Fix equation and typo

Author: Wang-yijun

cookbooks/CPO_induced_anisotropic_viscosity/plugin/anisotropic_stress.cc

@@ -112,15 +112,17 @@ namespace aspect
     {
       ASPECT_REGISTER_VISUALIZATION_POSTPROCESSOR(AnisotropicStress,
                                                   "Anisotropic stress",
-                                                  "A visualization output "
-                                                  "object that generates "
-                                                  "adiabatic temperature, pressure, "
-                                                  "density, and density derivative (with regard to depth)"
-                                                  "as produced by the \\texttt{AdiabaticConditions} class."
-                                                  "\n\n"
-                                                  "Physical units: $\\text{K}$, $\\text{Pa}$, "
-                                                  "$\\frac{\\text{kg}}{\\text{m}^3\\text{m}}$}, "
-                                                  "respectively, for the four components.")
+                                                  "A visualization output object that generates "
+                                                  "the 3 or 6 independent components (in 2d and 3d, respectively) "
+                                                  "of the anisotropic stress tensor. "
+                                                  "The anisotropic stress is defined as $2 \\eta "
+                                                  "(\\varepsilon(\\mathbf u) - \\tfrac 13 \\text{trace} "
+                                                  "\\varepsilon(\\mathbf u) \\mathbf 1) = 2\\eta (\\varepsilon(\\mathbf u) - "
+                                                  "\\frac 13 (\\nabla \\cdot \\mathbf u) \\mathbf I)$ * stress_strain_directors, "
+                                                  "and differs from the full stress by the absence of the pressure. "
+                                                  "The second term in the difference is zero if the model is incompressible. "
+                                                  "This particle property plugin should only be activated when using "
+                                                  "the CPO induced anisotropic viscosity material model from the cookbook.")
     }
   }
 }