**2. Atmospheric boundary layer flows and wind tunnel flow simulation**

The atmospheric boundary layer is the lowest part of atmosphere. Effects of the surface roughness, temperature and others properties are transmitted by turbulent movement in this layer. Under conditions of weak winds and very stable stratification, turbulent exchang‐ es are very weak and the atmospheric boundary layer is called surface inversion layer [8]. A distinction is usually made between atmospheric boundary layer over homogeneous and non-homogeneous terrain. In this last situation, the boundary layer is not well defined, and topographical features could cause highly complex flows.

The depth of the atmospheric boundary layer varies with the atmospheric condition, but it is typically 100 m during the night-time stable conditions and 1 km in daytime unstable or convective conditions. A detailed description related to the wind characteristics associated to the neutral condition is made by Blessmann [3] in his book on Wind in Structural Engi‐ neering. Several similarity theories have been proposed for different atmospheric stability conditions. Near the surface, from dimensional arguments the analysis leads to the Prandtl logarithmic law, Eq. (1), in the case of a neutral boundary layer:

$$\frac{\text{LI}(z)}{\mu^\*} = \frac{1}{0.4} \ln \frac{z - z\_d}{z\_0} \tag{1}$$

Where U is the mean velocity, *u\** is the friction velocity, *z0* is know as the roughness height and *zd* is defined as the zero-plane displacement for very rough surface. The depth of a wind tunnel boundary layer is defined as the height where mean velocity reaches 0.99 of the free stream velocity. This definition is used to characterize atmospheric flow simulations.

Wind tunnels are designed to obtain different air flows, so that similarity studies can be per‐ formed, with the confidence that actual operational conditions will be reproduced. Once a wind tunnel is built, the first step is the evaluation of the flow characteristics and of the pos‐ sibility of reproducing the flow characteristics for which the tunnel was designed. Many evaluation studies of wind tunnels are presented in the open literature. Some of which are the work of Cook [9] on the wind tunnel in Garston, Watford, UK, the presentation of the closed-return wind tunnel in London [10] and the Oxford wind tunnel, UK [11], the charac‐ terization of the boundary layer wind tunnel of the UFRGS, Brazil [12] and of the Danish Maritime Institute, Denmark [13].

Wind tunnel modeling of atmospheric boundary layer is generally oriented to neutrally sta‐ ble flows. Modeling of stratified boundary layer is more difficult to implement and less used in wind tunnel tests. Similarity criteria imply that a set of non-dimensional parameters should be the same in model and prototype. In general, the flow is governed by the boun‐ dary conditions and the Rossby, Reynolds, Strouhal, Froude, Eckert and Prandtl numbers, but in most of the situations of practical importance the effects of several non-dimensional numbers can be neglected. Later studies in atmospheric boundary layer simulations at‐ tempted to reproduce as closely as possible the mean velocity distribution and turbulence scales of the atmospheric flow. This is made by non-dimensional comparisons of mean and fluctuating velocity measurements in the wind tunnel flow and atmospheric data.

distinct configurations of three wind tunnels, a smooth pipe and atmospheric boundary lay‐ er are used. In addition, a different class of spectral representation proposed by Gagne et al. [7], which is based on local similarities and compatible with the multifractal formalism, is

**2. Atmospheric boundary layer flows and wind tunnel flow simulation**

The atmospheric boundary layer is the lowest part of atmosphere. Effects of the surface roughness, temperature and others properties are transmitted by turbulent movement in this layer. Under conditions of weak winds and very stable stratification, turbulent exchang‐ es are very weak and the atmospheric boundary layer is called surface inversion layer [8]. A distinction is usually made between atmospheric boundary layer over homogeneous and non-homogeneous terrain. In this last situation, the boundary layer is not well defined, and

The depth of the atmospheric boundary layer varies with the atmospheric condition, but it is typically 100 m during the night-time stable conditions and 1 km in daytime unstable or convective conditions. A detailed description related to the wind characteristics associated to the neutral condition is made by Blessmann [3] in his book on Wind in Structural Engi‐ neering. Several similarity theories have been proposed for different atmospheric stability conditions. Near the surface, from dimensional arguments the analysis leads to the Prandtl

0

Where U is the mean velocity, *u\** is the friction velocity, *z0* is know as the roughness height and *zd* is defined as the zero-plane displacement for very rough surface. The depth of a wind tunnel boundary layer is defined as the height where mean velocity reaches 0.99 of the free

Wind tunnels are designed to obtain different air flows, so that similarity studies can be per‐ formed, with the confidence that actual operational conditions will be reproduced. Once a wind tunnel is built, the first step is the evaluation of the flow characteristics and of the pos‐ sibility of reproducing the flow characteristics for which the tunnel was designed. Many evaluation studies of wind tunnels are presented in the open literature. Some of which are the work of Cook [9] on the wind tunnel in Garston, Watford, UK, the presentation of the closed-return wind tunnel in London [10] and the Oxford wind tunnel, UK [11], the charac‐ terization of the boundary layer wind tunnel of the UFRGS, Brazil [12] and of the Danish

Wind tunnel modeling of atmospheric boundary layer is generally oriented to neutrally sta‐ ble flows. Modeling of stratified boundary layer is more difficult to implement and less used


() 1 ln \* 0.4 *<sup>d</sup> U z z z u z*

stream velocity. This definition is used to characterize atmospheric flow simulations.

compared to traditional approaches.

