# Numerical Analysis on Spanwise Correlation of Vortex-Induced Force of Split Double-Box Beam
## 5 Comparison with Measured Data
### Pressure Distribution Law in Lock-In Range  
The average distribution of pressure \(\overline{C}_{P}\) around the deck of the inner bridge measured by [PERSON] et al. [26] is shown in Figure 14.  
Figure 13: Spanwise correlation of wake wind velocity of the main beam in the vibrational state. (**a**) Spanwise correlation of along-wind fluctuating wind velocity; (**b**) Spanwise correlation of across-wind fluctuating wind velocity; (**c**) Spanwise correlation of vertical-wind fluctuating wind velocity.  
\(\overline{C}_{P}\) at the measurement point is defined as:  
\[\overline{C}_{P_{i}}=\frac{2\left(\overline{P}_{i}-P_{\infty}\right)}{\rho U_{ \text{ref}}^{2}} \tag{18}\]  
where: \(\overline{P}_{i}\)--the mean pressure at the measurement point on the model surface, \(\rho\)--the air density, taken as 1.225 kg/m\({}^{3}\), \(P_{\infty}\)--the static pressure at the reference height, \(U_{ref}\)--the velocity in the downwind direction at the reference height.  
The pressure distribution of pressure measuring Section 1 in numerical simulation is shown in Figure 15 when the reduced wind speed is 6.21.  
It can be found from Figures 14a and 15a that:  
The trend of the pressure distribution law in general gained from numerical simulation in this paper is the same as that in the document [26]. The pressure coefficient on the lower surface of the upstream section is decreasing gradually and reaches the maximum negative value at the corner (0.3), and then resumes its upward trend. The pressure coefficients are all negative on the lower surface of the downstream section and those near the waking wind of the downstream section are all close to 0.  
It can be found from Figures 14b and 15b that:  
The pressure coefficient at the tail of the upstream section in the middle groove is negative, which decreases first and then increases gradually in this paper, while the pressure coefficient increases first and then decreases gradually at the leading edge of the downstream section. There is a vortex formed in the middle groove. That is to say that  
Figure 14: The average pressure distribution within 10 min in the lock-in range of vortex-induced vibration of the SI section in the document [26]. (**a**) Pressure distribution on section SI of the main beam; (**b**) Vertical pressure distribution of the upstream and downstream sections of the middle groove.  
Figure 15: Pressure distribution of pressure measuring Section 1 in numerical simulation of vortex-induced vibration. (**a**) Pressure distribution on the lower surface of pressure measuring Section 1 of the main beam; (**b**) Pressure distribution of the upstream and downstream sections of the middle groove.  
the airflow is shedding and forming complicatedly in the middle groove. Compared to Figure 14b, it can be seen that the results above are quite different from those obtained in the document [26]. The possible reason is that it takes 0.001 s as the time step to record the data in field measurement, while the pressure coefficient shown in Figure 14 is the average of the pressure in 10 min, of which the volume of data is larger, and the description of the vortex-vibration period is more comprehensive. It can also be caused by errors due to operation in the field measurement.