forked from lijiext/lammps
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
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@ -238,7 +238,7 @@ For example, the following information would specify a square particle
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whose edge length is sqrt(2) and rounded diameter is 1.0. The
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orientation of the square is aligned with the xy coordinate axes which
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is consistent with the 6 moments of inertia: ixx iyy izz ixy ixz iyz =
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1 1 4 0 0 0.
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1 1 4 0 0 0. Note that only Izz matters in 2D simulations.
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3 1 27
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4
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@ -253,6 +253,24 @@ is consistent with the 6 moments of inertia: ixx iyy izz ixy ixz iyz =
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3 0
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1.0 :pre
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A rod in 2D, whose length is 4.0, mass 1.0, rounded at two ends
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by circles of diameter 0.5, is specified as follows:
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1 1 13
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2
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1 1 1.33333 0 0 0
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-2 0 0
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2 0 0
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0.5 :pre
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A disk, whose diameter is 3.0, mass 1.0, is specified as follows:
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1 1 10
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1
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1 1 4.5 0 0 0
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0 0 0
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3.0 :pre
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The "pair_style body/rounded/polygon"_pair_body_rounded_polygon.html
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command can be used with this body style to compute body/body
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interactions. The "fix wall/body/polygon"_fix_wall_body_polygon.html
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@ -377,6 +395,24 @@ iyz = 0.667 0.667 0.667 0 0 0.
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3 0 4 7
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0.5 :pre
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A rod in 3D, whose length is 4.0, mass 1.0 and rounded at two ends
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by circles of diameter 0.5, is specified as follows:
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1 1 13
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2
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0 1.33333 1.33333 0 0 0
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-2 0 0
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2 0 0
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0.5 :pre
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A sphere whose diameter is 3.0 and mass 1.0, is specified as follows:
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1 1 10
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1
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0.9 0.9 0.9 0 0 0
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0 0 0
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3.0 :pre
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The "pair_style
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body/rounded/polhedron"_pair_body_rounded_polyhedron.html command can
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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
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Style {body/rounded/polygon} is for use with 2d models of body
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particles of style {rounded/polygon}. It calculates pairwise
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body/body interactions as well as interactions between body and point
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particles. See "Section 6.14"_Section_howto.html#howto_14 of the
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particles (modeled as disks with a specified diameter).
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See "Section 6.14"_Section_howto.html#howto_14 of the
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manual and the "body"_body.html doc page for more details on using
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body particles.
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body rounded/polygon particles.
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This pairwise interaction between rounded polygons is described in
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"Fraige"_#Fraige, where a polygon does not have sharp corners, but is
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@ -50,14 +51,8 @@ between two particles are defined with respect to the separation of
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their respective rounded surfaces, not by the separation of the
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vertices and edges themselves.
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This means that the specified cutoff in the pair_style command should
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be large enough to encompass the center-to-center distance between two
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particles (at any orientation) which would produce an overlap of the
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two surfaces. For example, consider two square particles with edge
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length = 1.0 and circle diameter 0.2. The maximum distance of one
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polygon's surface from its center is not sqrt(2)/2, but
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(sqrt(2)+0.1)/2. Thus the cutoff distance should be sqrt(2) + 0.1,
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since the surfaces of two particles that far apart could be touching.
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This means that the specified cutoff in the pair_style command is
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the cutoff distance, r_c, for the surface separation, \delta_n (see figure below).
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The forces between vertex-vertex, vertex-edge, and edge-edge overlaps
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are given by:
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@ -66,17 +61,6 @@ are given by:
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:c,image(JPG/pair_body_rounded.jpg)
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In "Fraige"_#Fraige, the tangential friction force between two
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particles that are in contact is modeled differently prior to gross
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sliding (i.e. static friction) and during gross-sliding (kinetic
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friction). The latter takes place when the tangential deformation
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exceeds the Coulomb frictional limit. In the current implementation,
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however, we do not take into account frictional history, i.e. we do
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not keep track of how many time steps the two particles have been in
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contact nor calculate the tangential deformation. Instead, we assume
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that gross sliding takes place as soon as two particles are in
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contact.
