MDOF Spring-Mass-Damper Problem

[5.trace]

Good afternoon,

I am working on simulating a 3 degree of freedom unforced spring-mass-damper system in MSC Nastran 2024.1. It consists of 4 masses combined together with a wall at each end and I want to find the displacement over time with initial conditions for displacement and velocity. I want to solve this problem using Solution 112 but I keep running in an issue where my initial displacement isn't matching my initial conditions. My code and response at time 0s is pasted below:

NASTRAN Input:

SOL 112
CEND
TITLE = Three Mass-Spring-Damper System
ECHO = BOTH
DISP (SORT1) = ALL
SPC = 44
IC(MODAL) = 20
METHOD = 55
TSTEP = 234
BEGIN BULK

GRID,1,,0.,0.,0.
GRID,2,,1.,0.,0.
GRID,3,,2.,0.,0.
GRID,4,,3.,0.,0.
GRID,5,,4.,0.,0.

CONM2, 22, 2,, 1.0
CONM2, 33, 3,, 1.0
CONM2, 44, 4,, 1.0

CELAS2, 111, 4.0, 1, 1, 2, 1
CELAS2, 222, 4.0, 2, 1, 3, 1
CELAS2, 333, 4.0, 3, 1, 4, 1
CELAS2, 444, 4.0, 4, 1, 5, 1

CDAMP2, 1111, 5.0, 1, 1, 2, 1
CDAMP2, 2222, 5.0, 2, 1, 3, 1
CDAMP2, 3333, 5.0, 3, 1, 4, 1
CDAMP2, 4444, 5.0, 4, 1, 5, 1

SPC1, 44, 123456, 1
SPC1, 44, 123456, 5

EIGRL, 55

TIC, 20, 1, -1, 0.3914, 0.9071
TIC, 20, 2, -1, 0.4950 , -2.1213
TIC, 20, 3, -1, -0.1086, 0.5071

TSTEP, 234, 200, 0.001, 1
ENDDATA

NASTRAN Output:

TIME = 0.000000E+00
D I S P L A C E M E N T V E C T O R

POINT ID. TYPE T1 T2 T3 R1 R2 R3
1 G 0.0 0.0 0.0 0.0 0.0 0.0
2 G -2.086179E-01 0.0 0.0 0.0 0.0 0.0
3 G 3.535534E-01 0.0 0.0 0.0 0.0 0.0
4 G 4.914179E-01 0.0 0.0 0.0 0.0 0.0
5 G 0.0 0.0 0.0 0.0 0.0 0.0

Thank you for the help

 

[dcascap]

Hi,
From a quick check of your code I see that the DOF entrance for the initial condition card (TIC) is -1, and it should be a number between 1 to 6 (1,2,3 --> tx, ty ,tz 3,4,5 --> rx,ry,rz), what if you change it to:

TIC, 20, 1, 1, 0.3914, 0.9071
TIC, 20, 2, 1, 0.4950 , -2.1213
TIC, 20, 3, 1, -0.1086, 0.5071

Then your grid points should 1,2,3 should have an initial condition for Tx.

I hope it helps!

 

[5.trace]

Hi dcascap,

Thank you for your response. I had those set to -1 since the initial conditions are applying to the modes for SOL 112, the qrg said I should use that if I'm working in modal coordinates.

 

[dcascap]

Oh yeah, sorry I checked the wrong qrg (Altair). But then, it seems that your output is in global coords, at least in Simcenter Nastran (I'm more familiar than with MSC Nastran) you use SDISPLACEMENT in the case control to ask for the modal displacement output.
In your code you're using DISP, so it seems that your results are in global coords. This would explain the difference between input to output.

 

[nastran_sol_200_web_app]

The displacement output is the displacement of physical coordinates, not modal coordinates. The initial conditions were defined on the modal coordinates, so I don't expect the displacement in physical coordinates to match.

Per your inquiry, I think you want the initial conditions applied to the physical coordinates of the grid.

This is how you do it. The BDF used and the confirmation are further down.

[ol 1]
[li]Use IC=20[/li]
[li]Apply the initial conditions to DOF 1[/li]
[li]Apply the initial conditions to grids 2, 3, 4. If you apply the IC to grid 1, you get an error because grid 1 is fixed.[/li]
[/ol]

Here is my displacement and velocity output and you can see I get the initial conditions defined in the BDF file.

