Double-diffusive convective motions for a saturated porous layer subject to modulated surface heating

We examine convective motions in a horizontal porous layer saturated with a binary mixture. The effect of variable solar radiation heating is introduc ed by allowing the upper surface temperature of the region to vary sinusoid ally with time. By modelling flow in the porous layer via Darcy's law and c arrying out a linear stability analysis using Floquet theory, we discuss th e response of the velocity, temperature and solute fields at the onset of c onvection. It is shown that each type of instability (synchronous, subharmo nic or at a frequency unrelated to the heating frequency) can be characteri zed by the behaviour of the vertical components of the perturbation fields. We demonstrate this by considering the time-dependent stationary points of the vertical components over several periods of heating. Phase shifts in t he Galerkin coefficients for the temperature and solutal perturbations are also compared with theoretical predictions.