**1. Introduction**

Wind tunnels are designed to realize similarity in model studies, with the confidence that actual operational conditions will be reproduced. The first step is the evaluation of the flow characteristics with the empty wind tunnel, and then, different flow characteristics are achieved or reproduced at the test section to be applied in the experimental tests. To perform aerodynamic studies of structural models, the distribution of the incident flow must be such that the atmospheric boundary layer (ABL) at the actual location of the structure is reproduced. This is obtained by surface roughness elements and vortex generators, so that natural wind simulations are performed.

The atmospheric boundary layer (ABL) is the lowest part of the atmosphere where the effects of the surface roughness, temperature, and others properties are transmitted by turbulent flows. Turbulent exchanges are very weak when there are conditions of weak winds and very stable stratification [1]. On the other hand, the atmospheric boundary layer over nonhomogeneous terrain is not well defined, and topographical features could cause highly complex flows. The depth of the atmospheric boundary layer is typically 100 m during the nighttime stable conditions, and this could reach 1 km in daytime unstable conditions [2]. The Prandtl logarithmic law (Eq. (1)), proposed from similarity theories, can be used near the surface in the case of a neutral boundary layer.

$$\frac{U(x)}{\mu\_{\ast}} = \frac{1}{0.4} \ln \frac{x - x\_d}{x\_0} \tag{1}$$

where *U* is the mean velocity, *u\** is the friction velocity, *z*0 is known as the roughness height, and *zd* is defined as the zero-plane displacement for very rough surface.

The potential law (Eq. (2)) is also widely used in wind engineering to characterize the vertical velocity distribution. The values for the exponent *α* vary between 0.10 and 0.43 and the boundary layer thickness *zg* between 250 and 500 m, according to the terrain type [2]. This law is verifiable in the case of strong winds and neutral stability conditions that must be considered for structural analysis.

$$\frac{U(x)}{U(x\_{\sharp})} = \begin{pmatrix} \frac{x}{x\_{\sharp}} \end{pmatrix}^{a} \tag{2}$$

Similarity requirements corresponding to studies of atmospheric flow in the laboratory can be obtained by the dimensional analysis. The equations are expressed in dimensionless form by means of reference parameters that lead to the following set of non-dimensional groups or numbers: Reynolds number, Prandtl number, Rossby number, and Richardson number. These dimensionless parameters must be in the same value with the model and prototype to obtain the exact similarity, and, in addition, there must be geometric similarity and similarity of the boundary conditions, including incident flow, surface temperature, heat flow, and longitudinal pressure gradient [3].

Geometric scales defined between the simulated laboratory boundary layer and the atmospheric boundary layer are generally <1:200, velocities in the model and prototype have values of the same order, and the viscosity is the same for both cases. This results in the impossibility of reproducing the Reynolds number in low-speed wind tunnels; however, the effects of Reynolds number variation can be taken into account according to the type of wind tunnel test. On the other hand, the equality of the Prandtl number is obtained simply by using the same fluid in model and prototype, as in this case. The equality of the Rossby and Richardson numbers may not be considered for simulation of neutral ABL since Coriolis forces and thermal effects are negligible.

In most laboratories it is more common to simulate the neutrally stratified boundary layer. This implies modeling the distribution of mean velocities, turbulence scales, and atmospheric spectrum [4]. The quality of these approximate models is simply evaluated by comparing the results expressed in dimensionless form with design values. Turbulence intensity distribution is commonly compared with values obtained by other authors [5] and by using Harris-Davenport formula for atmospheric boundary layer [2].

