Here, p is the pressure and μ is the fluid viscosity. It is important to note that, in absence of any external pressure driving force, the fluid motion is a result of the buoyancy forces that develop due to the density gradient caused by the volumetric expansion of the fluid with increasing temperature. Conventionally, for modelling free convection, the density in body force term, g  in equation ( ) is approximated by a linear function of temperature known as Boussinesq approximation :