# Wind Characteristics in Mountainous Valleys Obtained through Field Measurement
## 3 Results
### Turbulence Characteristics
#### 3.2.1 Turbulence Intensity  
The turbulence intensity is the ratio of the root mean square to the mean value of the fluctuating wind speed. This is often used to describe the degree of pulsation of natural wind. The turbulence intensity can be computed as  
\[I_{i}=\frac{\sigma_{i}}{U} \tag{3}\]  
Figure 12: Height relationship diagram of wind attack angle. Figure 13: Frequency statistics of wind attack angle: (**a**) frequency distribution of wind attack angle; (**b**) normal probability diagram of wind attack angle at a 30 m height. where \(I_{i}\) is the turbulence intensity in three directions; the subscript \(i\) denotes \(u\), \(v\), or \(w\); \(\sigma_{i}\) is the RMS value of wind speed fluctuations; and \(\overline{U}\) is the mean wind speed. In the Chinese specifications, the relationships of the three turbulence intensities are as follows:  
\[I_{u}:I_{v}:I_{w}=1:0.88:0.50 \tag{4}\]  
where \(I_{u}\), \(I_{v}\), and \(I_{w}\) represent the degree of turbulence in the three directions. The collected wind speed and direction data were filtered and segmented to obtain nearly 8000 wind samples in 10 min intervals, and the resulting turbulence was calculated by statistical analysis. Figure 14 shows that the mean value of the (10 min mean direction) turbulence \(I_{u}\) in the downwind direction on the northeast side was 13.07% with a standard deviation of 3.14. The maximum value exceeded 22.5%. The mean value of the turbulence \(I_{u}\) in the downwind direction on the southwest side was 12.57%, and the standard deviation was 3.67. The maximum value exceeded 25%. The mean value obtained by fitting the wind profile according to the exponential law was 0.21. According to the latest version of China's wind-resistant design specifications for highways and bridges, the recommended value for turbulence intensity in the height range of 30 m \(<\)\(z\)\(\leq\) 40 m on gentle hilly ground is 21%. The mean values of the turbulence on both the northeast and southwest sides are smaller than the recommended values for this type of surface, and the values are between those recommended for surfaces such as open water and flat open land. This may be related to the excessive wind speed threshold of 5 m/s chosen in this study. The test of whether the normal distribution is obeyed was performed with a 95% confidence level. From the normal probability plot of the turbulence \(I_{u}\) in the downwind direction on the northeast and southwest sides, we found that the percentile of the sample  
Figure 14: Probability statistics of turbulence \(I_{u}\) downwind: (**a**) turbulence degree \(I_{u}\) on the northeast side; (**b**) normal probability diagram of turbulence \(I_{u}\) on the northeast side; (**c**) turbulence degree \(I_{u}\) on the southwest side; (**d**) normal probability diagram of turbulence \(I_{u}\) on the southwest side. based on the assumption of normal distribution basically coincides with the normal distribution reference line. The percentile of the two-sided portion lies below the reference line and shows a typical right-handedness distribution. The degree of rightward bias is greater on the southwest side. That is, the frequency of low turbulence in the downwind turbulence \(I_{u}\) sample is higher on the southwest side than on the northeast side. In Figure 15, the mean value of the crosswind direction (the direction perpendicular to the 10 min mean direction) turbulence \(I_{v}\) on the northeast side is 6.67%, with a standard deviation of 1.99. The maximum value is close to 14%. The mean value of the crosswind direction turbulence \(I_{v}\) on the southwest side is 5.50% with a standard deviation of 1.81. The maximum value is more than 12%. The formula suggested by the design specification for wind resistance of highways and bridges in China is \(I_{v}=0.88I_{u}\). According to the statistical mean value of the measured windward turbulence, the crosswind turbulence \(I_{v}\) on the northeast and southwest sides is 11.5% and 11.06%, respectively.