Thus, to obtain representative (i.e., statistically steady) data that relate the flow within the building with the outdoor wind, one needs to perform air velocity measurements under steady conditions and for a sufficiently long time. However, steady  
Figure 1: Sketch of possible instantaneous (solid lines) and mean (dashed lines) flow characteristics of the turbulent flow impacting a naturally ventilated livestock building (NVLB) (blue), developed within the NVLB (red), and shed by the NVLB geometry (orange). The question marks represent the unknown ammonia emission and dispersion. The flow is from left to right. conditions are rare due to the atmospheric turbulence and, hence, longer time-averaging does not solve the issue [12]. [PERSON] et al. [13] pointed out a typical example of large scatter (by a factor of 2) in reported ammonia emissions of dairy cattle housings, where it was unclear whether the spread could be attributed to the effects of climatic conditions, housing layout, or different measurement concepts. Two research methods are available to improve the understanding of airflow and ammonia dispersion within and from an NVLB, concerning the in situ experiments mentioned above: computational fluid dynamics (CFD) and wind tunnel simulations. CFD is a numerical framework that can solve all processes (i.e., mechanical, thermodynamic, and chemical) of ammonia emission. However, the trustworthiness of the results needs to be established through proper validation. The validation process is even more crucial for predicting flows and pollutant dispersions around bluff bodies with sharp edges (the flow around NVLBs is an excellent example), where unsteady turbulent vortices of different length and timescales are shed into the surrounding flow [14; 15]. This shedding prediction is the central issue of most CFD turbulent models based on the so-called Reynolds-averaged Navier-Stokes equations (RANS), as unsteady fluctuations cannot be reproduced by these models [15]. Indeed, there exist CFD models, such as large-eddy simulations (LES) and direct numerical simulations (DNS), that can tackle this limitation by resolving the turbulent flow either through a filtered grid (LES) or without any filtering (DNS). However, these resolving models demand extraordinary computational effort and are, therefore, still object to more or less basic academic purposes [16]. On the contrary, wind tunnel simulations employ real fluids, real pollutants, and real geometries (though at a reduced scale). The conditions can be fully controlled and the specific parameters can be adjusted independently. Measurement techniques, such as time-resolved particle image velocimetry (TR-PIV) and planar laser-induced fluorescence (PLIF), can provide data with high spatiotemporal resolution and representativeness. Such data are vital for the validation and tuning of RANS models. Compared to in situ experiments, physical modelling in wind tunnels is more favourable in terms of costs. Time is also on the side of wind tunnel modelling. For instance, a reduction of the modelled case by 100 means that measuring 1 min in the wind tunnel corresponds to 100 min in situ. However, as mentioned above, the conditions during in situ measurements are rarely steady during these 100 min, and hence, insufficient representative data are usually obtained [11; 17]. Since the 1980s, wind tunnels have been used to understand the flow and dispersion processes inside NVLBs, not just due to the advantages mentioned above. Still, the physical modelling of such reduced-scale flows has limitations, and the obtained results must be interpreted accordingly. Therefore, this paper seeks to review previous wind tunnel studies that have addressed the natural ventilation of livestock buildings, as well as to outline the limitations and future improvements in this respect. It should be noted that the ammonia emission from an NVLB is also a specific factor related to animal production and activity within the building. However, these specifics fall outside the scope of the paper.