Initial Conditions in Nonlinear Transient Analysis

Yuri S.

Hi everybody,

I have to simulate the free oscillation of a system, which can be seen as a simple mass linked to the ground by means of a spring and a damper. 

I had in mind to impose a certain displacement to the mass though either Initial conditions or a nonlinear quasi static analysis and then release it and observe the free oscillation performing a nonlinear transient analysis.

Unluckly, I am facing the following problems:

• It seems like initial displacement conditions (imposed through a TIC(D) are not supported by nonlinear transient analysis
• If I perform a nonlinear quasi static analysis and let it follow (by means of CNTNLSUB) by a nonlinear transient analysis, the loads I imposend in the NLQS decrease linearly with time in the NLT. At this regard, the guide says:  
  
   "for NLSTAT(LOAD) + Nonlinear Transient, the load in the 2nd subcase is the combination of DLOAD of Nonlinear Transient and the (ramping down) LOAD of NLSTAT. Whereas, for NLSTAT(DLOAD) + Nonlinear Transient, the load in the 2nd subcase is only from DLOAD of Nonlinear Transient."
  
  Despite having tried in both ways (NLSTAT + DLOAD in the first place and with LOAD later on), I always find a ramp down of the load in the NLT analysis, which prevents me to correctly evaluate the free response of the system. 

My questions are:

1. Does anybody know a correct way to impose displacement initial conditions for a nonlinear transient analysis?
2. Does anybody know how to impose a displacement in a NLQS analysis which is not ramped down in the following NL Transient analysis? (in order to have a complete release of the oscillating mass).

Thank you very much for any help or suggestion.
Yuri

Adriano A. Koga

1) indeed Initial condition of Displacement is not allowed (it can be velocity or accel):

  *** ERROR # 4945 ***
  Nonlinear transient analysis does not support displacement initial
  condition. Abort due to initial displacement in subcase 1.

 

2) i was thinking about it: what if you create a group of MPCs in your initial loadcase, to pull-up the spring, and then in the second load case, instead of removing the imposed displacement, you just disable the MPC connecting both? maybe there's some better way of doing this, but this is what i could think about.

Adriano A. Koga

1) indeed Initial condition of Displacement is not allowed (it can be velocity or accel):

  *** ERROR # 4945 ***
  Nonlinear transient analysis does not support displacement initial
  condition. Abort due to initial displacement in subcase 1.

 

2) i was thinking about it: what if you create a group of MPCs in your initial loadcase, to pull-up the spring, and then in the second load case, instead of removing the imposed displacement, you just disable the MPC connecting both? maybe there's some better way of doing this, but this is what i could think about.

Yuri S.

Adriano A. Koga_21884 said:

1) indeed Initial condition of Displacement is not allowed (it can be velocity or accel):

  *** ERROR # 4945 ***
  Nonlinear transient analysis does not support displacement initial
  condition. Abort due to initial displacement in subcase 1.

 

2) i was thinking about it: what if you create a group of MPCs in your initial loadcase, to pull-up the spring, and then in the second load case, instead of removing the imposed displacement, you just disable the MPC connecting both? maybe there's some better way of doing this, but this is what i could think about.

Thank you for your kind help! 
I will try out your suggestion and see if it works.

Have a nice day.

Yuri S.

Adriano A. Koga_21884 said:

1) indeed Initial condition of Displacement is not allowed (it can be velocity or accel):

  *** ERROR # 4945 ***
  Nonlinear transient analysis does not support displacement initial
  condition. Abort due to initial displacement in subcase 1.

 

2) i was thinking about it: what if you create a group of MPCs in your initial loadcase, to pull-up the spring, and then in the second load case, instead of removing the imposed displacement, you just disable the MPC connecting both? maybe there's some better way of doing this, but this is what i could think about.

I confirm that the suggested method ensures an istantaneous drop of the load passing from the first to the second loadcase.

Thank you for your help.
Yuri