On the role of thermal boundary conditions in typical problems of buoyancy convection: A combined experimental-numerical analysis

•Heat loss through solid walls can exert a significant influence on buoyancy flow.•We explore the counter-intuitive relationship between heat loss and the hierarchy of bifurcations.•The problem is tackled experimentally and numerically considering a fluid with variable properties.•A kaleidoscope of solutions is obtained by exploring the space of relevant parameters.•The prevailing two-dimensional or three-dimensional nature of the emerging flow is examined. Buoyancy flows of thermal origin and related heat transfer problems are central in a variety of disciplines and technological applications. In the present study the classical case of a cavity heated from one side and cooled from the other (internal size 4 cm, filled with water) is tackled both experimentally and numerically for different circumstances (horizontal and inclined temperature gradients). The main objective is to fill a gap, namely, the surprising lack of knowledge relating to the role played by the effective heat loss taking place through the walls delimiting the fluid domain from above and from below and along the spanwise direction in influencing the instabilities of these flows and their progression towards chaos. We explore the response of such systems with respect to several parameters, including the inclination of the cavity with respect to gravity, the average temperature of the fluid, the applied temperature difference, the dependence of fluid properties on temperature and the intensity of heat transfer to the ambient. Experiments are supported by dedicated numerical simulations based on the Navier-Stokes and energy equations in their time-dependent and non-linear formulation (solved by means of the PISO method with a collocated-grid approach). It is shown that a kaleidoscope of states is possible depending on the considered conditions. The results reveal the counter-intuitive triadic relationship among heat loss through non-thermally active walls, the hierarchy of bifurcations displayed by the system and the prevailing two-dimensional or three-dimensional nature of the flow.