## 4 Summary  
This review showed the importance of wind tunnels for understanding the flow and pollutant dispersion processes within and from NVLBs. Several studies [49; 51; 52] have demonstrated the importance of data representativeness, in comparison with those obtained from in situ measurements, and that there is no reason to expect that the flow through the openings is uniform [46; 56] or that the pollutant within the building is well mixed even in quasi-stationary cases [57]. The focus of the previous wind tunnel studies was generally on studying the impact of the characteristics of sidewall openings on the flow within the building. Indeed, there is a consensus that the size and position of the sidewall openings are critical parameters for the building's flow behaviour, especially in the animal-occupied zone. In addition, both the opening size [44] and the building geometry [58] have been shown to impact the flow around the building, as well as obstacles surrounding the building [48; 50; 52]. The effect of obstacles on the flow characterises within an NVLB has attracted less attention as only two studies [53; 57] considered this effect during their experiments. While the first study [53] showed that the feeding alley changed the flow pattern and produced higher turbulence within the barn, the second [57] study demonstrated that the presence of cows (without heat production) had an insignificant effect on the flow and ventilation of an NVLB when the wind was perpendicular to the sidewall. Thus, further studies that will consider obstacles within their models of NVLBs as a function of the wind direction are of primary importance. Including heat production into the models of an NVLB will add another valuable, although qualitative, information about the role of non-isothermal effects on the building's flow characteristics or dispersion. However, from the reviewed papers, two primary deficiencies can be observed. First, there is a lack of studies that have properly modelled the atmospheric boundary layer (ABL) characteristics. It has been shown that due to the large openings of NVLBs, the outdoor (atmospheric) and indoor flows are in mutual interaction and that a better understanding of this interaction will improve the determination of the amount of ammonia emitted from such buildings. As was also demonstrated during this review, the integral length scales and spectrum of turbulence are the ABL parameters that influence the indoor flow appreciably. A wind tunnel study [57] has shown that despite the same mean wind speed at the building height, the flow and pollutant dispersion within the building differed for differently simulated characteristics of the ABL. Therefore, it is recommended to simulate all essential ABL parameters in forthcoming wind tunnel studies intended to model the natural ventilation of livestock buildings. This recommendation is from the perspective of comparability and the standpoint of reliability, which is crucial in CFD validations. Second, there is a lack of wind tunnel studies that have addressed the pollutant dispersion within or from NVLBs. This lack is a critical finding, as even the dispersion of a passive pollutant cannot be rigorously derived only from flow characteristics. Studies on NVLB [48] and generic building [60] ventilation have demonstrated that the mean concentration gradients are driven by the concentration fluctuations, not by the velocity fluctuations. Other important parameters are the position and type (point, line, or planar) of the source and mechanisms (advective and turbulent) of pollutant transport, which are crucial for understanding the ammonia emission from NVLBs. At the openings and near the bottom of the NVLB, turbulence will play a significant role in ammonia transport. However, there exists no wind tunnel study addressing the turbulent pollution flux (even a passive pollutant), neither near the bottom, nor at the openings of the NVLB.