# Experimental Simulation of Thunderstorm Profiles in an Atmospheric Boundary Layer Wind Tunnel
## 2 Literature Review  
### Experimental Simulation of Thunderstorm Downbursts  
The temperature and relative humidity variations at different levels play a crucial role in the development of downbursts [15]. However, climatic wind tunnels, despite their capability to control temperature and relative humidity, generally do not replicate the key aspects that are most relevant to structural loading, such as the mean velocity and longitudinal turbulence profiles. Consequently, when it comes to structural applications in the field of wind engineering, the experimental simulation of downbursts is typically constrained to reproducing the velocity field generated by these events. The simulation of the velocity field associated with downwards can be carried out in two groups of facilities. The first group can simulate the entire downburst event, reproducing the spatial and temporal variation of the associated wind field, while the second one is only able to replicate the main characteristics of outflows. Facilities from the first group can be called 3D wind simulators due to their unique capabilities of reconstructing the entire downburst event, creating a wall radial outflow and a vortex ring with the impinging jet technique. In these facilities, different experimental simulations have been carried out: isolated downdraft, i.e., stationary downburst without background flow [8; 16], translational downdraft [17; 18] and both cases embedded into ABL-like winds [19]. This type of experimental test requires the same length scale for the simulated downburst and the structure; thus, large facilities are needed, which makes them expensive and consequently not very common. Moreover, for the reasons specified, the scale of the structural model is usually very small (e.g., [20]). Therefore, this type of test is essentially carried out in research centers, and regular civil projects do not have easy access to them.  
Due to these issues, researchers have been focused on developing alternatives to recreate the main features of downburst outflows in the most common facilities, i.e., ABL Wind Tunnels. Therein, the downdraft phenomenon is not fully simulated: suitable devices are adopted in order to reproduce the downburst gust front. In single-fan wind tunnels, this type of simulation is based on redirecting the horizontal flow; thus, the facility has to be equipped with specific active devices, such as single- or multi-blade systems. Regarding the first case, [PERSON] and [PERSON] [21] carried out an experimental test using a manually controlled pivoted flat plate. Concerning the latter case, [PERSON] and [PERSON] [12] simulated the downburst outflows in a small-scale ABL wind tunnel proposing a simplified multi-blade flow device. [PERSON] et al. [11] suggested and calibrated in three stages a system of multiple louvers using numerical simulations. In this context, a downburst simulator was developed at the multiple-fan Wall of Wind (WoW) laboratory of Florida International University, consisting of two louvers located near the ground, able to simulate the gust front [22; 23], which was later used on an aeroelastic model by [PERSON] et al. [24] to evaluate its dynamic response. In the case of multiple-fan wind tunnels built with small (or very small) fans, in order to favor their time response without using any kind of blade system, the ability to control the performance of each of the different fans allows the generation of target wind speed time histories and profiles. Therefore, these new-generation facilities are, by their nature, capable of simulating the impact of two distinct features of gust front flows: the near-ground flow profile shape and the rapid, transient changes, e.g., [13; 25]. Recent developments include the study of unsteady effects due to sinusoidal streamwise gusts [26] and the simulation of transient wind fields through control schemes based on deep reinforcement learning [27].