“…Density di erences are induced, for instance, by gradients of temperature arising by non uniform heating of the uid. In the Boussinesq approximation of a large class of ow problems thermodynamical coe cients such as viscosity, speci c heat and thermal conductivity, can be assumed constant leading to a coupled system with linear second order operators in the Navier-Stokes and heat equations (see, e.g., [10], [11], [15], [21]). However, there are some uids like lubri cants or some plasma ow for which this is no longer an accurate assumption (see, e.g., [14], [19]).…”