## 3 Experiment Results
### Effect of Airflow Speed  
The far-field _SPL_ and _dSPL_ of the #2 and #3 cylinders (\(r\) = 40, 30) under different airflow speed are shown in Figure 11. The far-field _SPL_ is quite different at the #1 microphone under different airflow speeds. A higher airflow speed will lead to a larger far-field _SPL_. Furthermore, we analyze the relative difference between the wavy cylinders and the baseline cylinder under the same conditions. The _dSPL_ is the increment in the sound pressure level between the wavy cylinders and the baseline cylinders under the same airflow speed. Compared with the baseline cylinder, the far-field _dSPL_ of cylinders is similar under different airflow speeds. The main differences are the minimum far-field _dSPL_ and its corresponding frequency, which will become larger as the airflow speed is increased. Figure 12 presents the _OASPL_ distribution of the wavy cylinders (\(r\) = \(\infty\), 40, 30, and 24) and the baseline cylinder under different airflow speeds. The _OASPL_ is increased along with the increasing airflow speed. The increment in _OASPL_ between U = 30 m/s and U = 40 m/s is the largest for the same aspect ratio. Moreover, the increment in _OASPL_ between U = 60 m/s and U = 50 m/s is the lowest one for the same aspect ratio. Regardless of the aspect ratio, compared with the baseline cylinder, the wavy cylinder shows similar _OASPL_ variation trends when the airflow speed is varied. \begin{table}
\begin{tabular}{c c c} \hline \hline
**U (m/s)** & \(\mathbf{A}\) & \(\mathbf{B}\) \\ \hline
30 & 0.8467 & 0.001008 \\ \hline
40 & 0.7373 & 0.0009575 \\ \hline
50 & 0.7977 & 0.0008926 \\ \hline
60 & 0.804 & 0.0009734 \\ \hline \end{tabular}
\end{table} 
Table 2: Coefficients of the fitting formula between DOASPL and r.  
Figure 10: Averaged _DOASPL_ between the wavy cylinders with different aspect ratio and the baseline cylinder (U = 30, 40, 50, 60 m/s). Figure 11: Far-field _SPL_ and _dSPL_ of the #2 and 3 cylinders under different airflow speeds (\(r=30,24\)). Figure 12: _Cont_. The _DOASPL_ between the baseline cylinder and the wavy cylinders with different aspect ratios (\(r=40\), \(30\), and \(24\)) is shown in Figure 13. Obviously, the wavy cylinder with a large aspect ratio and large spanwise wavelength has a large noise reduction compared with the baseline cylinder. The noise reduction of the \(\#2\) wavy cylinder (\(\lambda=30\)) is the largest, but decreased slightly when \(\lambda\) is changed to \(40\) or \(24\). Figure 12: _OASPL_ distribution of the wavy cylinders and the baseline cylinder under different airflow speeds (\(r=\infty\), \(40\), \(30\), and \(24\)). Figure 13: _DOASPL_ between the wavy cylinders with different aspect ratios and the baseline cylinder (\(r=40\), \(30\), and \(24\)). Figure 14 shows the averaged _DOASPL_ between the wavy cylinder with different aspect ratios and the baseline cylinder (\(r\) = 40, 30, and 24). An obvious linear variation trend can be observed in Figure 14, i.e., the 1/2 power of the airflow speed divided by the averaged _DOASPL_ is linear with the 2 power of the airflow speed. The slope of the curves is almost the same for all the aspect ratios. \[DOASPL=U/\left(A+B\cdot U^{2}\right)^{2} \tag{9}\]  
where, U is the airflow speed, \(A\) and \(B\) are the coefficients of the fitting formula between _DOASPL_ and U, as shown in Table 3. Formula (9) is useful since it may be used to predict the _OASPL_ reduction using the wavy cylinders. The sound power reflects the ability of the object radiating noise. The sound power of all the cylinders with different aspect ratios (at U = 30 m/s) is presented in Figure 15, which is not related with the distance between the cylinder and microphone. There is no doubt that the #0 cylinder has the largest ability to radiate noise, followed by the #1 cylinder, #3 cylinder and #2 cylinder. The #2 cylinder has the smallest sound power to generate the flow-noise. The maximum sound power appears within the frequency between 150 Hz and 300 Hz.