Updated to the doc pages of body rounded/polygon and rounded/polyhedra and the pair style:

+ added examples for special cases with disks and rods for 2d, and spheres and rods for 3d,
+ corrected the definition of the cutoff distance in pair style command
This commit is contained in:
Trung Nguyen 2018-07-17 10:54:05 -05:00
parent 5abbea3606
commit d23788831c
3 changed files with 68 additions and 33 deletions

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@ -238,7 +238,7 @@ For example, the following information would specify a square particle
whose edge length is sqrt(2) and rounded diameter is 1.0. The
orientation of the square is aligned with the xy coordinate axes which
is consistent with the 6 moments of inertia: ixx iyy izz ixy ixz iyz =
1 1 4 0 0 0.
1 1 4 0 0 0. Note that only Izz matters in 2D simulations.
3 1 27
4
@ -253,6 +253,24 @@ is consistent with the 6 moments of inertia: ixx iyy izz ixy ixz iyz =
3 0
1.0 :pre
A rod in 2D, whose length is 4.0, mass 1.0, rounded at two ends
by circles of diameter 0.5, is specified as follows:
1 1 13
2
1 1 1.33333 0 0 0
-2 0 0
2 0 0
0.5 :pre
A disk, whose diameter is 3.0, mass 1.0, is specified as follows:
1 1 10
1
1 1 4.5 0 0 0
0 0 0
3.0 :pre
The "pair_style body/rounded/polygon"_pair_body_rounded_polygon.html
command can be used with this body style to compute body/body
interactions. The "fix wall/body/polygon"_fix_wall_body_polygon.html
@ -377,6 +395,24 @@ iyz = 0.667 0.667 0.667 0 0 0.
3 0 4 7
0.5 :pre
A rod in 3D, whose length is 4.0, mass 1.0 and rounded at two ends
by circles of diameter 0.5, is specified as follows:
1 1 13
2
0 1.33333 1.33333 0 0 0
-2 0 0
2 0 0
0.5 :pre
A sphere whose diameter is 3.0 and mass 1.0, is specified as follows:
1 1 10
1
0.9 0.9 0.9 0 0 0
0 0 0
3.0 :pre
The "pair_style
body/rounded/polhedron"_pair_body_rounded_polyhedron.html command can
be used with this body style to compute body/body interactions. The

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@ -29,9 +29,10 @@ pair_coeff 1 1 100.0 1.0 :pre
Style {body/rounded/polygon} is for use with 2d models of body
particles of style {rounded/polygon}. It calculates pairwise
body/body interactions as well as interactions between body and point
particles. See "Section 6.14"_Section_howto.html#howto_14 of the
particles (modeled as disks with a specified diameter).
See "Section 6.14"_Section_howto.html#howto_14 of the
manual and the "body"_body.html doc page for more details on using
body particles.
body rounded/polygon particles.
This pairwise interaction between rounded polygons is described in
"Fraige"_#Fraige, where a polygon does not have sharp corners, but is
@ -50,14 +51,8 @@ between two particles are defined with respect to the separation of
their respective rounded surfaces, not by the separation of the
vertices and edges themselves.
This means that the specified cutoff in the pair_style command should
be large enough to encompass the center-to-center distance between two
particles (at any orientation) which would produce an overlap of the
two surfaces. For example, consider two square particles with edge
length = 1.0 and circle diameter 0.2. The maximum distance of one
polygon's surface from its center is not sqrt(2)/2, but
(sqrt(2)+0.1)/2. Thus the cutoff distance should be sqrt(2) + 0.1,
since the surfaces of two particles that far apart could be touching.
This means that the specified cutoff in the pair_style command is
the cutoff distance, r_c, for the surface separation, \delta_n (see figure below).
The forces between vertex-vertex, vertex-edge, and edge-edge overlaps
are given by:
@ -66,17 +61,6 @@ are given by:
:c,image(JPG/pair_body_rounded.jpg)
In "Fraige"_#Fraige, the tangential friction force between two
particles that are in contact is modeled differently prior to gross
sliding (i.e. static friction) and during gross-sliding (kinetic
friction). The latter takes place when the tangential deformation
exceeds the Coulomb frictional limit. In the current implementation,
however, we do not take into account frictional history, i.e. we do
not keep track of how many time steps the two particles have been in
contact nor calculate the tangential deformation. Instead, we assume
that gross sliding takes place as soon as two particles are in
contact.
TRUNG: The diagram label "cohesive regions" confuses me. Are you
saying there is some distance d for which the force is attractive,
i.e. the particles are cohesive? I think when d > Ri + Rj, since Ri +
@ -89,6 +73,25 @@ is a single number, but depedning on the orientiation of the 2
particles they might have a suface/surface overlap at a much
smaller value of Ri + Rj. So what is Rc then?
Note that F_n and F_t are functions of the surface separation
\delta_n = d - (R_i + R_j).
In this model, when (R_i + R_j) < d < (R_i + R_j) + r_c, that is,
0 < \delta_n < r_c, the cohesive region of the two surfaces overlap
and the two surfaces are attractive to each other.
In "Fraige"_#Fraige, the tangential friction force between two
particles that are in contact is modeled differently prior to gross
sliding (i.e. static friction) and during gross-sliding (kinetic
friction). The latter takes place when the tangential deformation
exceeds the Coulomb frictional limit. In the current implementation,
however, we do not take into account frictional history, i.e. we do
not keep track of how many time steps the two particles have been in
contact nor calculate the tangential deformation. Instead, we assume
that gross sliding takes place as soon as two particles are in
contact.
The following coefficients must be defined for each pair of atom types
via the "pair_coeff"_pair_coeff.html command as in the examples above,
or in the data file read by the "read_data"_read_data.html command:

