Damping in *Modal Dynamic and *Steady State Dynamics Steps

[score33]

Hi all,

I'm trying to learn how to use the *Modal Dynamic and *Steady State Dynamics solutions. My trouble is that I can't figure out the proper way to apply damping.

For a test case I've got a spring mass damper system:
M=10kg
k=49 N/m
c=3 N-sec/m
x(0)=0
v(0)=0
F(t)=20cos(4t) N

Steady State Solution ~ 0.18sin(4t-1.46)

When I run the *Steady State Solution, I get U1=-0.1802 inches, which is correct, but the *Modal Dynamic results do not match the hand calculation at all (Used matlab and ODE45 to get exact solution).

I started out with the input below and have played with the *Global Damping and *Modal Damping parameters. My *Steady State Dynamics solution doesn't use either damping.

Any help on where to go to better understand damping, or what I might be doing wrong would be greatly appreciated.

Thanks in advance for the help.


*Amplitude, Name=Sine, Definition=Periodic
1, 4.0, 0.0, 0.0
1.0, 0.0
**===================================================
*STEP
*Frequency, eigensolver=lanczos
3,-0.1,2000.0
*END STEP
**
**==================================================
*STEP
*Modal Dynamic
0.01, 30.0
*CLOAD, OP=NEW, Amplitude=Sine
2, 1, 20.0