A volume with N_fore fores and N_rear rears.
Conceptually a Volume is a Sink and a Source. It therefore defines the level of inertial pressure r in a closed loop and serves as a loop breaker.
This mixing volume doesn't approximate mixing like a junction, but rather does it without approximation.
Since there is no formula to compute density_derp_h for this volume, an upper bound has to be set in the parameter density_derp_h_set. Alternativeley the derivative can be taken from the media model for all the media that implement the corresponding formula by setting density_derp_h_from_media=true (default:false).