Navier-Stokes equations

The incompressible Navier-Stokes equations are
$\rho {\frac {D\mathbf {u} }{Dt}}=-\nabla p+\mu \nabla ^{2}\mathbf {u} \,,$
$\nabla \cdot \mathbf {u} =0\;,$
something for density... put in the equations for the Boussinesq approximation.