wall/gran의 정보 빨아오기

wall/gran은 아래와 같이 per atom array를 제공한다고 한다.

https://docs.lammps.org/fix_wall_gran.html

fix wall/gran command — LAMMPS documentation

 

If the contacts option is used, this fix generates a per-atom array with at least 8 columns as output, containing the contact information for owned particles (nlocal on each processor). All columns in this per-atom array will be zero if no contact has occurred. The first 8 values of these columns are listed in the following table.

IndexValueUnits

1	1.0 if particle is in contact with wall, 0.0 otherwise
2	Force  exerted by the wall	force units
3	Force  exerted by the wall	force units
4	Force  exerted by the wall	force units
5	-coordinate of contact point on wall	distance units
6	-coordinate of contact point on wall	distance units
7	-coordinate of contact point on wall	distance units
8	Radius  of atom	distance units

 

wall/gran은 fix style로 만들어 지니까, 출력 형태를 보기 위해 variable로 가서 봤는데,

https://docs.lammps.org/variable.html

variable command — LAMMPS documentation

 

The allowed fix references in variable formulas for equal-, vector-, and atom-style variables are listed in the following table:

equal	f_ID	global scalar
equal	f_ID[I]	element of global vector
equal	f_ID[I][J]	element of global array
equal	F_ID[I]	element of per-atom vector (I = atom ID)
equal	F_ID[I][J]	element of per-atom array (I = atom ID)
vector	f_ID	global vector
vector	f_ID[I]	column of global array
atom	f_ID	per-atom vector
atom	f_ID[I]	column of per-atom array

Note that if an equal-style variable formula wishes to access per-atom data from a fix, it must use capital “F” as the ID prefix and not lower-case “f”.

 

per-atom array는 F_ID[][]라고 해서 했는데 안되더라.

 

그래서, lammps forum에서 찾은 예제를 해보니, 이런 스타일(f_ID[])을 추천함. 아 이거 뭐지?

fix              bottom_plate all wall/gran hertz/history 4.98E10 6.4E10 50.0 30.0 0.5 1 &
                 zplane ${zmin} NULL contacts

dump              7 all custom 1000 dump*.contact f_bottom_plate[1] f_bottom_plate[2] f_bottom_plate[3] f_bottom_plate[4] &
                  f_bottom_plate[5] f_bottom_plate[6] f_bottom_plate[7] f_bottom_plate[8]

 

출력결과를 보면 또 array 이다.

ITEM: TIMESTEP
3998000
ITEM: NUMBER OF ATOMS
4
ITEM: BOX BOUNDS ff ff ff
-7.4999999999999997e-03 7.4999999999999997e-03
-7.4999999999999997e-03 7.4999999999999997e-03
-7.4999999999999997e-03 2.0000000000000000e-02
ITEM: ATOMS f_bottom_plate[1] f_bottom_plate[2] f_bottom_plate[3] f_bottom_plate[4] f_bottom_plate[5] f_bottom_plate[6] f_bottom_plate[7] f_bottom_plate[8]
1 -3.24063e-80 2.42896e-80 0.000277371 0.00176423 -0.00158717 -0.005 0.0015
1 -2.96449e-80 -8.34283e-80 0.000277371 -0.00107295 -0.00061227 -0.005 0.0015
1 0 0 0.000277371 -0.00257804 0.00285807 -0.005 0.0015
1 -2.68885e-80 -4.4491e-80 0.000277371 -0.00281007 -0.00309978 -0.005 0.0015

 

Atom은 4개이고 모두 바닥 wall에 닿아서 z 방향 힘이 발생하고 있다.