# Wind Speed Measurement via Visual Recognition of Wind-Induced Waving Light Stick Target
## 5 Practical Application
### Results and Discussion  
The detected yaw angles of the light sticks, the corresponding calculated wind speeds, and the measured wind speed based on the T-tube pitot method at the four test points TP1#, TP2#, TP3#, and TP4# are given in terms of time-history curves (Figure 25). Encouraging agreement is observed in the variation trend between the proposed method and the T-tube pitot method.  
As seen above, during the first 8 s when no wind is blowing in the tunnel, the sticks stay in a constant yaw angle due to the steady flow driven by the inner blower from the outside environment. When the tunnel wind increases to 5 m/s, 10 m/s, 15 m/s, 20 m/s, and 25 m/s, the sticks displace to-and-fro, accordingly, with good response to prevailing wind fluctuation detected by the T-tube pitot. However, wind speed detection via the proposed method is smaller than that detected by the T-tube pitot method and there is also a slight time delay in the peak response of the proposed method. Because the motion of the stick is mainly influenced by the interplay between its deadweight, gravity, and wind pressure, as the wind speed increases, the response time also generally climbs because it takes longer for the stick to find a new equilibrium state from a higher starting speed, as gravity forces it to return. Furthermore, in general, the stick rises almost steadily from the first instance of a speed increase, fluctuates around the peak speed for the same equilibrium-finding reason, before it falls at the end of the test case. At every increase in wind speed, the stick rises almost steadily, which can also be observed in the pitot measurement. The T-tube method also appears to be more sensitive to sharp variations in the wind speed. While the proposed method follows the trend closely, it does not capture the peaks as sharply because, for some split-second, it needs to rise or fall, while continually pushing towards an equilibrium state that continues to change. In contrast, the T-tube pitot method has no moving parts and so directly senses the pressure as it varies over time.  
In addition, and as is evident from the yaw angle trend, increasing and decreasing according to the prevailing wind speed, measurement beyond critical angle limits did not arise since the model adopted was front-view dominated where limits are \(\pm\)90\({}^{\circ}\) (Equation (25)), which is the maximum.  
By calculating the absolute error between values detected by the proposed method and those expected by the T-tube pitot at each test point, the Root Mean Square Errors (RMSEs) are \(\pm\)0.8 m/s, \(\pm\)1.01 m/s, \(\pm\)1.2 m/s, and \(\pm\)1.4 m/s, respectively, for TP1#, TP2#,  
Figure 25: Wind speed time history of pitot T-tube method and the proposed method. (**a**) Test point TP1#; (**b**) Test point TP2#; (**c**) Test point TP3#; (**d**) Test point TP4#.  
TP3#, and TP4# (Figure 26) when the maximum yaw angles are detected at 52.1\({}^{\circ}\), 50.7\({}^{\circ}\), 53.5\({}^{\circ}\), and 55.1\({}^{\circ}\). The two methods agree well, although the proposed method is offset lower by about 0.73 m/s on average from the T-tube pitot method. This offset may be due to the inaccurate estimation of the T-tube pitot orientation [25; 26].  
The results comparison between the two methods at the four test points shows a mean absolute error of 1.1 m/s. The imperfectly uniform distribution of wind pressure, inaccurate estimation of the T-tube pitot orientation, and image-processing-related errors are suspected to account for the relative differences in the measurement results seen at the different test points. The low RMSE values demonstrate the robustness of the proposed method to measure wind speed with good response time.