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@ -29,9 +29,10 @@ pair_coeff 1 1 100.0 1.0 :pre
Style {body/rounded/polygon} is for use with 3d models of body
particles of style {rounded/polyhedron}. It calculates pairwise
body/body interactions as well as interactions between body and
point-particles. See "Section 6.14"_Section_howto.html#howto_14 of
point-particles (modeled as spheres with a specified diameter).
See "Section 6.14"_Section_howto.html#howto_14 of
the manual and the "body"_body.html doc page for more details on using
body particles.
body rounded/polyhedron particles.
TRUNG: I think we need a paragraph here about how body/sphere
interactions are handled. Does this pair style only do body/body but
@ -47,7 +48,7 @@ this pair style file just a couple lines about which part of the
interactions this pair style computes. Ditto in the pair body polygon
file.
This pairwise interaction between rounded polyhedra is described in
This pairwise interaction between the rounded polyhedra is described in
"Wang"_#Wang, where a polyhedron does not have sharp corners and
edges, but is rounded at its vertices and edges by spheres centered on
each vertex with a specified diameter. The edges if the polyhedron
@ -57,6 +58,7 @@ in the data file read by the "read data"_read_data.html command. This
is a discrete element model (DEM) which allows for multiple contact
points.
Note that when two particles interact, the effective surface of each
polyhedron particle is displaced outward from each of its vertices,
edges, and faces by half its sphere diameter. The interaction forces
@ -64,14 +66,8 @@ and energies between two particles are defined with respect to the
separation of their respective rounded surfaces, not by the separation
of the vertices, edges, and faces themselves.
This means that the specified cutoff in the pair_style command should
be large enough to encompass the center-to-center distance between two
particles (at any orientation) which would produce a surface-surface
overlap. For example, consider two cubic particles with edge length =
1.0 and sphere diameter 0.2. The maximum distance of one polygon's
surface from its center is not sqrt(3)/2, but (sqrt(3)+0.1)/2. Thus
the cutoff distance should be sqrt(3) + 0.1, since the surfaces of two
particles that far apart could be touching.
This means that the specified cutoff in the pair_style command is
the cutoff distance, r_c, for the surface separation, \delta_n (see figure below).
The forces between vertex-vertex, vertex-edge, vertex-face, edge-edge,
and edge-face overlaps are given by: