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read_surf command

Syntax:

read_surf filename keyword args ... 

Examples:

read_surf surf.sphere
read_surf surf.sphere group sphere2 typeadd 1
read_surf surf.file trans 10 5 0 scale 3 3 3 invert clip
read_surf surf.file trans 10 5 0 scale 3 3 3 invert clip 1.0e-6 

Description:

Read the geometry of a surface from the specified file. In SPARTA, a "surface" is a collection of surface elements that represent the surface of one or more physical objects which will be embedded in the global simulation box. Surface elements are triangles in 3d or line segments in 2d. Surface elements for each physical object are required to be a complete, connected set that tile the entire surface of the object. See the discussion of watertight surfaces below.

Particles collide with surface elements as they advect. Each surface element is assigned to a collision model, specified by the surf_collide command which affects how a particle bounces off the surface. Each surface elements can optionally be assigned to a reaction model, specified by the surf_react command which determines if any surface chemistry occurs during a collision. Statistics for each surface element due to their interactions with particles can be tallied via the compute surf command, time-averaged via the fix ave/surf command, and ouput via the dump surface command.

Surface elememts can be assigned to surface groups via the group surf command. Surface group IDs are used by other commands to operate on selected sets of elements. This command has group and typeadd keywords which can be used to help assign different elements or different objects to different groups.

Note that the read_surf command can be used multiple times to read multiple objects from multiple files and add them to the simulation domain. The format of a surface file is discussed below. Optional keywords allow the vertices in the file to be translated, scaled, and rotated in various ways. This allows a single surface file, e.g. containing a unit sphere, to be used multiple times in a single simulation or in different simulations.

The tools directory contains tools that can create surface files with simple geometric objects (spheres, blocks, etc). It also has tools that can convert surface files in other formats to the SPARTA format, e.g. for files created by a mesh-generation program.


The specified file can be a text file or a gzipped text file (detected by a .gz suffix).

A surface file has a header and a body. The header appears first. The first line of the header is always skipped; it typically contains a description of the file. Then lines are read one at a time. Lines can have a trailing comment starting with '#' that is ignored. If the line is blank (only whitespace after comment is deleted), it is skipped. If the line contains a header keyword, the corresponding value is read from the line. If it doesn't contain a header keyword, the line begins the body of the file.

The body of the file contains one or more sections. The first line of a section has only a keyword. The next line is skipped. The remaining lines of the section contain values. The number of lines in a section depends on the section keyword as described below. Zero or more blank lines can be used between sections. Sections can appear in any order.

The formatting of individual lines in the surface file (indentation, spacing between words and numbers) is not important except that header and section keywords must be capitalized as shown and can't have extra white space between their words.

These are the recognized header keywords. Header lines can come in any order. The value(s) are read from the beginning of the line. Thus the keyword points should be in a line like "1000 points".

These are the recognized section keywords for the body of the file.

The Points section consists of N consecutive entries, where N = # of points, each of this form:

point-ID x y z    (for 3d) 
point-ID x y      (for 2d) 

The point-ID is ignored; it is only added to assist in examining the file. The point-IDs should run consecutively from 1 to N. X,y,z are the coordinates of the point in distance units. Note that for 2d simulations, z should be omitted.

The Lines section is only allowed for 2d simulations and consists of N entries, where N = # of lines, each of this form:

line-ID (type) p1 p2 

The line-ID is ignored; it is only added to assist in examining the file. Type is an optional integer value which must be specified for all or none of the lines. If used, it must be a positive integer value for each line. If not specified, the type of each line is set to 1. Line types can be used to assign lines to surface groups via the group surf command.

P1 and p2 are the point-IDs of the 2 end points of the line segment. Each is a value from 1 to Npoints, as described above.

The ordering of p1 and p2 is important as it defines the direction of the outward normal for the line segment when a particle collides with it. Molecules only collide with the "outer" edge of a line segment. This is defined by a right-hand rule. The outward normal N = (0,0,1) x (p2-p1). In other words, a unit z-direction vector is crossed into the vector from p1 to p2 to determine the normal.

The Triangles section is only allowed for 3d simulations and consists of N entries, where N = # of triangles, each of this form:

tri-ID (type) p1 p2 p3 

The tri-ID is ignored; it is only added to assist in examining the file. Type is an optional integer value which must be specified for all or none of the triangles. If used, it must be a positive integer value for each triangle. If not specified, the type of each triangle is set to 1. Triangle types can be used to assign triangles to surface groups via the group surf command.

