## 3 Results and Discussion
### Analysis of PIV Experimental Results  
As described above, a PIV system was used to measure the flow field to further understand the underlying mechanism of the bypass passive jet flow control. Similar to the analysis of the pressure distribution, the PIV experimental results also focus on Case 4 and Case 0 under two wind attack angles of \(-20^{\circ}\) and \(15^{\circ}\). Figures 9 and 10 plot the mean value of \(u\) and \(v\) distributions around test model. The positive \(u\) value denotes an in-flow direction and positive \(v\) the upward direction. When the angle of wind attack is \(-20^{\circ}\), the mean \(u\) and \(v\) distributions are plotted in Figure 9a,b. It can be seen that the negative \(u\) on lower surface and wake flow in Case 4 is smaller than in Case 0, and the positive \(v\) on lower surface wake flow in Case 4 is also smaller than in Case 0. This is due to the passive jet formed by Case 4 into the wake. The jet flow interacts with the backward flow and results in a modification to the wake flow of Case 0. Figure 8: Distributions of mean and RMS values of pressure coefficient at the wind attack angle of \(15^{\circ}\): (a,b) are the upper surface and (c,d) are the lower surface. Figure 9: _Cont._When the wind attack angle is \(15^{\circ}\), the mean \(u\) and \(v\) distributions are plot in Figure 10a,b. It is shown that negative \(u\) on upper surface and wake flow in Case 4 is smaller than Case 0, and positive \(v\) on upper surface wake flow in Case 4 is also smaller than Case 0. The reason remains the same. Figure 10: The PIV measurement results for Case 0 and by-pass jet control at wind attack angle of \(15^{\circ}\). (**a**) Case 0; (**b**) Case 4. Figure 9: The particle imaging velocimetry (PIV) measurement results for Case 0 and by-pass jet control at wind attack angle of \(-20^{\circ}\). (**a**) Case 0; (**b**) Case 4. At the wind attack angle of \(-20^{\circ}\), the instantaneous and time-averaged flow fields are shown in Figure 11. A reverse vortex structure formed on the upper and lower surfaces of the test model of Case 0, which can be seen in Figure 11a. The vortex structure of the upper surface separated from the upper tail of the test model, while the vortex structure of the lower surface separated from the lower leading edge of the test model. The detachment of the unsteady wake vortex structure on the upper and lower surfaces of the model will result in obvious changes in the surface pressure around the model structure. In particular, this happens in the area of flow separation on the lower surface of the test model, as indicated by the surface pressure measurement results given in Figure 7c. For Case 4, the results of the vortex structure around the test model were similar to Case 0, as shown in Figure 11b. However, the vortex strength of Case 4 is lower than that of Case 0, while the wake width is decreased. This is consistent with the fluctuating pressure coefficient distributions, as shown in Figure 7d. The time-averaged flow fields of both cases are given in Figure 11c,d, respectively. The turbulence kinetic energy (TKE) in the flow fields around the model can be used to indicate the instability of the surface pressure of the bridge model [29], which in turn can represent the instability of the aerodynamic forces in the model. Therefore, TKE values in the flow fields around the test model were used as an indicator to assess the efficiency of the bypass passive jet flow control in this study. It was found that TKE distribution values in the wake of Case 4 are much lower than those in the model without control. Therefore, there will be reduced fluctuations in the aerodynamic forces, which is consistent with the above pressure and aerodynamic measurement results. Moreover, the recirculation region of Case 4 was suppressed to a smaller size. Figure 11: The PIV measurement results for Case 0 and bypass jet control at a wind attack angle of \(-20^{\circ}\).