# Wind Reversal in Bubbly Natural Convection
## 1 Introduction  
The standard Rayleigh-Benard convection occurs in a Newtonian single-phase fluid, bounded by lower (hot) and upper (cold) horizontal plates both at constant temperature, and an adiabatic side-wall. The thermal-induced buoyancy force pushes the stream toward the hotter region where the fluid is lighter. In the colder region, it happens in exactly the opposite way. The thermal convection undergoes significant changes with phase transitions [1, 2] and when polymers [3], bubbles [4, 5], and particles [6] disperse in the continuous phase. The most relevant effect is the heat transfer enhancement of the multi-phase convection respect to the case of single phase convection. The multi-phase Rayleigh-Benard convection plays a leading role in a wide range of physical phenomena such as the formation of atmospheric precipitation [7], magma chambers [8, 9] boiling of liquid [10], and counterflow cooling towers [11]. It is well known that suspended particles or bubbles have a very strong effect on turbulent flows. A considerable amount of experimental evidence has been accumulating for more than 40 years, showing that small particles or bubbles dampen the turbulence, while larger ones increase it, due to vortex shedding behind them [12]. The particles selectively interact with the coherent structures and, in turbulent flows, they preferentially distribute avoiding strong vortical regions whereas they clusteringtight regions [13; 14]. The near-wall turbulent vortical structures control the transport of the particles and the mechanisms that finally segregated them within the viscous boundary layer [15]. All these insights can be used only to roughly depict the physics leading the bubbles turbulence interaction in convective flows. Indeed, the multi-phase convection shows a wide richness of multi-scale physics due to the interplay of the isotropic turbulence in the bulk [16] with the viscous and thermal boundary layers [17] and with the local intermittent events such as the thermal plumes [18]. In such a system, the dispersed phase interacts with the various temporal and spatial scales of the flow topologies according to the coupling of local thermal and mechanical interact between each particle and the surrounding fluid. The physics of particles and bubbles dispersion in convective flows is a key feature in understanding the turbulence dampening and the enhancement of the thermal convection [4; 5; 6]. The control parameters governing the Rayleigh-Benard convection under the Boussinesq approximation are the Rayleigh number Ra (ratio between buoyancy and viscous forces), the Prandtl number Pr (ratio between the thicknesses of the thermal and viscous boundary layers), and the aspect ratio of the geometrical domain; the response parameter is, instead, the Nusselt number (heat transfer rate). In the turbulent regime, the flow is characterized by a large-scale circulation (LSC). The LSC, often called mean wind, whose scale is of the same order of magnitude of the container, overlaps with the background turbulence [19]. The mean wind seems to persist at least for several dozens of the Rayleigh number and for a variety of Prandtl numbers. This wind is fed and driven by the thermal plumes detaching from the boundary layers but also acts back onto the plume formation process [20; 21]. The temperature fluctuations in the thermal layer is responsible for the generation of the thermal plumes [22]. In the literature, there is not a precise definition for the thermal plume, but we can follow what we find in [PERSON] et al. [23], that defined the plume like a coherent structures arising from the thermal boundary layers by buoyancy. The plume makes the strain high because of the sweeping connected to the rising of hot fluid. The operative methodologies, able to identify the thermal plume, are mainly based on: (i) the characterization of the coherent structures in the boundary layers and (ii) the analysis of the thermal fluctuations of the local measurements. In the 1990s, the fly-wheel picture was developed for the large-scale wind.