*2.2. ROSEO-BIWT Design*

2.2.1. The Location on the Upper Edge of the Building

The effect of wind against buildings has been largely studied by the architectural sector for the purpose of studying the dynamic loads generated by air streams. Because of their work, there is considerable knowledge about the behavior of wind in urban environments. Much of the information has been obtained through experiments with scale models and CFD simulations, similar to the depiction generated by the authors in Figure 3, which was re-created based on the CFD simulation in [38].

**Figure 3.** Re-creation on the basis of Mertens [38] for wind acceleration over the windward upper edge of a building.

Most of the studies that have analyzed the behavior of air streams around buildings agree that the upper edge of the windward face of a building has grea<sup>t</sup> wind energy potential. This is because the wind has to surround an object. The effect is even more intense when the building is taller and when the wind direction is perpendicular to the building facade. For example, in a five-story building, the wind velocity increases by 1.2 times at the windward edge [38].

According to the CFD simulations of Balduzzi et al. [5,6], the wind speed increment at the edge can be between 10% and 30%, but the turbulence intensity increases considerable. This is not the worst inconvenience for Savonius turbines, since it is demonstrated that, when turbulence increases, the separation of boundary layer takes place on the lower side of returning blade of the rotor reducing the negative torque [39,40].

Areas of high turbulent intensity create more frequent and stronger gusts [9], but the inertia of a relatively long Savonius rotor (high aspect ratio between the length of the axis and the diameter) can keep the rotation of the turbine without relevant variations. Additionally, it is demonstrated that, as a consequence of blade tips, aerodynamic losses are reduced in Savonius turbines with high aspect ratios [41].
