# An Experimental Investigation of Passive Jet Control Method on Bridge Tower Wake
## 1 Introduction
### Experiment Details and Measurement Point Arrangement  
In this experiment, a four-hole pressure probe (Series 100 Coba Probe, Turbulent Flow Instrumentation Pty Ltd., Victoria, Australia) was used to measure the characteristics of the bridge tower wake flow. There are 172 measurement points in the zone of tower wake, distributed on a 2.4 m \(\times\) 1.8 m horizontal plane 1.5 m above the ground (half of the wind tunnel section height), as shown in Figure 1. In this plane, nine rows perpendicular to the incoming flow distribute at the interval of 300 mm, and nineteen columns parallel to the incoming flow distribute at the interval of 100 mm. The middle measurement point in the first row is 200 mm away from the back of the tower model. Taking the same scale ratio as the tower model, the realistic locations of four suspenders behind the bridge tower correspond to the middle points in rows 3, 5, 7, and 9. The measurement points distribution  
Figure 2: Passive jet ring structure schematic diagram.  
can be seen in Figure 3. At each measurement point, the sampling frequency is 625 Hz, and the sampling time is 20 s.  
The cobra probe is fixed on the bracket on the three-dimensional traverse measurement system, with its probe head straight towards the incoming flow. The traverse measurement system has a built-in coordinate system, by which we can precisely change the position of the probe. It also has a jack system to firmly fix the whole traverse measurement system when data are collected. The position of the probe is changed to the measurement points mentioned above one by one during the experiment. The incoming wind speed \(U_{0}\) is set to 10 m/s, corresponding to the range of Reynolds numbers (\(Re=\rho U_{0}L/\mu\), where \(\rho\) is the air density, \(\mu\) is the dynamic viscosity coefficient of air, \(L\) is the projection length of the tower cross-section) from 1.38 \(\times\) 10\({}^{5}\) to 2.27 \(\times\) 10\({}^{5}\), which varies by the projection length (\(L=B\)in \(\alpha+D\)cos \(\alpha\)) when the yaw angle \(\alpha\) changes. As the yaw angle changes, the corresponding positions of the suspenders vary, revolving the same degrees around the bridge tower. The locations of the first four imaginary suspenders behind the tower are labeled A1 to A4. When the yaw angle continues to increase, the imaginary suspenders behind the other side tower come into the wake zone of the tower model, the locations of which are labeled B1 to B4. There are five test conditions in this study, recorded as Case 0-Case 4. Case 0 is the baseline condition, in which only a tower model is tested, without passive jet control. Case 1-Case 4, corresponding to the category of the passive jet ring from 1 to 4, respectively, are controlled conditions, in which fourteen passive jet rings are installed on the tower model, distributed at the interval of 100 mm. The ring in the middle is installed 1.5 m from the ground, the same height as the probe. The yaw angle changes from 0 to 90 in Case 0 to Case 2, and from 0 to 30 in Case 3 to Case 4, because few suction slits work at a large angle in Case 3 and 4. The test conditions are shown in Table 1, and the value of suction coefficients at different yaw angles in each case are listed in Table 2.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Designation** & **Type of Passive Jet Ring** & **Slits Number at Long Side** & **Slits Number at Short Side** & **Length of Slits** \\ \hline Case 0 & without control & - & - & - \\ Case 1 & Category 1 & 9 & 7 & 80 mm \\ Case 2 & Category 2 & 7 & 5 & 60 mm \\ Case 3 & Category 3 & 0 & 7 & 80 mm \\ Case 4 & Category 4 & 0 & 5 & 60 mm \\ \hline \end{tabular}
\end{table}
Table 1: Test conditions of the experiment.  
Figure 3: Measurement points arrangement sketch map.