2.20 Natural convection
In Sec. 2.13 , a set of equations — Eq. (2.47 ) and Eq. (2.48 ) — was derived for ﬂow of an incompressible ﬂuid. They are a sample set of equations for mass and momentum conservation that can be solved using methods described in this book.
The example set of equations can be extended to include energy conservation and associated models of heat conduction and heat capacity, described in Sec. 2.15  2.18 .
The set of equations for mass, momentum and energy can be combined to simulate natural convection, e.g. for ﬂow of air around a room. In natural convection, a nonuniform temperature causes density variations which generate associated forces due to gravity. Colder air is driven downwards and hot air rises, creating buoyancy. Small temperature variations, e.g. due to a heat source , can cause buoyancy to be the dominant force.
A simple, approximate equation for can be derived, starting from internal energy conservation in the form of Eq. (2.57 ). The approximations of constant (with ) and zero viscosity reduce the stress/pressure work terms to zero.
Assuming constant, we can apply Eq. (2.63 ), which reduces to substituting by since derivatives of constants and are zero. Applying Fourier’s law Eq. (2.54) leads to

(2.65) 
This is another example, similar to Eq. (2.49 ), of a transport equation containing a time derivative, advection, diﬀusion and a source of heat. Applying suitable boundary conditions, the equation can be solved for .
Buoyancy force
The eﬀect of buoyancy can be simulated by setting the body force in Eq. (2.47 ) for an incompressible, Newtonian ﬂuid. While the assumption constant is applied across all the governing equations generally, it cannot be applied to this force. Therefore we apply

(2.66) 
The density is a function of and, optionally, provided by some equation of state, e.g. the ideal gas Eq. (2.55 ). The ﬁnal momentum equation including this buoyancy force, and assuming constant is:

(2.67) 