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TRUNG: The diagram label "cohesive regions" confuses me. Are you
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saying there is some distance d for which the force is attractive,
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i.e. the particles are cohesive? I think when d > Ri + Rj, since Ri +
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@ -89,6 +73,25 @@ is a single number, but depedning on the orientiation of the 2
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particles they might have a suface/surface overlap at a much
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smaller value of Ri + Rj. So what is Rc then?
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Note that F_n and F_t are functions of the surface separation
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\delta_n = d - (R_i + R_j).
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In this model, when (R_i + R_j) < d < (R_i + R_j) + r_c, that is,
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0 < \delta_n < r_c, the cohesive region of the two surfaces overlap
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and the two surfaces are attractive to each other.
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In "Fraige"_#Fraige, the tangential friction force between two
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particles that are in contact is modeled differently prior to gross
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sliding (i.e. static friction) and during gross-sliding (kinetic
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friction). The latter takes place when the tangential deformation
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exceeds the Coulomb frictional limit. In the current implementation,
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however, we do not take into account frictional history, i.e. we do
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not keep track of how many time steps the two particles have been in
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contact nor calculate the tangential deformation. Instead, we assume
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that gross sliding takes place as soon as two particles are in
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contact.
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The following coefficients must be defined for each pair of atom types
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via the "pair_coeff"_pair_coeff.html command as in the examples above,
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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
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Style {body/rounded/polygon} is for use with 3d models of body
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particles of style {rounded/polyhedron}. It calculates pairwise
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body/body interactions as well as interactions between body and
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point-particles. See "Section 6.14"_Section_howto.html#howto_14 of
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point-particles (modeled as spheres with a specified diameter).
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See "Section 6.14"_Section_howto.html#howto_14 of
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the manual and the "body"_body.html doc page for more details on using
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body particles.
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body rounded/polyhedron particles.
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TRUNG: I think we need a paragraph here about how body/sphere
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interactions are handled. Does this pair style only do body/body but
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@ -47,7 +48,7 @@ this pair style file just a couple lines about which part of the
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interactions this pair style computes. Ditto in the pair body polygon
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file.
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This pairwise interaction between rounded polyhedra is described in
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This pairwise interaction between the rounded polyhedra is described in
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"Wang"_#Wang, where a polyhedron does not have sharp corners and
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edges, but is rounded at its vertices and edges by spheres centered on
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each vertex with a specified diameter. The edges if the polyhedron
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@ -57,6 +58,7 @@ in the data file read by the "read data"_read_data.html command. This
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is a discrete element model (DEM) which allows for multiple contact
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points.
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Note that when two particles interact, the effective surface of each
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polyhedron particle is displaced outward from each of its vertices,
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edges, and faces by half its sphere diameter. The interaction forces
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@ -64,14 +66,8 @@ and energies between two particles are defined with respect to the
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separation of their respective rounded surfaces, not by the separation
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of the vertices, edges, and faces themselves.
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This means that the specified cutoff in the pair_style command should
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be large enough to encompass the center-to-center distance between two
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particles (at any orientation) which would produce a surface-surface
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overlap. For example, consider two cubic particles with edge length =
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1.0 and sphere diameter 0.2. The maximum distance of one polygon's
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surface from its center is not sqrt(3)/2, but (sqrt(3)+0.1)/2. Thus
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the cutoff distance should be sqrt(3) + 0.1, since the surfaces of two
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particles that far apart could be touching.
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This means that the specified cutoff in the pair_style command is
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the cutoff distance, r_c, for the surface separation, \delta_n (see figure below).
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The forces between vertex-vertex, vertex-edge, vertex-face, edge-edge,
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and edge-face overlaps are given by:
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