  We numerically study the propagation of reacting fronts in a shallow and
horizontal layer of fluid with solutal feedback and in the presence of a
thermally driven flow field of counter-rotating convection rolls. We solve the
Boussinesq equations along with a reaction-convection-diffusion equation for
the concentration field where the products of the nonlinear autocatalytic
reaction are less dense than the reactants. For small values of the solutal
Rayleigh number the characteristic fluid velocity scales linearly, and the
front velocity and mixing length scale quadratically, with increasing solutal
Rayleigh number. For small solutal Rayleigh numbers the front geometry is
described by a curve that is nearly antisymmetric about the horizontal
midplane. For large values of the solutal Rayleigh number the characteristic
fluid velocity, the front velocity, and the mixing length exhibit square-root
scaling and the front shape collapses onto an asymmetric self-similar curve. In
the presence of counter-rotating convection rolls, the mixing length decreases
while the front velocity increases. The complexity of the front geometry
increases when both the solutal and convective contributions are significant
and the dynamics can exhibit chemical oscillations in time for certain
parameter values. Lastly, we discuss the spatiotemporal features of the complex
fronts that form over a range of solutal and thermal driving.