Atmospheric velocity fluctuations with frequencies upper than 0.0015 Hz define the micrometeorological spectral region. Interest of wind engineering is concentrated on this spectral turbulence region. von Kármán suggested an expression for the turbulence spectrum in 1948, and today this spectral formula is still used for wind engineering applications. According to Reference [2], the expression for the dimensionless spectrum of the longitudinal component of atmospheric turbulence is given by Eq. (3):

**127**

**Figure 1.**

*Physical Models of Atmospheric Boundary Layer Flows: Some Developments and Recent…*

<sup>2</sup> <sup>=</sup> 1.6(*fzref*/*U*) \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

[1 + 11.325 (*fzref*/*U*)

where *Su* is the spectral density function of the longitudinal component, *f* is the

Different boundary layer flows are experimentally analyzed in this work. First,

Next, measurement results obtained at the Prof. Jacek Gorecki wind tunnel of the UNNE (**Figure 1**) in three different boundary layer flows are analyzed. The UNNE wind tunnel is a 39.56-m-long channel where the air enters through a contraction to reach the test section. This is connected to the velocity regulator and to the blower, and then, the air passes through a diffuser before leaving the wind tunnel. The contraction has a honeycomb and a screen to uniform the airflow. The test section is a 22.8-m-long rectangular channel (2.40 m width, 1.80 m height) where two rotating tables are located to place test models. Conditions of zero-pressure-gradient boundary layers can be obtained by the vertical displacement of the upper wall. The blower

The first of these flows correspond to a boundary layer developed on the smooth

has a 2.25 m diameter and is driven by a 92 kW electric motor at 720 rpm.

the neutrally stable ABL and the second to a part-depth model.

*The Prof. Jacek Gorecki boundary layer wind tunnel of the UNNE.*

floor of the wind tunnel test section. Then, the results obtained for two ABL simulations are analyzed. The first model corresponds to a full-depth simulation of

2 ]

is the quadratic mean, or the variance, of the longitu-

. The dimensionless frequency *fzref*/*U* is defined

5/6 (3)

*DOI: http://dx.doi.org/10.5772/intechopen.86483*

(*uRMS*)

2

**2. Boundary layer flows at the UNNE wind tunnel**

/s2

using an appropriate *zref*, generally gradient height, and the mean velocity *U*.

three types of boundary layer flows developed at the UNNE wind tunnel: one corresponding to a naturally developed boundary layer with the empty wind tunnel and the other two generated by different ABL simulation methods. Then, simulated ABL flows obtained with different velocities at the UFRGS wind tunnel are analyzed, and the results are compared with each other. Finally, some recent applications of ABL simulations are described, among them wind effects in high-rise buildings considering the urban environment and the surrounding topography, low buildings, aerodynamics of cable-stayed bridges, pollutant atmospheric dispersion,

\_\_\_\_\_\_ *fSu*

frequency in Hertz, and *uRMS*

dinal velocity fluctuations in m<sup>2</sup>

and flow in the wake of wind turbines.

*Physical Models of Atmospheric Boundary Layer Flows: Some Developments and Recent… DOI: http://dx.doi.org/10.5772/intechopen.86483*

$$\frac{f\mathbb{S}\_u}{\left(\mu\_{RMS}\right)^2} = \frac{\text{1.6}\left(f\text{z}\_{ref}/U\right)}{\left[\text{1}\star\text{11.32S}\left(f\text{z}\_{ref}/U\right)^2\right]^{5/6}}\tag{3}$$

where *Su* is the spectral density function of the longitudinal component, *f* is the frequency in Hertz, and *uRMS* 2 is the quadratic mean, or the variance, of the longitudinal velocity fluctuations in m<sup>2</sup> /s2 . The dimensionless frequency *fzref*/*U* is defined using an appropriate *zref*, generally gradient height, and the mean velocity *U*.

Different boundary layer flows are experimentally analyzed in this work. First, three types of boundary layer flows developed at the UNNE wind tunnel: one corresponding to a naturally developed boundary layer with the empty wind tunnel and the other two generated by different ABL simulation methods. Then, simulated ABL flows obtained with different velocities at the UFRGS wind tunnel are analyzed, and the results are compared with each other. Finally, some recent applications of ABL simulations are described, among them wind effects in high-rise buildings considering the urban environment and the surrounding topography, low buildings, aerodynamics of cable-stayed bridges, pollutant atmospheric dispersion, and flow in the wake of wind turbines.