P1 and p2 and p3 are the point-IDs of the 3 corner points of the triangle. Each is a value from 1 to Npoints, as described above.

The ordering of p1 and p2 and p3 is important as it defines the direction of the outward normal for the triangle when a particle collides with it. Molecules only collide with the "outer" face of a triangle. This is defined by a right-hand rule. The outward normal N = (p2-p1) x (p3-p1). In other words, the edge from p1 to p2 is crossed into the edge from p1 to p3 to determine the normal.


The following optional keywords affect the geometry of the read-in surface elements. The geometric transformations they describe are performed in the order they are listed, which gives flexibility in how surfaces can be manipulated. Note that the order may be important; e.g. performing an origin operation followed by a rotate operation may not be the same as a rotate followed by a origin.

Most of the keywords perform a geometric transformation on all the vertices in the surface file with respect to an origin point. By default the origin is (0.0,0.0,0.0), regardless of the position of individual vertices in the file.

The origin keyword resets the origin to the specified Ox,Oy,Oz. This operation has no effect on the vertices.

The trans keyword shifts or displaces the origin by the vector (Dx,Dy,Dz). It also displaces each vertex by (Dx,Dy,Dz).

The atrans keyword resets the origin to an absolute point (Ax,Ay,Az) which implies a displacement (Dx,Dy,Dz) from the current origin. It also displaces each vertex by (Dx,Dy,Dz).

The ftrans keyword resets the origin to a fractional point (Fx,Fy,Fz). Fractional means that Fx = 0.0 is the lower edge/face in the x-dimension and Fx = 1.0 is the upper edge/face in the x-dimension, and similarly for Fy and Fz. This change of origin implies a displacement (Dx,Dy,Dz) from the current origin. This operation also displaces each vertex by (Dx,Dy,Dz).

The scale keyword does not change the origin. It computes the displacement vector of each vertex from the origin (delx,dely,delz) and scales that vector by (Sx,Sy,Sz), so that the new vertex coordinate is (Ox + Sx*delx,Oy + Sy*dely,Oz + Sz*delz).

The rotate keyword does not change the origin. It rotates the coordinates of all vertices by an angle theta in a counter-clockwise direction, around the vector starting at the origin and pointing in the direction Rx,Ry,Rz. Any rotation can be represented by an appropriate choice of origin, theta and (Rx,Ry,Rz).

The invert keyword does not change the origin or any vertex coordinates. It flips the direction of the outward surface normal of each surface element by changing the ordering of its vertices. Since particles only collide with the outer surface of a surface element, this is a mechanism for using a surface files containing a single sphere (for example) as either a sphere to embed in a flow field, or a spherical outer boundary containing the flow.

The clip keyword does not change the origin. It truncates or "clips" a surface that extends outside the simulation box in the following manner. In 3d, each of the 6 clip planes represented by faces of the global simulation box are considered in turn. Any triangle that straddles the face (with points on both sides of the clip plane), is truncated at the plane. New points along the edges that cross the plane are created. A triangle may also become a trapezoid, in which case it turned into 2 triangles. Then all the points on the side of the clip plane that is outside the box, are projected onto the clip plane. Finally, all triangles that lie in the clip plane are removed, as are any points that are unused after the triangle removal. After this operation is repeated for all 6 faces, the remaining surface is entirely inside the simulation box, though some of its triangles may include points on the faces of the simulation box. A similar operation is performed in 2d with the 4 clip edges represented by the edges of the global simulation box.

IMPORTANT NOTE: If a surface you clip crosses a periodic boundary, as specified by the boundary command, then the clipping that takes place must be consistent on both the low and high end of the box (in the periodic dimension). This means any point on the boundary that is generated by the clip operation should be generated twice, once on the low side of the box and once on the high side. And those two points must be periodic images of each other, as implied by periodicity. If the surface you are reading does not clip in this manner, then SPARTA will likely generate an error about mis-matched or inconsistent cells when it attempts to mark all the grid cells and their corner points as inside vs outside the surface.

If you use the clip keyword, you should check the resulting statistics of the clipped surface printed out by this command, including the minimum size of line and triangle edge lengths. It is possible that very short lines or very small triangles will be created near the box surface due to the clipping operation, depending on the coordinates of the initial unclipped points.

