**4.5 Wind tunnel tests of the flow in the wake of wind turbines**

The interaction between the incident wind and wind turbines in a wind farm causes mean velocity deficit and increased levels of turbulence in the wake. The turbulent flow is characterized by the superposition of wind turbine wakes. A research work that included a series of wind tunnel tests to evaluate experimentally the spectral characteristics of turbulence in the wake of a wind turbine. Longitudinal

**Figure 19.**

*Launch of a space vehicle at the ACL, Maranhão, Brazil, and simulation of the dispersion process in the wind tunnel of the LAC/UFRGS.*

velocity fluctuations were measured in the incident flow and in the wake of a wind turbine-reduced model in the test section of the UNNE wind tunnel. In these experiments, the adequacy of spectral technique and changes in the turbulence spectral composition of the incident wind and the wake were analyzed [25].

All longitudinal velocity fluctuation measurements were realized employing a neutral ABL flow obtained by the Counihan method similar to the full-depth simulation described in Section 2.2. The simulated incident wind corresponds to a power law profile with an exponent α = 0.27 and a gradient height zg = 1.20 m. Wind tunnel measurements were made using a hot-wire anemometer system. The wind turbine model corresponds to a three-bladed UNIPOWER wind turbine, with a tower height of 100 m and a rotor diameter of 100 m. The scale of the model is approximately 1/450, and the model height is 0.33 m. **Figure 20** shows the wind incident, making the turbine model rotate.

The rotational velocity of the wind turbine was estimated and remained nearly constant during the measurements, but the values of the dimensionless speed ratio *λ* ensure the similarity of phenomenon in the range of the proper operation of the generator. Vertical profiles of dimensionless mean longitudinal velocity *U*/*Uref* measured in the incident wind and in the wake generated by the wind turbine are

**141**

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

indicated in **Figure 21**. Two profiles measured at locations x = 225 and 1185 mm downwind of the plane determined by the rotor blades are included. The comparison of the characteristic spectra of the incident wind and those obtained in the wake are also shown in **Figure 21** and allow observing the changes in the energy fluctuation distribution. These changes are a product of the turbulence introduced by the wind generator. Measurements allowed to analyze the configuration of the spectra in different frequency ranges, the effect of analog signal filtering, and differences in the spectral behavior of the incident wind relative to wind in the wake of the turbine.

*Vertical profiles of dimensionless mean longitudinal velocity and spectral comparison of the incident wind and* 

In this work, different boundary layer flows were experimentally analyzed. The BLF developed at the UNNE wind tunnel include a naturally developed boundary layer with the empty wind tunnel, a full-depth ABL generated by the Counihan

A simplified analysis considering the depth of the neutral ABL of about 500 m compared with the gradient height (0.30 m) obtained for the empty tunnel implicates a scale factor of 1/1650. In addition, turbulence intensity values inside the boundary layer are always minor than 10%, concluding that this boundary layer

Full-depth and part-depth simulations developed in the UNNE wind tunnel seem to show adequate performance. It is observed that values of measured turbulence intensity are lower than the values in the neutral atmosphere, mainly in the positions above, but other authors obtained similar results. Dimensionless spectral comparison indicates a deviation of the experimental results with respect to design spectra, but it is possible that parameters used to normalize the spectrum are not

fully adequate. Some studies are being developed to verify this behavior.

Comparison of ABL flows obtained with low velocities and the ABL flow obtained with high velocity at the UFRGS wind tunnel indicates an acceptable behavior of the mean velocity and turbulence intensity distribution. Dimensionless spectra were not obtained for measurements with low velocities. However, a poor spectral definition was observed for measurements realized at the lowest velocity. Five recent applications of ABL simulations in both wind tunnels (UNNE and UFRGS) are presented. A study of local wind loads on a high-rise building

method, and a part-depth ABL simulated by the Standen method.

flow is not appropriate to wind engineering experiments.

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

**5. Concluding remarks**

**Figure 21.**

*the wake flow at z = 225 mm.*

**Figure 20.** *Wind turbine model spinning during the wind tunnel test.*

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

**Figure 21.**

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

velocity fluctuations were measured in the incident flow and in the wake of a wind turbine-reduced model in the test section of the UNNE wind tunnel. In these experiments, the adequacy of spectral technique and changes in the turbulence spectral composition of the incident wind and the wake were analyzed [25].

*Launch of a space vehicle at the ACL, Maranhão, Brazil, and simulation of the dispersion process in the wind* 

All longitudinal velocity fluctuation measurements were realized employing a neutral ABL flow obtained by the Counihan method similar to the full-depth simulation described in Section 2.2. The simulated incident wind corresponds to a power law profile with an exponent α = 0.27 and a gradient height zg = 1.20 m. Wind tunnel measurements were made using a hot-wire anemometer system. The wind turbine model corresponds to a three-bladed UNIPOWER wind turbine, with a tower height of 100 m and a rotor diameter of 100 m. The scale of the model is approximately 1/450, and the model height is 0.33 m. **Figure 20** shows the wind

The rotational velocity of the wind turbine was estimated and remained nearly constant during the measurements, but the values of the dimensionless speed ratio *λ* ensure the similarity of phenomenon in the range of the proper operation of the generator. Vertical profiles of dimensionless mean longitudinal velocity *U*/*Uref* measured in the incident wind and in the wake generated by the wind turbine are

**140**

**Figure 20.**

**Figure 19.**

*tunnel of the LAC/UFRGS.*

*Wind turbine model spinning during the wind tunnel test.*

incident, making the turbine model rotate.

*Vertical profiles of dimensionless mean longitudinal velocity and spectral comparison of the incident wind and the wake flow at z = 225 mm.*

indicated in **Figure 21**. Two profiles measured at locations x = 225 and 1185 mm downwind of the plane determined by the rotor blades are included. The comparison of the characteristic spectra of the incident wind and those obtained in the wake are also shown in **Figure 21** and allow observing the changes in the energy fluctuation distribution. These changes are a product of the turbulence introduced by the wind generator. Measurements allowed to analyze the configuration of the spectra in different frequency ranges, the effect of analog signal filtering, and differences in the spectral behavior of the incident wind relative to wind in the wake of the turbine.
