## 2 Issues with Wind Tunnel Modelling of Flow and Dispersion Processes in NVLBs
### Boundary Conditions  
The first process in any wind tunnel study is the setting of appropriate boundary conditions. A wind tunnel with cross-dimensions of 1 m \(\times\) 1 m will not develop a boundary layer bigger than 1 m. However, one of the critical measures of ABL turbulence, the longitudinal integral length scale (\(L_{ux}\)), is generally bigger than the depth of the ABL and other turbulent structures (e.g., very large organised structures), which may develop along with the flow. Such wind tunnels can adequately model the vital part of the ABL--the so-called surface layer (SL)--at the scale of 1:100 and higher. However, for a smaller scale ratio (e.g., 1:50), approximately the upper half of the modelled SL will be missing. One needs a wind tunnel of a 2 m \(\times\) 2 m cross section to properly model the full SL at a scale of 1:50. Due to the Re reduction, both wind tunnels cannot model the smallest full-scale eddies (observed at the so-called Kolmogorov microscale) properly, although a bigger wind tunnel may model the broader spectrum of turbulence. Fortunately, the most energy-containing eddies, which play a crucial role in the dispersion of pollutants within the SL, are those that are much larger (about hundreds and thousands of millimetres at full scale) than those at the Kolmogorov microscale. Thus, dispersion modelling does not suffer too much due to this turbulence spectra reduction [27; 28; 29].  
Developing appropriate boundary conditions to model the ABL (or SL) in a wind tunnel is not an easy task. Usually, a trial-and-error process is undertaken to match all  
Figure 3: Comparison of the velocity fields within the rooms (Room N and Room S) of an apartment house between different wind tunnel simulations (WT 1, 2, 3) and full-scale experiments. WT 1—simulation of simple wind without turbulence generators; WT 2—simulation of wind with turbulence generators; WT 3—full simulation of the appropriate ABL. Point P denotes the position where the wind profiles were obtained. Adapted from [21]—Copyright 1992, Elsevier.  
essential characteristics (e.g., roughness length, \(z_{0}\); displacement height, \(d_{0}\); friction velocity, \(u\); integral length scale; and spectrum of turbulence) of the simulated ABL with those recommended by practical guidelines (see, e.g., [30]). To do so, physical modellers have used a well-established combination of vortex generators and roughness elements at the bottom of their wind tunnels. The overall purpose of these devices is to retard the mean flow close to the wind tunnel bottom and induce vorticity turbulence into the boundary layer, due to the relative shortness of the wind tunnel.  
For the vortex generators, [PERSON]'s [31] or [PERSON]'s [32] spires are mostly used at the beginning of the wind tunnel development section. At the same time, the roughness elements are represented by various shapes and are usually distributed in squared or staggered form in the remaining part of the development section (Figure 4). The role of the vortex generators is to produce vortices with a vertical axis or large-scale (longitudinal) eddies. Further downstream, these vortices or eddies interact with those induced by roughness elements. To provide an initial momentum deficit, some laboratories have used a castellated barrier [33], or a wall [34] of height smaller than 5% of the wind tunnel size, in front of the vortex generators. The shape of the roughness elements is also critical to reproduce appropriate turbulent structures within a modelled ABL [35; 36].