## 2 Issues with Wind Tunnel Modelling of Flow and Dispersion Processes in NVLBs  
The physical modelling of flow and dispersion processes within an NVLB requires both a properly modelled ABL (into which the building is immersed) and indoor flow. If we restrict ourselves to a simple isothermal case, only two Reynolds numbers (Re) should be fulfilled: external (i.e., related to the ABL, where the representative length and velocity are the ABL height and the mean velocity at that height, respectively) and internal (i.e., associated with the indoor flow). [PERSON] et al. [18] demonstrated, on a civil building model with a scale of 1:25, that the internal Re (based on the mean velocity at the opening and the smallest dimension of the room) should be at least 2 \(\times\) 10\({}^{4}\) in order to ensure the independence of the internal flow on the internal Re. Such high Re might be reasonably achieved for livestock buildings due to their large openings (Figure 2). However, special attention should be paid to the modelled scale of turbulence in wind tunnels. [PERSON] [19] demonstrated that even though he modelled the mean velocity and turbulence intensity of the required ABL in the wind tunnel properly, a plume coming from a stack was three times broader than that modelled by [PERSON] and [PERSON] [20]. After proper modelling of turbulence spectra (in a way that the spectra modelled in the wind tunnel correspond to those observed at full scale), [PERSON] achieved the same plume breadth, within the limits of experimental uncertainties. The same issue might be expected in the case of modelling the dispersion process within an NVLB. [PERSON] et al. [21] compared the velocity fields within naturally ventilated rooms of apartment houses obtained from wind tunnel tests with those obtained from full-scale measurements (Figure 3). Their comparison showed that the modelling of the critical ABL characteristics, such as the vertical profiles of the mean velocity, the intensity of turbulence, and vertical momentum fluxes, in the wind tunnel produced similar internal flow fields.  
The simulation of non-isothermal flows (e.g., unstable ABLs or convective flows within NVLBs) in wind tunnels is challenging, as both the Reynolds number and the Froude number (Fr) should be fulfilled. The latter represents the ratio of internal forces (flows) to buoyancy forces (flows). The simulation of the dominance of convective flows above the inertial means that the Fr is lower than unity. Attaining such a low Fr entails increasing the difference between the approaching flow temperature and, for example, the wind tunnel floor in order to produce sufficiently high heat flux. However, due to technical constraints, this high flux is usually achieved under the sacrifice of lowering the internal forces--the wind tunnel wind speed. Hence, the Re independence is usually relaxed. Several wind studies have attempted to tackle this problem concerning environmental flows [22; 23; 24; 25]. However, the simulation of stable or unstable flows to study their impact on the dispersion within and from NVLBs is more challenging due to the additional internal Re independence. Indeed, there has been no wind tunnel study on such flows concerning the natural ventilation of any building. Even the modelling of isothermal flows in a wind tunnel is not a trivial task. Therefore, in the following sections, we restrict ourselves to the proper modelling of neutrally stratified flows in wind tunnels and discuss under which conditions such modelling can be achieved for the cases of flow and pollutant dispersion within and from NVLBs.  
Figure 2: Internal Reynolds number independence test within a dairy building for two wind tunnel freestream velocities: 8 m/s (left) and 12 m/s (right). Note that the streamlines (lines with arrows) do not change significantly with the change in the approach wind speed (freestream velocity). Adapted from [26]—Copyright 2018, Elsevier.