CFD Utilities Files

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Program USLOS Description:

   USLOS (unstructured grid lines of sight) combines the functions of the
   LINES_OF_SIGHT and HEMISPHERES_OF_SIGHT utilities for the case of 3-space
   volume grids that are NOT multiblock volume grids.  Since these are purely
   geometric functions, it is assumed that the needed inner and outer (shock)
   boundaries can be provided as triangulated surfaces, and that these are
   compatible with Tecplot (a de facto standard at NASA).  The lines of sight
   are expected to be used for radiative heat flux calculations.

   Anticipated usage is for either a list of forebody points or a single body
   point on the aft body or perhaps near the shoulder of a typical atmos-
   pheric entry vehicle.  More than one set of hemisphere lines of sight is
   NOT recommended, so if the body point list contains multiple entries, it
   is assumed that only the body-normal lines are required.

   The inner surface is expected to be right-handed.  This means that for
   vertices 1, 2, 3 of a triangular element, the body normal vector is de-
   fined by the cross product vector w = (v2 - v1) x (v3 - v1) and this
   should point into the flow field (i.e., outwards from what is presumed to
   be a mostly-convex surface).  Any concavities may cause difficulties such
   as those handled by ancillary utility ADJUST_HEMISPHERE_LOS for the
   structured surface case, which will need an analogue for the unstructured
   case.  If the outer surface is derived from the same (right-handed) volume
   grid, this means that a vector defined similarly to be normal to the outer
   surface would point away from the body. But no such normal is ever needed,
   so the outer surface need not be right-handed.  It is used only for line-
   surface intersection calculations.

Body Point Inputs (one entry per line; trailing comments ignored):

   Body-normal (tangent-slab radiation calculation) case:

      0.  0.  0.
      1.628659741950E-01  5.741545696080E-01  0.  ! Nose/cone tangency
      8.985316209400E-01  1.837811469800E+00  0.  ! Cone/shoulder tangency
       :                   :                   :

   Hemispherical lines (full angular integration) case:

      1.0742  -1.81  1.083  [10.]  ! x, y, z[, cone angle]

      Only one body point is handled for this case.  The optional semi-cone
      angle allows for writing fewer than the full hemisphere's worth of
      lines to avoid unnecessary radiation cone angles outside the viewing
      angle of a radiometer instrument.  Note that NEQAIR_INTEGRATION can
      still integrate over a narrower angle than the indicated data angle.
      If the optional angle is omitted, it is taken to be 90 degrees.

Input Surface Triangulation Format (inner and outer, in separate files):

   TITLE = "Dataset title"
   VARIABLES = "x", "y", "z"[, "f1"[, "f2"] ...]
   ZONE T="ZONE 1", NODES=450816, ELEMENTS=150272,
   DATAPACKING=POINT, ZONETYPE=FETRIANGLE
    3.5443134E-01  1.0169792E+00  5.8715301E-01 [optional functions, ignored]
    3.5443952E-01  1.0356432E+00  5.5356261E-01     "         "         "
         :              :              :
         :              :              :
    2.8716209E+00 -1.1242014E-01  6.4854415E-02  0.0000000E+00
         1       2       3
         4       5       6
         :       :       :
         :       :       :

    450814  450815  450816

   Multizone ASCII files are permitted.  Unformatted file input is not sup-
   ported.  Any function data are ignored.  One way of triangulating a multi-
   block PLOT3D-type structured PLOT3D-type surface is to run HYPER_AERO and
   use the Tecplotable output of Newtonian Cp predictions.  The Cps will be
   ignored.  They are vertex-centered, so tri_header%fileform = 1 here.

Unit Spherical Octant Triangulation Choice:

   This version of USLOS has the SLOS option to read a previously generated
   unit triangulation that the Equal_Area_Triangulation variant of the
   NPOPT_DRIVER framework can now produce.  If nedge is specified as 25
   (say), we look for the file unit_sphere_octant_25.dat (likewise for
   other nedge values).  If the file is not found, subroutine spherical_-
   triangulation is employed as originally.

   In principle, the integrations w.r.t. solid angle of radiances
   calculated for each line of sight will be more accurate if the solid
   angle elements are essentially equal, although it is expected that the
   two kinds of triangulation will produce very similar results for a given
   value of nedge.

Outputs:

   PLOT3D-type output is retained as for the structured case (one or more
   blocks, one line of sight per block).  Ensuing procedures will interpolate
   flow data onto the discretized lines of sight and write results compatible
   with the radiation solver.  The file name is hard-coded as 'los.g' as part
   of simplifying the original nomenclature.

