message: mention filter algorithm reference
diff: @@ -245,6 +245,13 @@ public:
  void SetCutoffModes(int c) { filter_cutoff_modes = c; }
  /// Set the interpolation filter parameter @a alpha
  + /**
  + * If the @a a is > 0, the filtering algorithm for the velocity field after
  + * every time step from [1] is used.
  + *
  + * [1] Paul Fischer, Julia Mullen: Filter-based stabilization of spectral
  + * element methods
  + */
  void SetFilterAlpha(double a) { filter_alpha = a; }
  protected:
  