message: add documentation for outflow boundary conditions with DGTraceIntegrator
diff: @@ -2652,10 +2652,14 @@ public:
  /** Integrator for the DG form:
  alpha < rho_u (u.n) {v},[w] > + beta < rho_u |u.n| [v],[w] >,
  where v and w are the trial and test variables, respectively, and rho/u are
  - given scalar/vector coefficients. The vector coefficient, u, is assumed to
  - be continuous across the faces and when given the scalar coefficient, rho,
  - is assumed to be discontinuous. The integrator uses the upwind value of rho,
  - rho_u, which is value from the side into which the vector coefficient, u,
  + given scalar/vector coefficients. {v} represents the average value of v on
  + the face and [v] is the jump such that {v}=(v1+v2)/2 and [v]=(v1-v2) for the
  + face between elements 1 and 2. For boundary elements, v2=0 such that
  + alpha=1, beta=0.5 corresponds to "outflow" on the external boundary. The
  + vector coefficient, u, is assumed to be continuous across the faces and when
  + given the scalar coefficient, rho, is assumed to be discontinuous. The
  + integrator uses the upwind value of rho, rho_u, which is value from the side
  + into which the vector coefficient, u,
  points. */
  class DGTraceIntegrator : public BilinearFormIntegrator
  {
  