Convection-dissipation instability in the horizontal plane Couette flow of a highly viscous fluid

The linear stability of the plane Couette flow against thermoconvective rolls is studied. The case of a flow without a boundary-imposed temperature gradient is investigated. The non-uniform, possibly unstable, basic temperature distribution is caused by the effect of the internal viscous heating. Asymmetric thermal boundary conditions are considered: the bottom boundary is adiabatic, while the top boundary is isothermal. The focus is on a fluid with a large, mathematically infinite, Prandtl number, although the two-dimensional transverse roll instability is discussed also for a finite Prandtl number. The transition to the instability is described through the governing parameter Ge*Pe^2, where Ge is the Gebhart number and Pe is the Péclet number. The response of the basic Couette flow to arbitrarily oriented oblique rolls is tested, so that a complete set of disturbance modes is taken into account. It is shown that the Couette flow is more unstable to longitudinal rolls than to any other oblique roll mode.