For urban climate studies in wind tunnels it is necessary to produce a wind profile corresponding to the natural conditions. In the wind tunnel this is produced when the air current, at first homogenous and low in turbulence when entering through the intake, is passed by so-called eddy generators and rough surface areas. The eddy generators block the lower cross-section of flow more than the upper and thereby form the profile of a typical boundary layer before the shearing turbulence produced by the roughness of the ground can intrude into the airflow (Figure 4/6). Because of these requirements, boundary layer wind tunnels are very long in relation to the useable test area (Figure 4/7).

Since the theory of wind tunnel modeling cannot be dealt with in greater detail in the context of this booklet, the professional literature should be referred to (e.g. PLATE, 1982; SCHATZMANN et al., 1986). In studies of airflow mechanics it is important that the corresponding model guidelines (i.e. similarity criteria) are fulfilled so that the transferability of the results to the natural conditions can be established. This is the case when the distances in the model have a constant ratio to the distances in nature, and the flowing boundary layer and properties of the obstacles are "similar" to the natural conditions (Figure 4/8a).

The actual quantities of the model to be studied depend on the individual wind tunnel(s) and the study scenario(s). At the University of Karlsruhe, for example, models up to 2 m in diameter can be studied. In order to simulate various wind directions, the model is rotated correspondingly in the wind tunnel (Figue 4/8b und Figure 4/8c).

The advantage of a wind tunnel versus measurements taken at a location is that future projects and alternatives along with their modifications can be measured very quickly, as soon as a suitable model of the project area is put together for the first time.