Present work performed numerical simulations for jet-like, plume-like and off-source heated jet-like vortex pairs with standard k- model under OpenFOAM environment. It is shown that the total circulation of the eddy, which is considered to contribute to entrainment, increases due to buoyancy. Although eddy seems to be disrupted by the addition of heat away from the source, entrainment rate is reconfirmed to be increased in off-source heated jet. LES was also performed for pure jet, forced plume and off-source heated jet. Coherent vortical structures obtained under LES support the results gained from simulations of vortex pairs, and give more information on the relation between buoyancy and eddies.