[pre] TIME = 0.000000E+00
D I S P L A C E M E N T V E C T O R

POINT ID. TYPE T1 T2 T3 R1 R2 R3
1 G 0.0 0.0 0.0 0.0 0.0 0.0
2 G 3.914000E-01 0.0 0.0 0.0 0.0 0.0
3 G 4.950000E-01 0.0 0.0 0.0 0.0 0.0
4 G -1.086000E-01 0.0 0.0 0.0 0.0 0.0
5 G 0.0 0.0 0.0 0.0 0.0 0.0



V E L O C I T Y V E C T O R

POINT ID. TYPE T1 T2 T3 R1 R2 R3
1 G 0.0 0.0 0.0 0.0 0.0 0.0
2 G 9.071000E-01 0.0 0.0 0.0 0.0 0.0
3 G -2.121300E+00 0.0 0.0 0.0 0.0 0.0
4 G 5.071000E-01 0.0 0.0 0.0 0.0 0.0
5 G 0.0 0.0 0.0 0.0 0.0 0.0
[/pre]

Here is a plot of the time history for grids 2, 3 and 4. Again, the same initial conditions occur at t=0.

Listing A - model.bdf

[pre]SOL 112
CEND
TITLE = Three Mass-Spring-Damper System
ECHO = BOTH
DISP (SORT1) = ALL
velocity (SORT1) = ALL
SPC = 44
IC = 20
METHOD = 55
TSTEP = 234
BEGIN BULK
hdf5out
GRID,1,,0.,0.,0.
GRID,2,,1.,0.,0.
GRID,3,,2.,0.,0.
GRID,4,,3.,0.,0.
GRID,5,,4.,0.,0.

CONM2, 22, 2,, 1.0
CONM2, 33, 3,, 1.0
CONM2, 44, 4,, 1.0

CELAS2, 111, 4.0, 1, 1, 2, 1
CELAS2, 222, 4.0, 2, 1, 3, 1
CELAS2, 333, 4.0, 3, 1, 4, 1
CELAS2, 444, 4.0, 4, 1, 5, 1

CDAMP2, 1111, 5.0, 1, 1, 2, 1
CDAMP2, 2222, 5.0, 2, 1, 3, 1
CDAMP2, 3333, 5.0, 3, 1, 4, 1
CDAMP2, 4444, 5.0, 4, 1, 5, 1

SPC1, 44, 123456, 1
SPC1, 44, 123456, 5

EIGRL, 55

TIC, 20, 2, 1, 0.3914, 0.9071
TIC, 20, 3, 1, 0.4950 , -2.1213
TIC, 20, 4, 1, -0.1086, 0.5071

TSTEP, 234, 200, 0.001, 1
ENDDATA[/pre]

 

[nastran_sol_200_web_app]

If you really want to apply the initial conditions to the modal coordinates, you can confirm the application of ICs by looking at the modal displacements.

If you add SDISP(SORT1)=ALL, you get the displacement output at the modal coordinates. If you want to see the velocities, you would use SVELOCITY(SORT1)=ALL.

You can see the ICs are reflected in these results at t=0.


[pre] TIME = 0.000000E+00
D I S P L A C E M E N T V E C T O R (SOLUTION SET)

POINT ID. TYPE T1 T2 T3 R1 R2 R3
1 M 3.914000E-01 4.950000E-01 -1.086000E-01
1 THREE MASS-SPRING-DAMPER SYSTEM OCTOBER 23, 2024 MSC Nastran 12/12/23 PAGE 14

0
TIME = 1.000000E-03
D I S P L A C E M E N T V E C T O R (SOLUTION SET)

POINT ID. TYPE T1 T2 T3 R1 R2 R3
1 M 3.923059E-01 4.928844E-01 -1.080953E-01
1 THREE MASS-SPRING-DAMPER SYSTEM OCTOBER 23, 2024 MSC Nastran 12/12/23 PAGE 15

0
TIME = 2.000000E-03
D I S P L A C E M E N T V E C T O R (SOLUTION SET)

POINT ID. TYPE T1 T2 T3 R1 R2 R3
1 M 3.932094E-01 4.907803E-01 -1.075953E-01
1 THREE MASS-SPRING-DAMPER SYSTEM OCTOBER 23, 2024 MSC Nastran 12/12/23 PAGE 16

0
TIME = 3.000000E-03
D I S P L A C E M E N T V E C T O R (SOLUTION SET)

POINT ID. TYPE T1 T2 T3 R1 R2 R3
1 M 3.941094E-01 4.886931E-01 -1.071022E-01

[/pre]