def _t_normals(self):
        r"""
        Update the throat normals from the voronoi vertices
        """
        verts = self['throat.vertices']
        value = sp.zeros([len(verts), 3])
        for i in range(len(verts)):
            if len(sp.unique(verts[i][:, 0])) == 1:
                verts_2d = sp.vstack((verts[i][:, 1], verts[i][:, 2])).T
            elif len(sp.unique(verts[i][:, 1])) == 1:
                verts_2d = sp.vstack((verts[i][:, 0], verts[i][:, 2])).T
            else:
                verts_2d = sp.vstack((verts[i][:, 0], verts[i][:, 1])).T
            hull = sptl.ConvexHull(verts_2d, qhull_options='QJ Pp')
            sorted_verts = verts[i][hull.vertices].astype(float)
            v1 = sorted_verts[-1]-sorted_verts[0]
            v2 = sorted_verts[1]-sorted_verts[0]
            value[i] = tr.unit_vector(sp.cross(v1, v2))
        return value
r"""
        Update the throat normals from the voronoi vertices