If this is the case, an optional fraction argument can be appended to the clip keyword. Fraction is a unitless value which is converted to a distance delta in each dimension where delta = fraction * (boxhi - boxlo). If a point is nearer than delta to the lo or hi boundary in a dimension, the point is moved to be on the boundary, before the clipping operation takes place. This can prevent tiny surface elements from being created due to clipping. If fraction is not specified, the default value is 0.0, which means points are not moved. If specified, fraction must be a value between 0.0 and 0.5.

Note that the clip operation may delete some surface elements and create new ones. Likewise for the points that define the end points or corner points of surface element lines (2d) or triangles (3d). The resulting altered set of surface elements can be written out to a file by the write_surf command, which can then be used an input to a new simulation or for post-processing and visualization.


The following optional keywords affect group and type settings of the read-in surface elements.

Surface groups are collections of surface elements. Each surface element belongs to one or more surface groups; all elements belong to the "all" group, which is created by default. Surface group IDs are used by other commands to identify a group of suface elements to operate on. See the group surf command for more details.

Every surface element also stores a type which is a positive integer. Type values are useful for flagging subsets of elements or different objects in the surface file. For example, a patch of triangles on a sphere. Or one sphere out of several that the file contains. Surface element types can be used to define surface groups. See the group surf command for details.

The group keyword specifies an extra surface group-ID to assign all the read-in surface elements to. All the read-in elements are assigned to the "all" group and to group-ID. If group-ID does not exist, a new surface group is created. If it does exist the read-in surface elements are added to that group.

The typeadd keyword defines an Noffset value which is added to the type of each read-in surface element. The default is Noffset = 0, which means the read-in type values are not altered. If type values are not included in the file, they default to 1 for every element, but can still be altered by the typeadd keyword.

Note that use of the group and typeadd keywords allow the same surface file to be read multiple times (e.g. with different origins, tranlations, rotations, etc) to define multiple objects, and assign their surface elements to different groups or different type values.

The particle keyword determines how particles in the simulation are affected by the new surface elements. If the setting is none, which is the default, then no particles can exist in the simulation. If the setting is check, then particles in grid cells that are inside the new watertight surface object(s) or in grid cells intersected by the new surface elements are deleted. This is to insure no particles will end up inside a surface object, which will typically generate errors when particles move. If the setting is keep then no particles are deleted. It is up to you to insure that no particles are inside surface object(s), else an error may occur later. This setting can be useful if a remove_surf was used to remove a surface object, and a new object is being read in, and you know the new object is smaller than the one it replaced. E.g. for a model of a shrinking or ablating object.


Restrictions:

This command can only be used after the simulation box is defined by the create_box command, and after a grid has been created by the create_grid command. If particles already exist in the simulation, you must insure particles do not end up inside the added surfaces. See the particle keyword for options with regard to particles.

To read gzipped surface files, you must compile SPARTA with the -DSPARTA_GZIP option - see Section 2.2 of the manual for details.

Every vertex in the final surface (after translation, rotation, scaling, etc) must be inside or on the surface of the global simulation box. Note that using the clip operation guarantees that this will be the case.

The surface elements in a single surface file must represent a "watertight" surface. For a 2d simulation this means that every point is part of exactly 2 line segments. For a 3d simulation it means that every triangle edge is part of exactly 2 triangles. Exceptions to these rules allow for triangle edges (in 3d) that lie entirely in a global face of the simulation box, or for line points (in 2d) that are on a global edge of the simulation box. This can be the case after clipping, which allows for use of watertight surface object (e.g. a sphere) that is only partially inside the simulation box, but which when clipped to the box becomes non-watertight, e.g. half of a sphere.

Note that this definition of watertight does not require that the surface elements in a file represent a single physical object; multiple objects (e.g. spheres) can be represented, provided each is watertight.

Another restriction on surfaces is that they do not represent an object that is "infinitely thin", so that two sides of the same object lie in the same plane (3d) or on the same line (2d). This will not generate an error when the surface file is read, assuming the watertight rule is followed. However when particles collide with the surface, errors will be generated if a particle hits the "inside" of a surface element before hitting the "outside" of another element. This can occur for infinitely thin surfaces due to numeric round-off.

When running a simulation with multiple objects, read from one or more surface files, you should insure they do not touch or overlap with each other. SPARTA does not check for this, but it will typically lead to unphysical particle dynamics.

Related commands:

write_surf

Default:

The default origin for the vertices in the surface file is (0,0,0). The defaults for group = all, type = no, toffset = 0, particle = none.