The ornamental design for a polyhedral model, as shown and described.
FIG. 1 illustrates a perspective view of a polyhedral model corresponding to a rotated elevational view rotated approximately 15°C about an x-axis and 45°C about a y-axis.
FIG. 2 illustrates a perspective view of the polyhedral model of FIG. 1 rotated approximately 45°C about an x-axis; and,
FIG. 3 illustrates a perspective view of the polyhedral model of FIG. 1 rotated approximately 70°C about an x-axis.