Hybrid forced-buoyancy convection in a channel with a backward facing step

•Hybrid forced-buoyant flow in a duct with a backward facing step is considered.•The sensitivity of this system to the thermal boundary conditions is investigated.•A variety of solutions is obtained for different values of the Rayleigh and Reynolds numbers.•The route to chaos displays a remarkable sensitivity on the considered conditions. The present work integrates and expands the line of inquiry started in Inam and Lappa (2021), Int. J. of Heat and Mass Transfer, 173, 121267 for unsteady mixed forced-buoyancy convection in a channel with a forward facing step by addressing the mirror case where the fluid flowing in a duct undergoes a sudden expansion (i.e., a channel with a backward facing step). By alternatively setting some portions of the lower boundary of the system as isothermal (heated) or adiabatic surfaces, the interplay of forced and buoyancy convection is investigated numerically for a fixed expansion ratio (ER = 2) and a variety of conditions corresponding to extended intervals of the Rayleigh number Ra and several values of the Richardson number Ri (Ra≥O(104) and Ri≥O(1)). It is shown that the emerging dynamics involve a variety of concurrent mechanisms, which range from the standard hydrodynamic phenomena typical of isothermal fluids, to specific effects of buoyant nature (driven by the instability of thermal boundary layers and the ensuing emergence of thermal plumes in various parts of the domain). Comparison with the equivalent forward facing step configuration indicates that, besides the differences, these two systems display interesting analogies.