198 Wind Tunnel Designs and Their Diverse Engineering Applications

Maritime Institute, Denmark [13].

topographical features could cause highly complex flows.

logarithmic law, Eq. (1), in the case of a neutral boundary layer:

In general, wind tunnel evaluation is performed at the highest flow velocity, the results be‐ ing presented in terms of mean velocity distributions, turbulence intensities and scales. However, many simulations are performed at low velocities to evaluate some specific prob‐ lems. This is the case of laboratory simulation of dispersion problems [14] and transmission line modeling [15].

Boundary-layer simulations are performed with help of grids, vortex generators and rough‐ ness elements, to facilitate the growth of the boundary layer and to define the mean velocity profile. This is used in the most applied simulation methods, namely the full-depth simula‐ tion [16] and part-depth simulation [17]. The use of jets and grids is also applied [12].

The "Jacek Gorecki" wind tunnel, located at the Universidad Nacional del Nordeste, UNNE at Resistencia (Chaco), Argentina, is a low velocity atmospheric boundary-layer wind tun‐ nel, built with the aim to perform aerodynamic studies of structural models. The atmospher‐ ic boundary layer is reproduced with help of surface roughness elements and vortex generators, so that natural wind simulations are performed. Fig. 1 shows a view of the "Ja‐ cek Gorecki" wind tunnel, which is a 39.56 m long channel. The air enters through a contrac‐ tion, passing a honeycomb prior to reach the test section, which is a 22.8 m long rectangular channel (2.40 m width, 1.80 m height). Two rotating tables are located in the test section to place structural models. Conditions of zero pressure gradient boundary layers can be ob‐ tained by vertical displacement of the upper wall. The test section is connected to the veloci‐ ty regulator and to the blower, which has a 2.25 m diameter and is driven by a 92 kW electric motor at 720 rpm. A diffuser decelerates the air before leaving the wind tunnel.

In this wind tunnel, many models of atmospheric boundary layer were implemented. In gener‐ al, the simulation of natural wind on the atmospheric boundary layer was performed by means of the Counihan and Standen methods [18, 19, 20]. To illustrate this type of flow model, an ex‐ ample of full-depth Counihan simulation with velocity distributions corresponding to a class III terrain is presented. According to Argentine Standards CIRSOC 102 [21], this type of terrain is designed as "ground covered by several closely spaced obstacles in forest, industrial or ur‐ ban zone". The mean height of the obstacles is considered to be about 10 m, while the boundary layer thickness is zg = 420 m. The power law for velocity distribution is given by

$$\frac{\text{LU}(z)}{\text{LU}(z\_{\\_\\_\nu})} = \left(\frac{z}{z\_{\\_\nu}}\right)^{\nu} \tag{2}$$

with suitable values for the exponent a between 0.23 and 0.28 [3]. This law is of good appli‐ cation in neutral stability conditions of strong winds, typical for structural analysis. For this Counihan full-depth simulation, where the complete boundary-layer thickness is simulated, four 1.42 m (Hv) high elliptic vortex generators and a 0.23 m (b) barrier were used, together with prismatic roughness elements placed on the test section floor along 17 m (l) (see Fig. 2). The wind tunnel test section and the simulation hardware are shown in Fig. 3.

**Figure 1.** "Jacek Gorecki" Wind Tunnel at UNNE.

In this work, measurements of wind velocity realized in three different wind tunnels will be used for the spectral analysis. The *"Jacek Gorecki"* wind tunnel [19] described above, the "*TV2"* wind tunnel of the Laboratorio de Aerodinámica, UNNE, smaller, also an open cir‐ cuit tunnel, and the closed return wind tunnel *"J. Blessmann"* of the Laboratório de Aerodi‐ nâmica das Construçoes, Universidade Federal de Rio Grande do Sul, UFRGS [12].

**Figure 2.** Arrangement of Counihan simulation hardware.

with suitable values for the exponent a between 0.23 and 0.28 [3]. This law is of good appli‐ cation in neutral stability conditions of strong winds, typical for structural analysis. For this Counihan full-depth simulation, where the complete boundary-layer thickness is simulated, four 1.42 m (Hv) high elliptic vortex generators and a 0.23 m (b) barrier were used, together with prismatic roughness elements placed on the test section floor along 17 m (l) (see Fig. 2).

The wind tunnel test section and the simulation hardware are shown in Fig. 3.

200 Wind Tunnel Designs and Their Diverse Engineering Applications

**Figure 1.** "Jacek Gorecki" Wind Tunnel at UNNE.

**Figure 3.** Test section and full-depth Counihan vortex generators.
