

6
Prestressed Modal Analysis

• For prestressed modal analysis, Simulation
  performs the two necessary iterations internally
• A linear static analysis with PSTRES,ON is run
• A modal analysis is then run right afterwards
  with PSTRES,ON to consider prestress effects

7
Prestressed Modal Analysis

• Other items useful for ANSYS users to keep in
  mind
• No large-deflection prestress effects are
  currently supported in Simulation, so enabling
  the Large Deflection On in the Solution branch
  is not permitted.
• The equation solver for the static analysis and
  the eigensolver for the modal analysis currently
  cannot be independently set. Both will be
  affected by the Solver Type setting in the
  Solution branch.
• If a Point Mass is present, rigid-body modes may
  be introduced in a prestressed modal analysis.
  This is due to the fact that the RBE3-type of
  surface constraint defined with CONTA174 and
  TARGE170 introduce 6 DOF but the MASS21 element
  has no rotary inertial terms (3 DOF).
• The user can usually ignore these rigid-body
  modes, as they are associated with the MASS21
  elements (verify by checking displacement scale
  of these mode shapes).
• No such problems exist for a regular modal with
  Point Masses.

8
(No Transcript)