**3. Boundary layer flows at the UFRGS wind tunnel**

Next, tests made at the wind tunnel of the UFRGS (**Figure 10**) are analyzed. The Prof. Joaquim Blessmann boundary layer wind tunnel at the Laboratório de Aerodinâmica das Construções of UFRGS, Brazil, is a closed-return circuit, and it has a cross-section of 1.30 m × 0.90 m at downstream end of the main working section that is 9.32 m long (**Figure 10**). A detailed description of the characteristics of the tunnel is indicated in Blessmann's previous work [11].

#### **3.1 Simulation of atmospheric boundary layers with different velocities**

Four perforated spires, a barrier, and surface roughness elements were used to simulate a full-depth boundary layer. The arrangement of the simulation hardware is shown/illustrated in **Figure 11**. Velocity and longitudinal velocity fluctuations were measured by means of a TSI hot-wire anemometer along a vertical line on the center of rotating table located downstream of the working section.

**Figure 10.** *The Prof. Joaquim Blessmann boundary layer wind tunnel of the UFRGS.*

*Boundary Layer Flows - Theory, Applications and Numerical Methods*

to Eq. (2) determines a value of the exponent *α* of 0.23.

for the Counihan method (**Figure 7**).

simulator has a trapezoidal shape of 1.50 m height, 0.53 and 0.32 m sides. The roughness elements distributed on the test section floor is the same that was used

Measurements of mean velocity and longitudinal velocity fluctuations were made along a vertical line on the center of rotating table and along lines 0.60 m to the right and left of this line. Vertical velocity distribution and the corresponding log-graph representation to verify the extension of the logarithmic behavior are shown in **Figure 8**. The three measured velocity profiles are quite similar, and the fit

*Dimensionless spectra obtained at different heights for the part-depth boundary layer simulation and the von* 

*Vertical mean velocity and turbulence intensity profiles measured for the part-depth boundary layer* 

**132**

**Figure 9.**

**Figure 8.**

*simulation.*

*Kármán spectrum.*

**Figure 12** shows the non-dimensional profiles obtained with low velocities *Uref* = 1 and 3.5 m/s, respectively. These profiles are compared with the values obtained with the highest mean velocity achievable in the wind tunnel (*Uref* ≈ 35 m/s). The mean velocity profile given by the power law expression (Eq. (2)) is also included in this graph, being the power law exponent *α* equal to 0.23 and the boundary layer thickness *H* = 0.60 m.

Also, turbulence intensities measured in correspondence to *Uref* = 1, 3.5, and 35 m/s are shown in **Figure 12**. Turbulence intensity values corresponding to 3.5 m/s are slightly higher than those obtained at high velocity, which is a behavior commonly observed at low velocities. For measurements at velocity *Uref* = 1 m/s, it is possible to observe even larger deviations in comparison with 3.5 and 35 m/s cases that can be attributed to extremely low velocity. It is worth noting that with these velocity magnitudes, the relative errors affecting the hot-wire anemometer technique are larger than for measurements at high velocities. This kind of measurement deviation was also observed in similar wind tunnel tests using three-dimensional laser Doppler velocimetry [12].

Power spectra of the velocity fluctuations obtained at two different positions, *z* = 0.15 and 0.35 m with low velocities *Uref* = 1 and 3.5 m/s, respectively, are presented in **Figure 13**. Sampling series used for the spectral analysis were obtained with an acquisition frequency of 1024 Hz. A poor definition of the Kolmogorov's inertial subrange is observed for the spectra measured at velocity *Uref* = 1 m/s.