Method:

   >  A handful of prompts serve to drive the program -- no control file.
   >  Read the indicated body point (x,y,z) file -- one or more points.
   >  If a single body point is found, prompt in case hemispherical lines are
      NOT intended.
   >  If all body points are on the centerline (y = 0.), the input surfaces
      are assumed to be the starboard (right) half.  That half (y > 0.)
      also serves for off-center body-normal lines, but for a hemisphere at
      an off-center body point, a full volume grid is expected, with the
      outer surface closed and convex.
   >  Read an inner surface triangulation then an outer surface triangulation
      from what is assumed to be a shock-aligned volume grid.
   >  Build a search tree from the inner surface triangulation, and search
      for each body point.
   >  Generate and store the corresponding unit body-normal vector(s).
   >  Read the indicated outer boundary triangulation, and build a new search
      tree as needed for line-surface intersection calculations.

   >  Body-normal line case:
         >  For each body point B:
            >  The body normal unit vector is already in hand.
            >  A search interval along the body-normal line for minimizing
               the distance to the outer surface is provided by s = 0 at the
               low end and s = the diagonal of the bounding box for the
               outer surface mesh.
            >  The intersection point is then conveniently calculated via
               a 1D minimization w.r.t. distance s from B along the body-
               normal line (INTSEC8).
            >  Impose a CFD-type point distribution on the intersected
               interval BP, as needed for interpolated flow field data.
               DPLR-type use of inputs nk, ds1, ds2fraction will suffice.
         >  Write the discretized line(s) of sight to the indicated PLOT3D-
            type output file.

   >  Hemisphere lines of sight case:
         >  The vertices of a triangulated unit hemispherical quadrant define
            the lines of sight between the body point and the outer grid
            boundary.  The number of uniform points specified along an edge
            determines the resolution.  Typically, 25 such points are used,
            giving 650 lines of sight for a centerline body point, or 1300
            lines if the body point is off-center.  Centerline points require
            just half the volume data, and the eventual integration over
            solid angle of radiances from the lines of sight can be doubled.
         >  The initial unit quadrant is transformed so that line 1/vertex 1
            corresponds to the body point normal. After The lines for the
            first quadrant are generated, the quadrant is rotated 90 degrees
            about the normal and the next set of lines is generated.  Then if
            the body point is off-center, this is repeated twice more.
         >  Each intersection involves a 2-point line from the body point
            through a quadrant vertex, of length derived from the bounding
            box of the outer shock surface.  This length is overkill for many
            intersection search intervals, but that doesn't matter.  Each
            hemisphere line is intersected and discretized as for the body
            normal (tangent-slab) case described above.
History:

   09/27/2018  D.A.Saunders  Initial design.
   10/20/2018     "    "     After a hiatus, finished the body-normal case.
   10/22/2018     "    "     Allowed for multizone surfaces.
   10/31/2018     "    "     Finished adding & testing the hemisphere option.
   11/01/2018     "    "     Filled in the missing hemisphere method outline.
   11/06/2018     "    "     Corrected some of the description above.  Note
                             SLOS, the structured-grid analogue of USLOS, is
                             now an alternative to the original utilities.
   11/09/2018     "    "     Use 'los.g' for the output file name as part of
                             adjusting NEQAIR_Integration to handle the
                             original nomenclature and this simplified form.
   10/22/2019     "    "     Updates analogous to SLOS refinements, where
                             failed intersections prompted another way of
                             retrying: discretize the part of the line beyond
                             the failed solution and evaluate line-surface
                             distances from those points; pick the interval
                             containing the smallest distance, and redo the
                             line-surface intersection.  REMEMBER THAT THE
                             ARC LENGTH INTERVAL PASSED TO INTSEC8 SHOULD BE
                             NORMALIZED BY THE LINE LENGTH IF THAT IS THE
                             INTERVAL EXPECTED TO CONTAIN THE INTERSECTION,
                             AS IT IS HERE, USING THE BOUNDING BOX DIAGONAL.
   03/07/2022     "    "     Subroutine intsec8 has been superseded by higher
                             level routine usintsec to handle possible con-
                             cavities in an inner or outer boundary.
   04/02/2022     "    "     Quad. surfaces are now an option.  They are
                             converted in place to triangulations, and a new
      and                    combine_zones in triangulation_io transcribes
                             %xyz and %conn for all zones to the header as
   04/15/2022                needed for rapid searching.
   04/25/2022     "    "     Cleared up confusion over %centers_to_vertices:
                             any functions are ignored by USLOS, so this
                             switch is irrelevant once %numf <-- 0.
   09/30/2022     "    "     The option to read an optimized (more nearly
                             equal cell areas) unit spherical octant tri-
                             angulation has been belatedly installed as in
                             SLOS.  See description above.

Author:  David Saunders, AMA, Inc. at NASA Ames Research Center, CA.

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