## 4 Results and Discussion  
### Flow Structure and Thermal Fields  
#### 4.1.1 Dynamic and Thermal Fields for \(\mathrm{Ra}=5\times 10^{7}\)  
The stream function visualization reveals a complex, multicellular flow structure, as indicated in Figure 5. For \(Re=10^{3}\), (Figure 5a) two convective cells fill the entire space between the heat sinks (hereafter referred to as micro-cavity) by keeping out fresh air. In fact, the air confined in this zone is set in motion by the thermal gradient imposed by the vertical walls of the blocks. As it rises towards the center of the cavity, it is forced to move downwards under the effect of the training by the forced jet on the one hand a cooling in contact with the latter which will increase its density and therefore its weight, on the other hand. Two other low-intensity cells are in the upper part of the cavity. They are a consequence of the interaction between the ambient air (relatively hot) and the jet of forced convection (cold). These cells have a favorable effect on the heat exchange between the cavity and the exterior, since they accentuate the mixing of the two airs and increase the residence time of the fresh air jet in the cavity. The open lines representing the ventilated air have a separation at the horizontal faces of the hot blocks. They are, moreover, distorted in the upper part of the cavity, because of the shear with the drive cells mentioned above. As the Reynolds number increases, the entrainment phenomenon disappears from the top of the cavity, giving way to fresh ventilation air. Only micro-cavity is the seat of closed convective cells, as shown in Figure 5b-d for \(Re=1.5\times 10^{3}\), \(2\times 10^{3}\), \(2.5\times 10^{3}\), respectively. Figure 4: Flow structures: Comparison of the present results (**a**) with those of [PERSON] (**b**) [40,41]. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & \multicolumn{3}{c}{\(Ra\) = \(10^{6}\)} & \multicolumn{3}{c}{\(Ra\) = \(10^{7}\)} \\ \hline \multirow{2}{*}{**Present Study**} & **[PERSON] et al.** & **Deviation** & **Present Study** & **[PERSON] et al.** & **Deviation** \\  & **[38]** & **\%** & **\%** & **[38]** & **\%** \\ \hline \(\mathbf{U_{max}}\) & 0.228 & 0.240 & 5 & & - & - \\ \hline \(\mathbf{V_{max}}\) & 0.340 & 0.356 & 4.49 & & - & - \\ \hline \(\mathbf{\Psi_{max}}\) & 0.0275 & 0.0290 & 5.172 & 0.0165 & 0.0174 & 5.172 \\ \hline \end{tabular}
\end{table} 
Table 3: Quantitative comparison between the present work and the results of [PERSON] et al. [40]. for \(Ra=10^{6}\) and \(Ra=10^{7}\). The rotation direction of two cells (in the micro-cavity) permits a part of the fresh air jet to enter the micro-cavity's core. The right-hand part cools more than the left because of a distortion at the base of the boundary layers and an entrainment cell on the left-hand horizontal section that pushes the jet of fresh air towards the right-hand horizontal section. Consequently, both the horizontal sides of the blocks and the vertical side of the apertures with respect to the micro-cavity experience excellent cooling. The thermal field indicated by the isothermal lines is shown in Figure 6. Generally, the two faces close to the openings are often quite cold. The heat exchange between these two vertical plates and ambient air is quite significant, as shown by the isothermal lines which are too tight indicating a development of thermal boundary layer heat transfer along these plates. For \(Re=10^{3}\), given in Figure 5a, a small amount of fresh air moves towards the bottom of the micro-cavity, driven by the rotating cells mentioned above. It increases as the Reynolds number increases, relatively improving heat transfer along the vertical faces adjacent to the micro-cavity, Figure 5b, \(Re=1.5\times 10^{3}\). This phenomenon disappears for \(Re\) up of \(1.5\times 10^{3}\). Indeed, when \(Re\) increases further, the jet becomes relatively intense compared to the downward drag phenomenon, Figure 5c-e, for \(Re=2\times 10^{3}\), \(2.5\times 10^{3}\) and \(3\times 10^{3}\), respectively.