Buoyancy term in openlb


I was trying to learn the 2D Rayleigh-Benard convection example. In the introduction of LBMethod.org (http://www.lbmethod.org/numerics:rb_2d), the buoyancy term is referred to as “the coupling between the two is implemented by means of a linear buoyancy term based on a Boussinesq approximation”, however I can not fine the parameter of buoyancy in the rayleigh_benard2D.cpp example. Whether it consists in the subroutine of

NavierStokesAdvectionDiffusionCouplingGeneratior2D <T, DESCRIPTOR>
coupling (0, nx-1,0,ny-1, converter.getGravity(), converter.getT0(), 
converter.getDeltaTemperature(), dir)

In addition, whether the buoyancy is produced automatically, because of the different density in two neighboring lattices. Thank you in advance. I am very appreciate for your response.