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Implicit: Dynamic relaxation

To run implicit Dynamic Relaxation (DR):

  1. Set IDRFLG=5 and set the dynamic relaxation termination time DRTERM in *CONTROL_DYNAMIC_RELAXATION
  2. Add *CONTROL_IMPLICIT commands as necessary to govern implicit analysis in DR phase. As a minimum, you'll need *CONTROL_IMPLICIT_GENERAL with DT0 set to a positive value.
  3. Ramp preload linearly using *DEFINE_CURVE command(s) with SIDR=1. The ramp time for preload is typically equal to DRTERM (see step 1).
  4. Include *DATABASE_BINARY_D3DRLF to write output from the implicit dynamic relaxation phase (recommended). The output interval is given in units of cycles, not time.
  5. *CONTROL_IMPLICIT_DYNAMICS governs whether implicit analyses are static or transient.
  6. If you need to initialize velocities at the conclusion of the dynamic relaxation phase, e.g., as in a spinning blade problem, set IPHASE=1 on *INITIAL_VELOCITY_GENERATION.

As an alternative to implicit DR is switching from implicit to explicit via a curve (1st parameter in *CONTROL_IMPLICIT_GENERAL is negative curve ID). This does NOT allow for initialization of velocities at the beginning of the explicit phase.


A few questions on dynamic relaxation (DR):

  1. What is the advantage of implicit method over explicit for DR?
    For DR or any analysis, the pros and cons of implicit and explicit are the same. Implicit may be particularly suited to DR because the loads imposed during DR are generally small and the response linear (except perhaps for the nonlinearity associated with contact).
  2. I am still trying to understand the purpose of DR.
    DR is a convenient way to preload a model prior to applying dynamic loads in the ensuing transient analysis. It's a way to clearly distinguish the preloading phase from the transient phase. DR serves the purpose of reaching a steady state preloaded condition (nearly) free from dynamic oscillation.
  3. What happens if I do not use DR for gravity load?
    You start the transient analysis in an unstressed state. The gravity load, if applied suddenly, will excite dynamic oscillation.